Async methods return null

Question

If I try to mock a type containing an async method such as :

interface Foo
{
    Task<int> Bar();
}

Then the mock's Bar method is returning null. I guess Moq is choosing default(Task<int>) as default return value for my method, which is indeed null. However Moq should rather choose something like Task.FromResult(default(int)) as default value. Can I force Moq to make async methods returning non-null Tasks ?

Answer 1

If someone is interested, I made an extension class which makes async methods stubing less verbose :

public static class SetupExtensions
{
    public static IReturnsResult<TMock> ReturnsTask<TMock, TResult>(
        this ISetup<TMock, Task<TResult>> setup) where TMock : class
    {
        return setup.Returns(() => Task.FromResult(default(TResult)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, TResult>(
        this ISetup<TMock, Task<TResult>> setup, TResult value) where TMock : class
    {
        return setup.Returns(() => Task.FromResult(value));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, TResult>(
        this ISetup<TMock, Task<TResult>> setup, Func<TResult> func) where TMock : class
    {
        return setup.Returns(Task.Factory.StartNew(func));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T, TResult>(
        this ISetup<TMock, Task<TResult>> setup, Func<T, TResult> func) where TMock : class
    {
        return setup.Returns<T>(arg => Task.Factory.StartNew(() => func(arg)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2, TResult>(
        this ISetup<TMock, Task<TResult>> setup, Func<T1, T2, TResult> func) where TMock : class
    {
        return setup.Returns<T1, T2>((arg1, arg2) => Task.Factory.StartNew(() => func(arg1, arg2)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2, T3, TResult>(
        this ISetup<TMock, Task<TResult>> setup, Func<T1, T2, T3, TResult> func) where TMock : class
    {
        return setup.Returns<T1, T2, T3>((arg1, arg2, arg3) => Task.Factory.StartNew(() => func(arg1, arg2, arg3)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2, T3, T4, TResult>(
        this ISetup<TMock, Task<TResult>> setup, Func<T1, T2, T3, T4, TResult> func) where TMock : class
    {
        return setup.Returns<T1, T2, T3, T4>((arg1, arg2, arg3, arg4) => Task.Factory.StartNew(() => func(arg1, arg2, arg3, arg4)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock>(this ISetup<TMock, Task> setup, Action action) where TMock : class
    {            
        return setup.Returns(Task.Factory.StartNew(action));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T>(this ISetup<TMock, Task> setup, Action<T> action) where TMock : class
    {            
        return setup.Returns<T>(arg => Task.Factory.StartNew(() => action(arg)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2>(this ISetup<TMock, Task> setup, Action<T1, T2> action) where TMock : class
    {            
        return setup.Returns<T1, T2>((arg1, arg2) => Task.Factory.StartNew(() => action(arg1, arg2)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2, T3>(this ISetup<TMock, Task> setup, Action<T1, T2, T3> action) where TMock : class
    {            
        return setup.Returns<T1, T2, T3>((arg1, arg2, arg3) => Task.Factory.StartNew(() => action(arg1, arg2, arg3)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock, T1, T2, T3, T4>(this ISetup<TMock, Task> setup, Action<T1, T2, T3, T4> action) where TMock : class
    {            
        return setup.Returns<T1, T2, T3, T4>((arg1, arg2, arg3, arg4) => Task.Factory.StartNew(() => action(arg1, arg2, arg3, arg4)));
    }

    public static IReturnsResult<TMock> ReturnsTask<TMock>(this ISetup<TMock, Task> setup) where TMock : class
    {
        return setup.Returns(Task.Factory.StartNew(delegate {  }));
    }
}

Some examples :

//Example 1 :
public interface IFoo
{
    Task Bar();
}

var mock = new Mock<IFoo>();

mock.Setup(m => m.Bar()).ReturnsTask(); //await Bar() will return void

//Example 2 :
public interface IFoo
{
    Task<int> Bar();
}

var mock = new Mock<IFoo>();

mock.Setup(m => m.Bar()).ReturnsTask(); //await Bar() will return default(int)

//Example 3 :
public interface IFoo
{
    Task<int> Bar();
}

var mock = new Mock<IFoo>();

mock.Setup(m => m.Bar()).ReturnsTask(4); //await Bar() will return 4;

//Example 4 :
public interface IFoo
{
    Task<int> Bar(int x, int y);
}

var mock = new Mock<IFoo>();

mock.Setup(m => m.Bar(It.IsAny<int>(), It.IsAny<int>()))
                     .ReturnsTask<IFoo, int, int, int>((x,y) => x + y); //await Bar(x, y) will return x + y;

Answer 2

You will just have to stub the Bar method, and make it return Task.FromResult(default(int))

Answer 3

Looks like this problem is fixed in Moq 4.2. So you just need to upgrade to the newest version of Moq (at least it started returning non-empty Tasks in my case)