E&C: Using a lambda-captured variable in an async method throws NRE at runtime & debugger

[SLaks]

Version Used: 2017 RC

static async Task<IEnumerable<int>> M(int x) {
	return new int[1].Select(i => x);
}
static void Main(string[] args) {
	for (int i = 0; i < 4; i++) {
		try {
			M(1).Wait();
		} catch (Exception ex) {
			Console.WriteLine(ex);
		}
	}
}

Set a breakpoint on the return statement, then paste x = x;.

Continuing execution will throw the following exception on the first call only:

System.AggregateException: One or more errors occurred. ---> System.ArgumentException: Delegate to an instance method cannot have null 'this'.
   at System.MulticastDelegate.CtorClosed(Object target, IntPtr methodPtr)
   at ConsoleApplication1.Program.<M>d__0.MoveNext()
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at ConsoleApplication1.Program.Main(String[] args)
---> (Inner Exception #0) System.ArgumentException: Delegate to an instance method cannot have null 'this'.
   at System.MulticastDelegate.CtorClosed(Object target, IntPtr methodPtr)
   at ConsoleApplication1.Program.<M>d__0.MoveNext()<---

In addition, the x parameter will throw an NRE in the Locals window on all subsequent calls until the debugger is restarted.

[SLaks]

If you use x directly before the breakpoint, you'll get a simple NRE from MoveNext at runtime instead.

Specifically,

static async Task<IEnumerable<int>> M(int x) {
	x = x;

	return new int[1].Select(i => x);
}

Set a breakpoint on the first line and duplicate it.