function of type void from class to form........

You can use an output parameter (as described here[^]), if you dont want to return a value.

Your method does nothing.
Yes, it adds the two numbers together, and puts the result in a variable called "sum1".
But then, the method ends. Since "sum1" is a local variable, it is thrown away when the method exits.

If you want to preserve the result for later use, there are only three practical ways:
1) Return the result as a datatype - I know, you don't want to, but it is the most sensible way to do it:

public static double sum (double a,double b)
   {
   return a + b;
   }

2) Return the result as a static variable (since your method is static, you can't return it as a class variable)

private static double result;
public static double sum (double a,double b)
   {
   result = a + b;
   }

This has the disadvantage that if you have two instances of your class, they will both overwrite the same variable.
3) Change your method to a non-static method, and return the result via a class level variable:

private double result;
public double sum (double a,double b)
   {
   result = a + b;
   }

This is a clumsy solution, particularly if there are many routines doing this for different purposes - each needs it's own variable.

Use option (1) - it is the easiest, most flexible, and most friendly!

If i get you correctly you need to do something like this

class myClass
    {
        public static void sum(double a, double b, out double sum1)
        {
            sum1 = a + b;
        }//in class1
    }

then you call it like this

double a = 2, b = 3, sum1 = 0;
           myClass.sum(a, b,out sum1);
           MessageBox.Show(sum1.ToString());

and that should give you the correct result.

Hope this helps.