As from comments you have 2 options, ref and out
1)
private void assign(ref int num)
{
   num = 10;
}
private void doSomething()
{
   int num = 0;
   assign(ref num);
   MessageBox.Show(num.ToString());
}
2)
private int assign(out int num)
{
   num = 10;
}
private void doSomething()
{
   var num = 0;
   assign(out num);
   MessageBox.Show(num.ToString());
}
Normally this is done by returning the value:
private int assign(int input)
{
   //some complicated calculation on input.
   return 10;
}
private void doSomething()
{
   var num = 0;
   num = assign();
   MessageBox.Show(num.ToString());
}