Plz answer them

When do we call garbage collector explicitly and why do we   call it ?  
       How to load assembly at runtime.? when to load ?
       how to load asp.net file at runtime ?
       how to load c# file at runtime ?

ANSWERS:
You call GC when you think some resources may not be collected by .NET Runtime automatically such as IO resources, Files, and unmanaged objects. Here are some articles on GC:
http://www.c-sharpcorner.com/UploadFile/DipalChoksi/UnderstandingGarbageCollectioninNETFramework11292005051110AM/UnderstandingGarbageCollectioninNETFramework.aspx

http://www.c-sharpcorner.com/UploadFile/Ashish1/dispose02152006095441AM/dispose.aspx
http://www.c-sharpcorner.com/UploadFile/tkagarwal/MemoryManagementInNet11232005064832AM/MemoryManagementInNet.aspx

You load an assembly at runtime when you don't know what's in the assembly and to read it dynamically you load it and read its classes and their members. See Assembly class and Load method to learn how to load an assembly at runtime.

I am not sure what do you mean by loading asp.net file at runtime.

You can load and compile a C# file using the Compiler namespace and its classes.