Scope of objects in lambda expressions

Question

I am new to C# and trying to understand lambda expressions along with delegates. This is the code I am running:

delegate bool D1();
delegate bool D2(int i);

namespace Console
{
    class Program
    {
        D1 d1;
        D2 d2;

        public void testMethod(int input)
        {
            int j = 0;
            d1 = () => { j = 10; return j < input; };
            d2 = (x) => { return x == j; };
            System.Console.WriteLine("j = {0}", j);

            bool res = d1();
            System.Console.WriteLine("res={0}, j ={1}", res, j);

        }

        static void Main(string[] args)
        {
            Program p = new Program();
            p.testMethod(10);
            System.Console.WriteLine(p.d2(10));
            System.Console.ReadKey();
        }
    }
}

What I don't understand is the invocation of d2 prints true. When d2 was constructed value of j was 0. It was changed only after d1 was invoked later in the testMethod. So how it is printing True ? what am I missing here?

Answer 1

d1 and d2 both refer to the same instance of j. When you set j by calling d1 you're also altering the value of the variable that d2 can see.

If you want them do have different instance of j you'll need to scope the variable:

{
  int j = 0;
  d1 = () => { j = 10; return j < input; };
}

{
  int j = 0;
  d2 = (x) => { return x == j; };
}

However, if you're going to do this then you might as well have two different variables, say j1 and j2 instead. This will be more readable.

Answer 2

When you create a delegate, any variable outside its scope that it uses is captured. In this case, the delegates assigned to d1 and d2 both capture the variable j declared inside testMethod(); they don't create copies of j, they capture the actual variable. In this sense, during the lifetime of d1 and d2, j behaves inside them as if it was a global variable (though there is no such thing as a global variable in C#, of course).

If you want those methods to get separate instances of j, you would need to provide each method with a copy of j instead of just using it. For instance:

public void testMethod(int input)
{
    int j = 0;
    int k = j;
    d1 = () => { j = 10; return j < input; };
    d2 = (x) => { return x == k; };
    System.Console.WriteLine("j = {0}", j);

    bool res = d1();
    System.Console.WriteLine("res={0}, j ={1}", res, j);

}