ASP.NET Interview questions and answers for web user controls

What are ASP.NET Custom controls?
Custom controls extend the tools available to Web developers. Using custom controls, you can encapsulate key aspects of the visual interface and program logic that you want to reuse throughout your application, or throughout your organization.

What are the 3 types of custom controls in ASP.NET?
Microsoft Visual Studio .NET provides three types of custom control for use on Web forms.
1. Web user controls
These combine existing server and HTML controls by using the Visual Studio .NET Designer to create functional units that encapsulate some aspect of the user interface. User controls reside in content files, which must be included in the project in which the controls are used.
2. Composite custom controls
These create new controls from existing server and HTML controls. Although similar to user controls, composite controls are created in code rather than visually, and therefore they can be compiled into an assembly (.dll), which can be shared between multiple applications and used from the Toolbox in Visual Studio .NET.
3. Rendered custom controls
These create entirely new controls by rendering HTML directly rather than using composition. These controls are compiled and can be used from the Toolbox, just like composite controls, but you must write extra code to handle tasks that are performed automatically in composite controls.

What are the limitations of user controls in ASP.NET?
As the user controls are not compiled into assemblies, they have the following limitations:
1. A copy of the control must exist in each Web application project in which the control is used.
2. User controls can’t be loaded in the Visual Studio .NET Toolbox; instead, you must create them by dragging the control from Solution Explorer to the Web form.
3. User control code is initialized after the Web form loads, which means that user control property values are not updated until after the Web form’s Load event.

What are the steps to follow for creating and using a user control in a Web application?
1. Add a Web user control page (.ascx) to your project.
2. Draw the visual interface of the control in the designer.
3. Write code to create the control’s properties, methods, and events.
4. Use the control on a Web form by dragging it from Solution Explorer to the Web form on which you want to include it.
5. Use the control from a Web form’s code by declaring the control at the module level and then using the control’s methods, properties, and events as needed within the Web form.

How do you identify user controls?
User controls are identified by their .ascx file extensions.

What is the base class from which user controls derive?
User controls derive from System.Web.UI.UserControl base class. This base class provides the base set of properties and methods you use to create the control.

What are the steps to follow to create properties and methods for the user control that you can use from a Web form?
To create properties and methods for the user control that you can use from a Web form, follow these steps:
1. Create the public property or method that you want to make available on the containing Web form.
2. Write code to respond to events that occur for the controls contained within the user control. These event procedures do the bulk of the work for the user control.
3. If the property or method needs to retain a setting between page displays, write code to save and restore settings from the control’s ViewState.

What happens when you drag a user control from solution explorer and drop it on a web form?
When you drag a user control from solution explorer and drop it on a web form, Visual Studio .NET generates a @Register directive and HTML tags to create the control on the Web form.

ASP.NET Interview questions and answers for custom control

What are composite custom controls?
Composite custom controls combine one or more server or HTML controls within a single control class, which can be compiled along with other control classes to create an assembly (.dll) that contains a custom control library. Once created, the custom control library can be loaded into Visual Studio .NET and used in the same way as the standard server and HTML controls.

Composite custom controls are functionally similar to user controls, but they reside in their own assemblies, so you can share the same control among multiple projects without having to copy the control to each project, as you must do with user controls. However, composite controls are somewhat more difficult to create because you can’t draw them visually using the Visual Studio .NET Designer.

What are the steps to follow create and use a custom control in a Web application?
1. Create a solution containing a custom control project.
2. Add a Web application project to the solution, and set it as the startup project. You will use the Web application project to test the custom control during development.
3. Add a project reference from the Web application to the custom control project, and add an HTML @Register directive and control element to use the custom control on a Web form.
4. Create the custom control’s visual interface by adding existing controls to it through the custom control’s CreateChildControls method.
5. Add the properties, methods, and events that the custom control provides.
6. Build and test the custom control.

In general what is the base class for every composite custom control?
System.Web.UI.WebControls.WebControl

Which directive is used to add a custom control to a Web form?
Register directive.

