Question

I was experimenting with the the Type.GetMethod method the other day when I ran into a problem.

My program is a Windows application. There is one textbox in the form used for output and there are only these two blocks of code in the Form1_Load method:

   MethodInfo info2 = typeof(IEnumerable<int>).GetMethod("GetEnumerator");
   textBox1.Text += info2.ToString();

   MethodInfo info1 = typeof(IEnumerator<int>).GetMethod("MoveNext");
   textBox1.Text += info1.ToString();

The first GetMethod returned the correct information, but the second one returned nothing.

So I went one step further and included a try/catch statement in the code:

        try
        {
            MethodInfo info2 = typeof(IEnumerable<int>).GetMethod("GetEnumerator");
            textBox1.Text += info2.ToString();

            MethodInfo info1 = typeof(IEnumerator<int>).GetMethod("MoveNext");
            textBox1.Text += info1.ToString();
        }
        catch (Exception v)
        {
            textBox1.Text += v.Message;
        }

The second GetMethod now gives me the error:

Object reference not set to an instance of an object.

I am confused. Neither method is static and neither takes any parameter. Why does MoveNext return the error while GetEnumerator doesn’t?

Answer 1

It is because the interface IEnumerator<T> does not have a definition for MoveNext, that is in the interface IEnumerator.

Example:

   MethodInfo info1 = typeof(IEnumerator).GetMethod("MoveNext");
   textBox1.Text += info1.ToString();

This will work fine,

You can loop though the Interfaces assigned to IEnumerator<T> and find MoveNext

MethodInfo info1 = typeof(IEnumerator<int>).GetInterfaces()
                  .Where(i => i.GetMethod("MoveNext") != null)
                  .Select(m => m.GetMethod("MoveNext"))
                  .FirstOrDefault();
textBox1.Text += info1 != null ? info1.ToString() : string.Empty;