Each class definition should live in its own file.

Reasons:

Easy to locate class definitions outside the Visual Studio IDE (e.g. SourceSafe, Windows Explorer)

The only exception should be - classes that collectively forms one atomic unit of reuse should live in one file. For example:

class MyClass

 
{


        ...


}




class MyClassAEventArgs


{


        ...


}




class MyClassBEventArgs


{


        ...


}




class MyClassAException


{


        ...


}




class MyClassBException


{


        ...


}

Bad example - 1 project, 1 file.