What are the 3 Register directive's attributes?
TagPrefix
This name identifies the group that the user control belongs to. For example, the tag prefix for ASP.NET server controls is “asp”. You use this prefix to create a naming convention to organize your custom controls.
Namespace
This is the project name and namespace within the custom control assembly that contains the controls to register. Microsoft Visual Basic .NET uses the project name as an implicit namespace, so for controls written in Visual Basic .NET, use the project name.
Assembly
This is the name of the assembly (.dll) containing the custom controls. The control assembly must be referenced by the Web application. Referencing the assembly maintains a copy of it in the Web application’s /bin directory.

What are the differences between User Controls and Custom Controls?
1. User Controls are easy to create where as Custom Controls are difficult to create.
2. User Controls cannot be compiled into an assembly, where as Custom Controls can be compiled into an assembly.
3. User Controls cannot be added to tool box, where as Custom controls can be added to the toolbox.
4. You need to have a copy of user control in every project where you want to use it, where as this is not the case with custom controls. You can install a single copy of the Web custom control in the global assembly cache and share it between applications, which makes maintenance easier.
5. User controls are used for reusing existing user interface elements and code, but are not useful for developing reusable components for multiple web applications.

ASP.NET Interview questions and answers for cascading style sheets

What are Cascading style sheets?
Cascading style sheets (CSS) collect and organize all of the formatting information applied to HTML elements on a Web form. Because they keep this information in a single location, style sheets make it easy to adjust the appearance of Web applications.

What are the 3 levels at which formatting can be applied with in a web application?
1. Styles can be defined in a style sheet file. Styles in a style sheet can be applied to all webforms referencing the style sheet.

2. You can also define styles in the page’s head element. These styles can be applied to all elements on the current page.

3. You can also define styles inline, in the HTML tag itself. Inline styles are applicable only to the HTML element in which these styles are defined.

Inline formatting takes precedence over local formatting, which, in turn, takes precedence over global formatting. These precedence rules are the reason style sheets are referred to as cascading.

What are the advantages of storing style definitions in a style sheet file (.css) rather than locally in each Web form or inline with each HTML element?
1. Formatting can be maintained in one location so that you make changes only once for an entire application.

2. Several sets of parallel formatting rules can be maintained in separate style sheets for formatting output on different devices or for different user needs. For example, an application might provide standard, enlarged-type, and printer-friendly style sheets that the user can select at run time.

3. In general, you should use page and inline styles only when you have a really good reason to override the global styles. Relying heavily on page and inline styles can make it difficult to maintain the formatting in a Web application.

What HTML element is used to reference a style sheet on webform?
To reference a style sheet on webform you must add a link element to the page’s head element, as shown below.

link href="Styles.css" type="text/css" rel="st"

What is the use of Style Builder?
Style Builder is used to change the appearance of any of the styles in a style sheet. Changes to the style sheet change the appearance of all Web forms that reference that style sheet.

How do you modify a style sheet using style builder?
To modify a style sheet using style builder, follow these steps:

1. Open the style sheet in Visual Studio. Visual Studio .NET displays the style definitions in the Document window and an outline of the style sheet in the Tool window

2. Select the style to modify from the Tool window. Visual Studio .NET displays the definition for that style in the Document window.

3. Right-click in the style definition or right-click the style in the Tool window, and select Build Style from the shortcut menu. Visual Studio .NET displays the Style Builder Wizard.

4. Use the Style Builder to compose the formatting that you want to add or modify in the selected style, and then click OK.

5. When you have finished, you’ll see that the Style Builder adds the new or modified style attributes to the style definition.

Can you apply styles using class names or element IDs?
Yes, Using class names allows you to apply a single style to a number of different elements or to style the same element differently, depending on how the element is used. Using element IDs allows you to apply a style to a unique element on one or more Web forms.

When you create a style rule for a class, Visual Studio .NET adds a style definition to the style sheet using a .classified identifier.

You apply the style class to HTML elements by using the class attribute. You apply the style to server controls by using the Cssclass attribute.

Can you change style sheets at run time?
Yes.

