Code Contracts + Async in .NET 4.5: "The method or operation is not implemented"

Question

I receive the following compilation error from ccrewrite when using Code Contracts 1.4.51019.0 in VS2012 on Windows 7 x64: "The method or operation is not implemented."

It appears to be caused by a combination of property accessors and the use of async methods which lack an inner await.

Reproduction steps:

Create a new class library with 'Full' Runtime Contract Checking enabled:

namespace CodeContractsAsyncBug
{
    using System.Threading.Tasks;

    public class Service
    {
        // Offending method!
        public async Task ProcessAsync(Entity entity)
        {
            var flag = entity.Flag;
        }
    }

    public class Entity
    {
        public bool Flag { get; set; }
    }
}

Has anyone else experienced this?

This is Mike Barnett's project. He likes getting email so don't hesitate to ping him about this bug. mbarnett _at_ microsoft _dot_ com — Hans Passant, Oct 27 '12 at 18:59
Have raised this here: http://social.msdn.microsoft.com/Forums/pl-PL/codecontracts/thread/bdb88f77-469a-40c7-ace6-2c1840fb1a8e — Lawrence Wagerfield, Oct 30 '12 at 10:13

score 2 · Answer 1 · answered Jan 13 '13 at 16:46

2

An async method that does not await is usually indicative of a programming error. There is a compiler warning that will inform you of this situation.

If you wish to synchronously implement a method with an asynchronous signature, the normal way to do this is to implement a non-async method and return a Task, such as Task.FromResult<object>(null). Note that with this approach, exceptions are raised synchronously instead of being placed on the returned Task.

answered Jan 13 '13 at 16:46

Stephen Cleary

437,863
77
675
810

1

I am aware of its correct use and you're right in saying that fixing up non-async code resolves the issue. It is still a Code Contracts bug, however :) – Lawrence Wagerfield Jan 14 '13 at 11:55

score 2 · Accepted Answer · answered Oct 09 '13 at 16:38

2

This appears to be fixed in version 1.5 of Code Contracts.

answered Oct 09 '13 at 16:38

Adrian Sureshkumar

666
5
3

score 1 · Answer 3 · answered Apr 25 '13 at 16:42

1

Over the last several months, we have fixed many issues with rewriting async methods. I would suggest you try your code again on the latest installer and if you are still having a problem, please provide a complete repro.

answered Apr 25 '13 at 16:42

Manuel Fahndrich

665
4
12

score 0 · Answer 4 · answered Jan 13 '13 at 14:17

0

I believe async keyword just stands for that - either you have a await during the code, by which it will be generated a Task and handled when the method is called, or you need to return a Task explicitly.

answered Jan 13 '13 at 14:17

slepox

792
4
9

2

You certainly *can* have `async` method without `await`. It usually means that the author of the code doesn't understand how `async` works (and it produces a warning because of that), but it does work and I think it can be useful in some rare cases. – svick Jan 14 '13 at 00:33
There are some other subtle differences too. For example, an exception thrown from an async method will always be applied to the task, and never raise synchronously. – Lawrence Wagerfield Jan 14 '13 at 11:53

Code Contracts + Async in .NET 4.5: "The method or operation is not implemented"

4 Answers4