AutoMapper unable to cast TestDbAsyncEnumerable to IQueryable

Question

I've implemented the TestDbAsync fakes from https://msdn.microsoft.com/en-us/library/dn314429(v=vs.113).aspx and I want to be able to use AutoMapper to project to a different type before calling the Async EF methods (ToListAsync, CountAsync, etc.).

I get a cast exception in ProjectionExpression.To

Example code that throws the exception.

_userRepository.GetAll().OrderBy(x => x.Id).ProjectTo<User>.ToListAsync();

This works fine in a non-test scenario, but when I mock the DbSet using the TestDbAsyncEnumerable I get

: Unable to cast object of type 'Namespace.TestDbAsyncEnumerable`1[UserEntity]' to type 'System.Linq.IQueryable`1[User]'.

Right now to get around this I have to ProjectTo after the call to the Async EF extensions. Is there any way to keep the ProjectTo call before the EF extensions?

Reference code:

public class TestDbAsyncEnumerable<T> : EnumerableQuery<T>, IDbAsyncEnumerable<T>, IQueryable<T>
{
    public TestDbAsyncEnumerable(IEnumerable<T> enumerable)
        : base(enumerable)
    { }

    public TestDbAsyncEnumerable(Expression expression)
        : base(expression)
    { }

    public IDbAsyncEnumerator<T> GetAsyncEnumerator()
    {
        return new TestDbAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
    }

    IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
    {
        return GetAsyncEnumerator();
    }

    IQueryProvider IQueryable.Provider => new TestDbAsyncQueryProvider<T>(this);
}

public static Mock<DbSet<T>> ToAsyncDbSetMock<T>(this IEnumerable<T> source)
        where T : class
    {

        var data = source.AsQueryable();

        var mockSet = new Mock<DbSet<T>>();

        mockSet.As<IDbAsyncEnumerable<T>>()
            .Setup(m => m.GetAsyncEnumerator())
            .Returns(new TestDbAsyncEnumerator<T>(data.GetEnumerator()));

        mockSet.As<IQueryable<T>>()
            .Setup(m => m.Provider)
            .Returns(new TestDbAsyncQueryProvider<T>(data.Provider));

        mockSet.As<IQueryable<T>>().Setup(m => m.Expression).Returns(data.Expression);
        mockSet.As<IQueryable<T>>().Setup(m => m.ElementType).Returns(data.ElementType);
        mockSet.As<IQueryable<T>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());

        return mockSet;
    }

score 25 · Accepted Answer · edited May 28 '19 at 08:36

Edit your TestDbAsyncQueryProvider<>.CreateQuery() so that it returns the right type of the expression passed by ProjectTo<>.

Here is my sample implementation.

public IQueryable CreateQuery(Expression expression)
{
    switch (expression)
    {
        case MethodCallExpression m:
            {
                var resultType = m.Method.ReturnType; // it shoud be IQueryable<T>
                var tElement = resultType.GetGenericArguments()[0];
                var queryType = typeof(TestDbAsyncEnumerable<>).MakeGenericType(tElement);
                return (IQueryable)Activator.CreateInstance(queryType, expression);
            }
    }
    return new TestDbAsyncEnumerable<TEntity>(expression);
}

https://gist.github.com/masaedw/95ab972f8181de6bbe48a20ffe9be113

I have written also unit test. It's working.

https://github.com/masaedw/AutoMapper/blob/TestDbAsync/src/IntegrationTests/MockedContextTests.cs

Nice :) Good to see someone tracking the root cause. – Lucian Bargaoanu Aug 19 '17 at 08:49 — Lucian Bargaoanu, Aug 19 '17 at 08:49
I like your product! Thanks! – Masayuki Muto Aug 21 '17 at 08:42 — Masayuki Muto, Aug 21 '17 at 08:42

score 6 · Answer 2 · answered Aug 29 '17 at 09:37

I ran into this same problem, in addition to the accepted answer you might also have the generic version of CreateQuery like I do - I fixed that too like this:

public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
    var queryType = typeof(TestDbAsyncEnumerable<>).MakeGenericType(typeof(TElement));
    return (IQueryable<TElement>)Activator.CreateInstance(queryType, expression);
}

The type is being provided by TElement, so its a simplier implementation on the generic version.

score 4 · Answer 3 · answered Jul 10 '17 at 20:42

4

I was getting this same error in my tests after upgrading from Automapper 6.0.2 to 6.1.1. Downgrading back to 6.0.2 fixed the issue.

Not sure if this is a regression or a breaking change in Automapper. I haven't had time to pursue it further than reviewing the change log and github issues. Nothing jumps out.

answered Jul 10 '17 at 20:42

jslatts

9,307
5
35
38

Didn't think to downgrade. We ended up abstracting away the EF calls behind an interface for testing. – Dylan Musil Jul 11 '17 at 01:06
As @Masayuki Muto discovered below, the problem is in the mock provider. If anyone is interested in contributing, we could also work around it inside AM. – Lucian Bargaoanu Aug 19 '17 at 08:57

score 2 · Answer 4 · answered Jan 24 '20 at 13:42

By using a combination of the two mentioned answers above helped me use .ProjectTo across our unit tests.

In my TestDbAsyncQueryProvider implementation I replaced these two methods:

public IQueryable CreateQuery(Expression expression)
{
    switch (expression)
    {
        case MethodCallExpression m:
            {
                var resultType = m.Method.ReturnType; // it shoud be IQueryable<T>
                var tElement = resultType.GetGenericArguments()[0];
                var queryType = typeof(TestDbAsyncEnumerable<>).MakeGenericType(tElement);
                return (IQueryable)Activator.CreateInstance(queryType, expression);
            }
    }
    return new TestDbAsyncEnumerable<TEntity>(expression);
}

public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
    var queryType = typeof(TestDbAsyncEnumerable<>).MakeGenericType(typeof(TElement));
    return (IQueryable<TElement>)Activator.CreateInstance(queryType, expression);
}

It works fine with the latest EF6 as of today.

score 1 · Answer 5 · answered Apr 02 '20 at 16:59

1

in case someone on his Entities has other object, the ProjectTo will crash or throw an exception with Null Reference Object.

Solution for that, is to make sure your Entity has all its objects initialized with a default() or something.

Answer found on the last comment / answer here -> Why is Automapper.ProjectTo() throwing a null reference exception?

P.S thanks for the IAsyncQueryProvider implementation :)

answered Apr 02 '20 at 16:59

Mircea Mihai

167
1
13

Thank you. It took me a long time to find this solution, so here is something for Google to latch onto for people in the future: System.NullReferenceException Object reference not set to an instance of an object. at lambda_method(Closure , UserEntity ) at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext() at System.Linq.Enumerable.Count[TSource](IEnumerable`1 source) at TestDbAsyncQueryProvider`1.Execute[TResult](Expression expression) – Onosa Sep 29 '21 at 19:23

AutoMapper unable to cast TestDbAsyncEnumerable to IQueryable

5 Answers5