ASP.NET Interview questions and answers for building application

What are the 2 build options for an ASP.NET web application?
1. Debug
2. Release

What are the 3 levels at which we can have a configuration file for an ASP.NET web application?
1. At the web server level : The Machine.config file located in the Windows\Microsoft.NET\Framework\version\config directory. This sets the base configuration for all .NET assemblies running on the server.
2. At the application or web site level : The Web.config file located in the IIS root directory.This sets the base configuration for all Web applications and overrides settings in Machine.config.
3. In the sub directory of an application root folder : These settings override the settings in the root folder's web.config file.

What happens when you make changes to an application’s Web.config file?
When you make changes to an application’s Web.config file, IIS automatically restarts the application and applies the changes. This has the side effect of resetting current Application or Session state variables, which can adversely affect users.

What happens when you access the Web.config file from a browser?
For security reasons, you can’t access the Web.config file from a browser. If a user requests the Web.config file from your Web site, he or she will receive an "This type of page is not served" error message.

What happens when you access the Global.asax file from a browser?
For security reasons, you can’t access the Global.asax file from a browser. If a user requests the Global.asax file from your Web site, he or she will receive an "This type of page is not served" error message.

What are the steps to follow to host a web application on a web server?
1. Use IIS to set up a virtual folder for the application.
2. Copy the Web application to the virtual directory.
3. Add any shared .NET components to the server’s global assembly cache (GAC).
4. Set the security permissions on the server to allow the application to access required resources.

What are the 2 components that should be installed on a web server where, the ASP.NET web application runs?
ASP.NET Web applications run under IIS, so both IIS and the Microsoft .NET Framework must be installed on the server before that server can host Web applications.

What is the purpose of default start page in IIS?
A default page enables IIS to display a page if the user does not specify one in his or her request. For example, a user might make a request using only your domain name, such as http://www.cinterviews.blogspot.com/. If a default page is enabled, IIS will respond with http://www.cinterviews.blogspot.com/default.aspx.

Don’t confuse IIS default pages with the Web Forms application start page in Visual Studio .NET. Visual Studio .NET requires that you set a start page for each project so that the development environment knows which page to display first during debugging. This setting has no effect on the IIS default page settings.

How do you copy the COM component to the server and register?
COM components generally provide a setup program to install or remove them from the system. If the component doesn’t provide a setup program, you can copy it to the server and register it using the MFC RegSvr32.exe utility, as shown below:
RegSvr32 MyComname.dll

What is GAC?
GAC stands for Global Assembly Cache. The global assembly cache (GAC) is a special subfolder within the Windows folder that stores the shared .NET components.

What is the difference between weak-named .NET components and strong-named .NET components?
This difference is how the names are stored within the assembly. Weak names are not guaranteed to be unique and thus cannot be shared without potentially causing conflicts. Strong names are digitally signed and provide a public key that ensures there are no conflicts. Furthermore, .NET components with strong names can’t call unmanaged code (such as COM components) and thus avoid potential conflicts with dependencies.

Weak-named .NET components must be individually copied to the /bin directories of the Web applications where they are used. Strong-named .NET components can be copied into the server’s GAC

What is the account under which the ASP.NET worker process run?
By default, the ASP.NET worker process runs using the ASPNET account, which is created when you install the .NET Framework. This account has limited privileges, which can cause permission-denied errors if your application writes files or tries to read files outside the current Web application’s boundaries.

What are the 3 ways in which you can modify the additional permissions required by your application.?
1. Grant the ASPNET user access to the required files. To use this option, the server must be using the Windows NT file system (NTFS).
2. Change the group the ASPNET user belongs to.
3. Use impersonation to run the process as another user.

Why is it not a good idea to add ASPNET user to the Administrators group?
Adding the ASPNET user to the Administrators group gives your Web application full privileges on the server; however, it also poses a potential security risk because outside users might be able to manipulate your application to hack your server.

How do you impersonate the ASP.NET worker process?
To use impersonation to run the ASP.NET worker process as a user other than ASPNET, set the identity element’s impersonation attribute in the application’s Web.config file.

ASP.NET Interview questions and answers for caching

What is caching?
High-performance Web applications should be designed with caching in mind. Caching is the technique of storing frequently used items in memory so that they can be accessed more quickly. Caching is important to Web applications because each time a Web form is requested, the host server must process the Web form’s HTML and run Web form code to create a response. By caching the response, all that work is bypassed. Instead, the request is served from the reponse already stored in memory.

