Bug 15719 - InvalidProgramException when executing async event handler that doesn't await
Summary: InvalidProgramException when executing async event handler that doesn't await
Alias: None
Product: Compilers
Classification: Mono
Component: C# ()
Version: unspecified
Hardware: PC Mac OS
: Normal normal
Target Milestone: ---
Assignee: Marek Safar
Depends on:
Reported: 2013-10-25 17:50 UTC by Frank A. Krueger
Modified: 2013-10-29 06:52 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Frank A. Krueger 2013-10-25 17:50:49 UTC
I got this exception today when I commented out an await line in an async event handler. This is running on the simulator.

System.InvalidProgramException: Invalid IL code in Calca.iOS.DocumentListController/<HandleTitleTap>c__async45:MoveNext (): IL_0013: ldloc.0   

  at System.Runtime.CompilerServices.AsyncVoidMethodBuilder.Start[<HandleTitleTap>c__async45] (Calca.iOS.<HandleTitleTap>c__async45& stateMachine) [0x0001b] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Runtime.CompilerServices/AsyncVoidMethodBuilder.cs:106 
  at Calca.iOS.DocumentListController.<HandleTitleTap>m__15 (System.Object ss, System.EventArgs ee) [0x00000] in /Users/fak/Dropbox/Projects/Calca/Calca.iOS/DocumentListController.cs:226 
  at Praeclarum.UI.TextInputController+<HandleDone>c__async2C.MoveNext () [0x00198] in /Users/fak/Dropbox/Projects/Praeclarum/Praeclarum.iOS/UI/TextInputController.cs:90 

void HandleTitleTap (object sender, EventArgs e)
			var name = DirectoryName;

			var c = new TextInputController () {
				Title = "Rename",
				InputText = name,
				ValidateFunc = n => AppDelegate.ValidateNewName (n, name),

			var nc = new UINavigationController (c);
			nc.ModalPresentationStyle = UIModalPresentationStyle.FormSheet;

			c.Cancelled += (ss, ee) => {
				nc.DismissViewController (true, null);
			c.Done += async (ss, ee) => {
				nc.DismissViewController (true, null);

				if (c.InputText != name) {
//                                      WHEN THIS IS COMMENTED OUT, bad IL is generated 
//					await Rename (c.InputText);

			PresentViewController (nc, true, null);

TextInputController is just a UITableViewController that fires Cancelled and Done when UIBarButtonItems are clicked.

Here is the IL for MoveNext():

.method public final hidebysig newslot virtual 
	instance void MoveNext () cil managed 
	// Method begins at RVA 0x8614
	// Code size 69 (0x45)
	.maxstack 3

	IL_0000: ldarg.0
	IL_0001: ldfld int32 Calca.iOS.DocumentListController/'<HandleTitleTap>c__async45'::$PC
	IL_0006: ldarg.0
	IL_0007: ldc.i4.m1
	IL_0008: stfld int32 Calca.iOS.DocumentListController/'<HandleTitleTap>c__async45'::$PC
	IL_000d: brtrue IL_0044

	IL_0012: nop
	IL_0013: ldloc.0
	IL_0014: ldfld class [monotouch]MonoTouch.UIKit.UINavigationController Calca.iOS.DocumentListController/'<HandleTitleTap>c__AnonStorey44'::nc
	IL_0019: ldc.i4.1
	IL_001a: ldnull
	IL_001b: callvirt instance void [monotouch]MonoTouch.UIKit.UIViewController::DismissViewController(bool, class [monotouch]MonoTouch.Foundation.NSAction)
	IL_0020: ldloc.1
	IL_0021: callvirt instance string Praeclarum.UI.TextInputController::get_InputText()
	IL_0026: ldloc.0
	IL_0027: ldfld string Calca.iOS.DocumentListController/'<HandleTitleTap>c__AnonStorey44'::name
	IL_002c: call bool [mscorlib]System.String::op_Inequality(string, string)
	IL_0031: brfalse IL_0038

	IL_0036: nop
	IL_0037: nop

	IL_0038: nop
	IL_0039: ldarg.0
	IL_003a: ldflda valuetype [mscorlib]System.Runtime.CompilerServices.AsyncVoidMethodBuilder Calca.iOS.DocumentListController/'<HandleTitleTap>c__async45'::$builder
	IL_003f: call instance void [mscorlib]System.Runtime.CompilerServices.AsyncVoidMethodBuilder::SetResult()

	IL_0044: ret

There is no .locals.
Comment 1 Miguel de Icaza [MSFT] 2013-10-28 12:09:11 UTC

Is this the actual full source code for this?

Because the method contains no "async" on the declaration, but it still produced a MoveNext method, which indicates that perhaps this is a code snippet?

We suspect maybe this was a caching issue, but wanted to confirm.
Comment 2 Frank A. Krueger 2013-10-28 12:25:11 UTC
This is the full code. Notice the async on the Done event handler:

    c.Done += async (ss, ee) => { ...
Comment 3 Marek Safar 2013-10-28 13:13:23 UTC
I can reproduce it with similar code

using System;

class Y

class X
	public event Action<int, string> E;

	void Foo ()
		var nc = new Y ();

		E += async (arg1, arg2) => {
			nc = null;

	public static void Main ()
Comment 4 Marek Safar 2013-10-29 06:52:00 UTC
Fixed in master