Question

I am trying to write a simple QueryProvider for a selection and keep failing. This is my example query:

var query = from elem in MyIQueryableImplementation
            select new {Name = elem.Name};

It is followed by a foreach statement that calls:

public IEnumerator<T> GetEnumerator()
{
    return ((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
}

Which invokes (ignoring the expression tree for now):

public override object Execute(Expression expression)
{
       return Exec();
}

And then:

public static IEnumerable<object> Exec()
{
     for(int i = 0; i < 10; i++)
     {
        yield return new {Name = "TEST"};
     } 
     yield break;
}

Running this, it throws the exception (in the GetEnumerator() call):

Unable to cast object of type '<Exec>d__0' to type 'System.Collections.Generic.IEnumerable`1[<>f__AnonymousType0`1[System.String]]'.

How do I fix this?

Answer 1

The IEnumerable returned from Exec is an IEnumerable<object>. It happens to return elements of a type derived from object, but its interface type is just IEnumerable<object>. That’s why you can’t cast it to IEnumerable1[<>f__AnonymousType01[System.String]].

You can cast it element-wise, though:

return ((IEnumerable)this.provider.Execute(this.expression)).Cast<T>().GetEnumerator();