Caching an item incurs considerable overhead, so it’s important to choose the items to cache wisely. A Web form is a good candidate for caching if it is frequently used and does not contain data that frequently changes. By storing a Web form in memory, you are effectively freezing that form’s server-side content so that changes to that content do not appear until the cache is refreshed.

What directive is used to cache a web form?
The @OutputCache page directive is used to cache a Web form in the server’s memory.

What is the use of duration attribute of @OutputCache page directive?
The @OutputCache directive’s Duration attribute controls how long the page is cached. For example if you set the duration attribute to 60 seconds, the Web form is cached for 60 seconds.

The first time any user requests the Web form, the server loads the response in memory and retains that response for 60 seconds. Any subsequent requests during that time receive the cached response.

After the cache duration has expired, the next request for the Web form generates a new response, which is then cached for another 60 seconds. Thus the server processes the Web form once every 60 seconds at most.

What are the 2 required attributes of the @OutputCache directive?
The @OutputCache directive has two required attributes:
1. Duration
2. VaryByParam. How do you cache multiple responses from a single Web form?
The VaryByParam attribute lets you cache multiple responses from a single Web form based on varying HTTP POST or query string parameters. Setting VaryByParam to None caches only one response for the Web form, regardless of the parameters sent.

You can also cache multiple responses from a single Web form using the VaryByHeaders or VaryByCustom attribute.

The VaryByCustom attribute lets you cache different responses based on a custom string. To use VaryByCustom, override the GetVaryByCustomString method in the Web application’s Global.asax file.

Is it possible to cache a web form without using @OutputCache directive?
Yes, you can cache a web form using the Response object’s Cache property, which returns an HttpCachePolicy object for the response. The HttpCachePolicy object provides members that are similar to the OutputCache directive’s attributes.

Give a simple example to show how to cache a web form without using @OutputCache directive?
For example, the following code caches the Web form’s response for 60 seconds:
private void Page_Load(object sender, System.EventArgs e)
{
// Cache this page
DateTimeLabel.Text = System.DateTime.Now.ToString();
// Set OutputCache Duration. Response.Cache.SetExpires(System.DateTime.Now.AddSeconds(60));
// Set OutputCache VaryByParams.
Response.Cache.VaryByParams["None"] = true;
// Set OutputCache Location.
Response.Cache.SetCacheability(HttpCacheability.Public);
}

The preceding code is equivalent to the following OutputCache directive:
@ OutputCache Duration="5" VaryByParam="None" Location="Any"

What is @OutputCache directive’s Location attribute and the HttpCachePolicy object’s SetCacheability property used for?
The @OutputCache directive’s Location attribute and the HttpCachePolicy object’s SetCacheability property determine where Microsoft ASP.NET stores cached responses. By default, ASP.NET caches responses at any available location that accepts cache items - the client, proxy servers, or the host server. In practice, those locations might or might not allow caching, so you can think of the Location/SetCacheability setting as more of a request than a command.

What is HttpCachePolicy object’s SetAllowResponseInBrowserHistory method used for?
You can override the cache location settings using the HttpCachePolicy object’s SetAllowResponseInBrowserHistory method. Setting that method to True allows the response to be stored in the client’s history folder even if the location setting is None or Server.

ASP.NET Interview questions and answers for cache

Which object can used to store frequently used items in the server’s memory for quick retrieval?
Cache object can be used to store frequently used items in the server’s memory for quick retrieval.

Is the cache object available for all web forms with in a web application?
Yes, the Cache object is global, that is, data stored in the Cache object is available anywhere within a Web application. In this way, the Cache object is very similar to the intrinsic Application object.

What are the 3 different ways to store data in the Cache object?
Use assignment.
Assigning a value to an unused key in the Cache object automatically creates that key and assigns the value to that key. Assigning a value to a key that already exists replaces the cached value with the assigned value.
Use the Insert method.
The Insert method uses parameters rather than assignment to create or change cached data. Insert optionally accepts parameters to establish dependencies and set expiration policy.
Use the Add method.
The Add method is similar to Insert; however, it requires all parameters and returns an object reference to the cached data.

