Is it possible to change the static field value inside different function of same class in c#

A static variable allows you to count instances of a class, by adding 1 to the variable each time you pass through the constructor.

class Foo
{
    static int counter;
    foo()
    {
        counter++;
    }

    static void main()
    {
        Foo bar = new Foo();
        Foo poo = new Foo();
        Console.writeline(counter());
   }
}

NB: Did you also post this question: http://www.codeproject.com/Messages/5167614/Using-static-var-on-a-class.aspx[^], under a different user id?

The question is not clear, by the following reason: "change the static field value inside different function of same class" main mean different things. One understanding of such "inside" situation would be: "There is a static field if a class A. Can I change it in the code another function?" This formulation would be invalid, because there is no "the same" or "another" function. What is "not another"? A field can be defined only in a class, not in a function. If you mean some object defined inside a function, it is never a field. So, the other understanding of this question would be even less valid: "I have a static field defined in one function, can I modify it in another function?" — there is no such thing.

So, the only answer would be: all functions of the same class have the same access to any member of this class. The "static" makes no difference. A static member belongs to the class and not to any particular instance. A static member can be accessed in both static and instance members of a class, but non-static members (they are called instance members all need some instance to operate. For a method, it is done be passing the instance as an implicit method parameters called "this". Static methods don't have it.

If you wanted to ask "can I access a local variable defined in one function, from the code of another function?" (as I already said, it cannot be a field), the answer would be "no". The local variables are created on the stack in each separate stack frame, and a stack frame is discarded after each call. All variables declared in a method leave only inside the call, before return.

—SA