Using C# method group executes code

Question

While updating my UI code (C# in a .NET 4.0 application), I ran into a strange crash due to a call to the UI being executed in the wrong thread. However, I was invoking that call on the main thread already, so the crash made no sense: MainThreadDispatcher.Invoke(new Action(View.Method)) crashed with "The calling thread cannot access this object because a different thread owns it." on the View property.

Upon further investigation I found the cause: I was invoking via a method group. I had thought that using a method group or a delegate/lambda are essentially the same thing (see also this question and this question). Instead, converting the method group to a delegate causes code to execute, checking the value of View. This is done immediately, i.e. on the original (non-UI) thread, which caused the crash. If I use a lambda instead, checking the property is done later, and thus in the correct thread.

That seems interesting, to say the least. Is there anyplace in the C# standard where this is mentioned? Or is that implicit due to the need to find the correct conversion?

Here's a test program. First, the direct way. Second, in two steps, which better shows what happens. For additional fun, I then modify Item after the delegate has been created.

namespace ConsoleApplication1 // Add a reference to WindowsBase to a standard ConsoleApplication
{
    using System.Threading;
    using System.Windows.Threading;
    using System;

    static class Program
    {
        static Dispatcher mainDispatcher;
        static void Main()
        {
            mainDispatcher = Dispatcher.CurrentDispatcher;
            mainDispatcher.Thread.Name = "Main thread";
            var childThread = new Thread(() =>
                {
                    Console.WriteLine("--- Method group ---");
                    mainDispatcher.Invoke(new Action(Item.DoSomething));

                    Console.WriteLine("\n--- Lambda ---");
                    mainDispatcher.Invoke(new Action(() => Item.DoSomething()));

                    Console.WriteLine("\n--- Method group (two steps) ---");
                    var action = new Action(Item.DoSomething);
                    Console.WriteLine("Invoking");
                    mainDispatcher.Invoke(action);

                    Console.WriteLine("\n--- Lambda (two steps) ---");
                    action = new Action(() => Item.DoSomething());
                    Console.WriteLine("Invoking");
                    mainDispatcher.Invoke(action);

                    Console.WriteLine("\n--- Method group (modifying Item) ---");
                    action = new Action(Item.DoSomething);
                    item = null;
                    mainDispatcher.Invoke(action);
                    item = new UIItem();

                    Console.WriteLine("\n--- Lambda (modifying Item) ---");
                    action = new Action(() => Item.DoSomething());
                    item = null;
                    Console.WriteLine("Invoking");
                    mainDispatcher.Invoke(action);

                    mainDispatcher.InvokeShutdown();
                });
            childThread.Name = "Child thread";
            childThread.Start();

            Dispatcher.Run();
        }

        static UIItem item = new UIItem();
        static UIItem Item
        {
            get
            {
                // mainDispatcher.VerifyAccess(); // Uncomment for crash.
                Console.WriteLine("UIItem: In thread: {0}", Dispatcher.CurrentDispatcher.Thread.Name);
                return item;
            }
        }

        private class UIItem
        {
            public void DoSomething()
            {
                Console.WriteLine("DoSomething: In thread: {0}", Dispatcher.CurrentDispatcher.Thread.Name);
            }
        }
    }
}

Short version:

namespace ConsoleApplication1 // Add a reference to WindowsBase to a standard ConsoleApplication
{
    using System.Threading;
    using System.Windows.Threading;
    using System;

    static class Program
    {
        static Dispatcher mainDispatcher;
        static void Main()
        {
            mainDispatcher = Dispatcher.CurrentDispatcher;
            mainDispatcher.Thread.Name = "Main thread";
            var childThread = new Thread(() =>
                {
                    Console.WriteLine("--- Method group ---");
                    mainDispatcher.Invoke(new Action(Item.DoSomething));

                    Console.WriteLine("\n--- Lambda ---");
                    mainDispatcher.Invoke(new Action(() => Item.DoSomething()));    

                    mainDispatcher.InvokeShutdown();
                });
            childThread.Name = "Child thread";
            childThread.Start();

            Dispatcher.Run();
        }

        static UIItem item = new UIItem();
        static UIItem Item
        {
            get
            {
                mainDispatcher.VerifyAccess();
                Console.WriteLine("UIItem: In thread: {0}", Dispatcher.CurrentDispatcher.Thread.Name);
                return item;
            }
        }

        private class UIItem
        {
            public void DoSomething()
            {
                Console.WriteLine("DoSomething: In thread: {0}", Dispatcher.CurrentDispatcher.Thread.Name);
            }
        }
    }
}

Answer 1

The fact that the property will be eagerly accessed is not special to method-group members in any way; it's a characteristic of member-expressions in general.

It's actually the lambda that's creating the special case: its body (and thus the property-access) will be deferred until the delegate is actually executed.

From the specification:

7.6.4 Member access

[...] A member-access is either of the form E.I or of the form E.I, where E is a primary-expression.

[...] if E is a property or indexer access, then the value of the property or indexer access is obtained (§7.1.1) and E is reclassified as a value.