For example, the following Cache statements all add the same item to the cache:

using System.Web.Caching;
private void Page_Load(object sender, System.EventArgs e)
{
if(!IsPostBack)
{
Cache["NewItem"] = "Some string data";
Cache.Add("NewItem", "Some string data", null, Cache.NoAbsoluteExpiration, System.TimeSpan.FromMinutes(1), CacheItemPriority.Default, null);
Cache.Insert("NewItem", "Some string data");
}
}

What are absoluteExpiration and slidingExpiration parmeters of the Insert and Add methods?
absoluteExpiration
A DateTime object that identifies when the data should be removed from the cache. If you’re using sliding expiration, specify Cache.NoAbsoluteExpiration for this parameter.
slidingExpiration
A TimeSpan object that identifies how long the data should remain in the cache after the data was last accessed. If you’re using absolute expiration, specify Cache.NoSlidingExpiration for this parameter.

Which delegate can be used to notify the application when items are removed from the cache?
onRemoveCallback is used to notify the application when items are removed from the cache.

How do you retrieve the value of a cache item stored in the servers memory?
You can retrieve the value of a cache item stored in the servers memory through the item’s key, just as you do with the Application and Session objects. Because cached items might be removed from memory, you should always check for their existence before attempting to retrieve their value, as shown in the following code:

private void Button1_Click(object sender, EventArgs e)
{
if (Cache["ChachedItem"] == null)
{
Lable1.Text = "Cached Item not found.";
}
else
{
Lable1.Text = Cache["ChachedItem"].ToString();
}
}

Which method can be used to remove data from the cache?
Cache object’s Remove method can be used to remove data from the cache as shown in the following code example / sample.

private void RemoveButton_Click(object sender, System.EventArgs e)
{
Cache.Remove("CachedItem");
}

How do you control how long data is cached?
The Cache object’s Add and Insert method parameters allow you to control how long an item is stored in the server’s memory. In practice, these parameter settings provide only indirect control of how long data remains in memory. If your server runs low on available memory, ASP.NET recovers as much memory as possible from expired cache items. If that’s not enough, ASP.NET will unload unexpired items from the cache based on their priority and when they were last accessed.

What is CacheItemPriority enumeration used for?
CacheItemPriority enumeration is used to set the relative importance of cached items. CacheItemPriority.NotRemoveable has the highest priority and CacheItemPriority.Low has the lowest priority.

Which is the only "event” provided by Cache object?
CacheItemRemoved "event” is the only "event” provided by Cache object.

How do you update the Cache object when data changes?
Items stored in the cache are often copies of data that is stored and maintained elsewhere, such as records in a database. Use the Add and Insert methods’ dependency parameter to establish a relationship between a cached data item and an external source, such as a file, a folder, or a group of files.

The dependency parameter accepts a CacheDependency object, which in turn identifies the file, folder, or set of files to watch for changes. ASP.NET checks the time stamp of the items in the CacheDependency object, if one of those time stamps is later than the DateTime entered for the cached item, ASP.NET unloads that item from the cache.

C# interview questions on Destructors

C# interview questions on Destructors

What is a Destructor?
A Destructor has the same name as the class with a tilde character and is used to destroy an instance of a class.

Can a class have more than 1 destructor?
No, a class can have only 1 destructor.

Can structs in C# have destructors?
No, structs can have constructors but not destructors, only classes can have destructors.

Can you pass parameters to destructors?
No, you cannot pass parameters to destructors. Hence, you cannot overload destructors.

Can you explicitly call a destructor?
No, you cannot explicitly call a destructor. Destructors are invoked automatically by the garbage collector.

Why is it not a good idea to use Empty destructors?
When a class contains a destructor, an entry is created in the Finalize queue. When the destructor is called, the garbage collector is invoked to process the queue. If the destructor is empty, this just causes a needless loss of performance.

