Creating variable of type <derived class> to store <base class> object in C#

in what case i can use this.

You can't. Ever.

The main point of static typing is so that you can rely on the value of a variable being compatible with the type you've declared. So if you declare a variable as:

DerivedClass C;

then any value for C must be able to be treated as a DerivedClass. In particular, all the members of DerivedClass must be available. That simply wouldn't be the case if the value is actually a reference to an instance of BaseClass which doesn't have all those members.

Fortunately the language rules (and therefore the compiler) understand that this would be a Very Bad Thing and so you'll get a compile-time error.