1. What should you do to store an object in a Viewstate?

Do serialization of convert the object to string

2. What should one do to make class serializable?

To make a class serializable is to mark it with the Serializable attribute as follows.
public class MyObject {
public int n1 = 0;
public int n2 = 0;
public String str = null;

3. What is Viewstate in .NET?

A server control's view state is the accumulation of all its property values. In order to preserve these values across HTTP requests, ASP.NET server controls use this property, which is an instance of the StateBag class, to store the property values.

4. What is the use of ErrorProvider Control in .NET?

The ErrorProvider control is used to indicate invalid data on a data entry form. Using this control, you can attach error messages that display next to the control when the data is invalid, as seen in the following image. A red circle with an exclamation point blinks, and when the user mouses over the icon, the error message is displayed as a tooltip.

5. What is the Difference Between Response.write & response.output.Write?

In ASP.NET the Response object is of type HttpResponse and when you say Response.Write you're really saying (basically) HttpContext.Current.Response.Write and calling one of the many overloaded Write methods of HttpResponse. Response.Write then calls .Write() on it's internal TextWriter object:
public void Write(object obj){ this._writer.Write(obj);}
HttpResponse also has a Property called Output that is of type, yes, TextWriter, so:
public TextWriter get_Output(){ return this._writer; }
Which means you can to the Response whatever a TextWriter will let you. Now, TextWriters support a Write() method ala String.Format, so you can do this:
Response.Output.Write(”Scott is {0} at {1:d}”, “cool”,DateTime.Now);
But internally, of course, this this is happening:
public virtual void Write(string format, params object[] arg)
this.Write(string.Format(format, arg));

6. Which DLL translate XML to SQL in Internet Information Server (IIS)?

DLL used to translate XML to SQL in Internet Information Server (IIS)

7. Why The JavaScript Validation Not Run on the Asp.Net Button But Run Successfully On The HTML Button?

The Asp.Net Button Is post backed on the server & not yet Submit & when It goes to the server its states is lost So if we r using JavaScript in our application so we always use the Input Button in the asp Button

8. When we go for html server controls and when we go for web server controls?

Server controls are a part of ASP.net. When a server control is used there will be an extra overhead on the server to create the control at run time and accordingly set the values. HTML controls are static controls and are easy to use. They are supported is ASP.net.
As a rule, if there is a corresponding HTML control available instead of the server control, you should always go for the HTML control as it enhances the server performance and ensures faster response. Server controls should be used when it is found that the available HTML controls are not sufficient to achieve the task.

9. what is the difference between user control an custom control? advantages/disadvantages?

Web user controls Vs Web custom controls Easier to create Vs Harder to create
Limited support for consumers who use a visual design tool Vs Full visual design tool support for consumers
A separate copy of the control is required in each application Vs Only a single copy of the control is required, in the global assembly cache
Cannot be added to the Toolbox in Visual Studio Vs Can be added to the Toolbox in Visual Studio
Good for static layout Vs Good for dynamic layout

http://msdn.microsoft.com/library/default.asp?url=/library/ en-us/vbcon/html/vbconwebusercontrolsvscustomwebcontrols.asp

10. Which dll is required to translate XML to SQL in Internet Information Server (IIS)?

Microsoft.data.sqlxml.dll used to translate XML to SQL using Internet Information Server IIS