Is it possible to force garbage collector to run?
Yes, it possible to force garbage collector to run by calling the Collect() method, but this is not considered a good practice because this might create a performance over head. Usually the programmer has no control over when the garbage collector runs. The garbage collector checks for objects that are no longer being used by the application. If it considers an object eligible for destruction, it calls the destructor(if there is one) and reclaims the memory used to store the object.

Usually in .NET, the CLR takes care of memory management. Is there any need for a programmer to explicitly release memory and resources? If yes, why and how?
If the application is using expensive external resource, it is recommend to explicitly release the resource before the garbage collector runs and frees the object. We can do this by implementing the Dispose method from the IDisposable interface that performs the necessary cleanup for the object. This can considerably improve the performance of the application.

When do we generally use destructors to release resources?
If the application uses unmanaged resources such as windows, files, and network connections, we use destructors to release resources.

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ASP.NET Interview questions and answers for themes and skins

ASP.NET Interview questions and answers for themes and skins

What is a "theme" in ASP.NET?
A "theme" is a collection of property settings that allow you to define the look of pages and controls, and then apply the look consistently across pages in a Web application, across an entire Web application, or across all Web applications on a server.
What is the extension for a skin file?
.skin

What are the 2 types of control skins in ASP.NET?
1. Default skins
2. Named skins
What is the difference between Named skins and Default skins?
A default skin automatically applies to all controls of the same type when a theme is applied to a page. A control skin is a default skin if it does not have a SkinID attribute. For example, if you create a default skin for a Calendar control, the control skin applies to all Calendar controls on pages that use the theme. (Default skins are matched exactly by control type, so that a Button control skin applies to all Button controls, but not to LinkButton controls or to controls that derive from the Button object.)
A named skin is a control skin with a SkinID property set. Named skins do not automatically apply to controls by type. Instead, you explicitly apply a named skin to a control by setting the control's SkinID property. Creating named skins allows you to set different skins for different instances of the same control in an application.
What are the 3 levels at which a theme can be applied for a web application?
1. At the page level - Use the Theme or StyleSheetTheme attribute of the @ Page directive.

2. At the application level - Can be applied to all pages in an application by setting the element in the application configuration file.

3. At the web server level - Define the element in machine.config file. This will apply the theme to all the web applications on that web server.

What is the name of the folder that contains the application themes?
App_Themes
What is a global theme?
A global theme is a theme that you can apply to all the Web sites on a server. Global themes allow you to define an overall look for your domain when you maintain multiple Web sites on the same server.
What is the difference between themes and CSS?
1. Themes can define many properties of a control or page, not just style properties. For example, using themes, you can specify the graphics for a TreeView control, the template layout of a GridView control, and so on.

2. Themes can include graphics.

3. Themes do not cascade the way style sheets do. By default, any property values defined in a theme referenced by a page's Theme property override the property values declaratively set on a control, unless you explicitly apply the theme using the StyleSheetTheme property.

4. Only one theme can be applied to each page. You cannot apply multiple themes to a page, unlike style sheets where multiple style sheets can be applied.
What are the security concerns to keep in mind when using themes?
Themes can cause security issues when they are used on your Web site. Malicious themes can be used to:

1. Alter a control's behavior so that it does not behave as expected.

2. Inject client-side script, therefore posing a cross-site scripting risk.

3. Expose sensitive information.

4. The mitigations for these common threats are:

5. Protect the global and application theme directories with proper access control settings. Only trusted users should be allowed to write files to the theme directories.

6. Do not use themes from an untrusted source. Always examine any themes from outside your organization for malicious code before using them on you Web site.

7. Do not expose the theme name in query data. Malicious users could use this information to use themes that are unknown to the developer and thereby expose sensitive information.

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ASP.NET Interview questions and answers for interfaces

ASP.NET Interview questions and answers for  interfaces

Explain what is an Interface in C#?
An Interface in C# is created using the interface keyword. An example is shown below.

using System;
namespace Interfaces
{
   interface IBankCustomer
   {
      void DepositMoney();
      void WithdrawMoney();
   }
   public class Demo : IBankCustomer
   {
      public void DepositMoney()
      {
         Console.WriteLine("Deposit Money");
      }

      public void WithdrawMoney()
      {
         Console.WriteLine("Withdraw Money");
      }

      public static void Main()
      {
         Demo DemoObject = new Demo();
         DemoObject.DepositMoney();
         DemoObject.WithdrawMoney();
      }
   }
}

In our example we created IBank Customer interface. The interface declares 2 methods.
1. void DepositMoney();
2. void WithdrawMoney();

Notice that method declarations does not have access modifiers like public, private, etc. By default all interface members are public. It is a compile time error to use access modifiers on interface member declarations. Also notice that the interface methods have only declarations and not implementation. It is a compile time error to provide implementation for any interface member. In our example as the Demo class is inherited from the IBankCustomer interface, the Demo class has to provide the implementation for both the methods (WithdrawMoney() and DepositMoney()) that is inherited from the interface. If the class fails to provide implementation for any of the inherited interface member, a compile time error will be generated. Interfaces can consist of methods, properties, events, indexers, or any combination of those four member types. When a class or a struct inherits an interface, the class or struct must provide implementation for all of the members declared in the interface. The interface itself provides no functionality that a class or struct can inherit in the way that base class functionality can be inherited. However, if a base class implements an interface, the derived class inherits that implementation.

Can an Interface contain fields?
No, an Interface cannot contain fields.

What is the difference between class inheritance and interface inheritance?
Classes and structs can inherit from interfaces just like how classes can inherit a base class or struct. However there are 2 differences.
1. A class or a struct can inherit from more than one interface at the same time where as A class or a struct cannot inherit from more than one class at the same time. An example depicting the same is shown below.

using System;
namespace Interfaces
{
   interface Interface1
   {
      void Interface1Method();
   }
   interface Interface2
   {
      void Interface2Method();
   }
   class BaseClass1
   {
      public void BaseClass1Method()
      {
         Console.WriteLine("BaseClass1 Method");
      }
   }
   class BaseClass2
   {
      public void BaseClass2Method()
      {
         Console.WriteLine("BaseClass2 Method");
      }
   }

   //Error : A class cannot inherit from more than one class at the same time
   //class DerivedClass : BaseClass1, BaseClass2
   //{
   //}

   //A class can inherit from more than one interface at the same time
   public class Demo : Interface1, Interface2
   {
      public void Interface1Method()
      {
         Console.WriteLine("Interface1 Method");
      }

      public void Interface2Method()
      {
         Console.WriteLine("Interface2 Method");
      }

      public static void Main()
      {
         Demo DemoObject = new Demo();
         DemoObject.Interface1Method();
         DemoObject.Interface2Method();
      }
   }
}

2. When a class or struct inherits an interface, it inherits only the method names and signatures, because the interface itself contains no implementations.

Can an interface inherit from another interface?
Yes, an interface can inherit from another interface. It is possible for a class to inherit an interface multiple times, through base classes or interfaces it inherits. In this case, the class can only implement the interface one time, if it is declared as part of the new class. If the inherited interface is not declared as part of the new class, its implementation is provided by the base class that declared it. It is possible for a base class to implement interface members using virtual members; in that case, the class inheriting the interface can change the interface behavior by overriding the virtual members.

Can you create an instance of an interface?
No, you cannot create an instance of an interface.

If a class inherits an interface, what are the 2 options available for that class?
Option 1: Provide Implementation for all the members inheirted from the interface.

namespace Interfaces
{
   interface Interface1
   {
      void Interface1Method();
   }

   class BaseClass1 : Interface1
   {
      public void Interface1Method()
      {
         Console.WriteLine("Interface1 Method");
      }
      public void BaseClass1Method()
      {
         Console.WriteLine("BaseClass1 Method");
      }
   }
}

Option 2: If the class does not wish to provide Implementation for all the members inheirted from the interface, then the class has to be marked as abstract.

namespace Interfaces
{
   interface Interface1
   {
      void Interface1Method();
   }

   abstract class BaseClass1 : Interface1
   {
      abstract public void Interface1Method();
      public void BaseClass1Method()
      {
         Console.WriteLine("BaseClass1 Method");
      }
   }
}

A class inherits from 2 interfaces and both the interfaces have the same method name as shown below. How should the class implement the drive method for both Car and Bus interface?

namespace Interfaces
{
   interface Car
   {
      void Drive();
   }
   interface Bus
   {
      void Drive();
   }

   class Demo : Car,Bus
   {
      //How to implement the Drive() Method inherited from Bus and Car
   }
}

To implement the Drive() method use the fully qualified name as shown in the example below. To call the respective interface drive method type cast the demo object to the respective interface and then call the drive method.

using System;
namespace Interfaces
{
   interface Car
   {
      void Drive();
   }
   interface Bus
   {
      void Drive();
   }

   class Demo : Car,Bus
   {
      void Car.Drive()
      {
         Console.WriteLine("Drive Car");
      }
      void Bus.Drive()
      {
         Console.WriteLine("Drive Bus");
      }

      static void Main()
      {
         Demo DemoObject = new Demo();
         ((Car)DemoObject).Drive();
         ((Bus)DemoObject).Drive();
      }
   }
}

What do you mean by "Explicitly Implementing an Interface". Give an example?
If a class is implementing the inherited interface member by prefixing the name of the interface, then the class is "Explicitly Implemeting an Interface member". The disadvantage of Explicitly Implemeting an Interface member is that, the class object has to be type casted to the interface type to invoke the interface member. An example is shown below.

using System;
namespace Interfaces
{
   interface Car
   {
      void Drive();
   }

   class Demo : Car
   {
      // Explicit implementation of an interface member
      void Car.Drive()
      {
         Console.WriteLine("Drive Car");
      }

      static void Main()
      {
         Demo DemoObject = new Demo();

         //DemoObject.Drive();
         // Error: Cannot call explicitly implemented interface method
         // using the class object.
         // Type cast the demo object to interface type Car
         ((Car)DemoObject).Drive();
      }
   }
}

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ASP.NET Interview questions Data security

ASP.NET Interview questions Data security

What are the best practices to follow to secure connection strings in an ASP.NET web application?1. Always store connection strings in the site's Web.config file. Web.config is very secure. Users will not be able to access web.config from the browser.
2. Do not store connection strings as plain text. To help keep the connection to your database server secure, it is recommended that you encrypt connection string information in the configuration file.
3. Never store connection strings in an aspx page.
4. Never set connection strings as declarative properties of the SqlDataSource control or other data source controls.

Why is "Connecting to SQL Server using Integrated Security" considered a best practice?
Connecting to SQL Server using integrated security instead of using an explicit user name and password, helps avoid the possibility of the connection string being compromised and your user ID and password being exposed.
What is the advantage of storing an XML file in the applications App_Data folder? 
The contents of the App_Data folder will not be returned in response to direct HTTP requests.
What is Script injection?
A script injection attack attempts to send executable script to your application with the intent of having other users run it. A typical script injection attack sends script to a page that stores the script in a database, so that another user who views the data inadvertently runs the code.

What is SQL injection?
A SQL injection attack attempts to compromise your database by creating SQL commands that are executed instead of, or in addition to, the commands that you have built into your application.

What are the best practices to keep in mind when accepting user input on a web application?
1. Always use validation controls whenever possible to limit user input to acceptable values.
2. Always check the IsValid property of the aspx page. Run the server side code only if the IsValid property value is true. A value of false means that one or more validation controls have failed a validation check.
3. Always perform server side validation irrespective of client side validation being performed or not. This will protect your web application even if the client has by passed the client side validation by disabling javascript in the web browser.
4. Also make sure to re validate user input in the business logic layer of your application.
What are the steps to follow to avoid Script Injection attacks?
1. Encode user input with the HtmlEncode method. This method turns HTML into its text representation.
2. If you are using the GridView control with bound fields, set the BoundField object's HtmlEncode property to true. This causes the GridView control to encode user input when the row is in edit mode.

What are the steps to follow to avoid SQL Injection attacks?
Always use parameterized queries or stored procedures instead of creating SQL commands by concatenating strings together.

Can you encrypt view state data of an aspx page?
Yes, you encrypt view state data of an aspx page by setting the page's ViewStateEncryptionMode property to true.

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