C# Method overload resolution not selecting concrete generic override

Question

This complete C# program illustrates the issue:

public abstract class Executor<T>
{
    public abstract void Execute(T item);
}

class StringExecutor : Executor<string>
{
    public void Execute(object item)
    {
        // why does this method call back into itself instead of binding
        // to the more specific "string" overload.
        this.Execute((string)item);
    }

    public override void Execute(string item) { }
}

class Program
{
    static void Main(string[] args)
    {
        object item = "value";
        new StringExecutor()
            // stack overflow
            .Execute(item); 
    }
}

I ran into a StackOverlowException that I traced back to this call pattern where I was trying to forward calls to a more specific overload. To my surprise, the invocation was not selecting the more specific overload however, but calling back into itself. It clearly has something to do with the base type being generic, but I don't understand why it wouldn't select the Execute(string) overload.

Does anyone have any insight into this?

The above code was simplified to show the pattern, the actual structure is a bit more complicated, but the issue is the same.

Answer 1

Looks like this is mentioned in the C# specification 5.0, 7.5.3 Overload Resolution:

Overload resolution selects the function member to invoke in the following distinct contexts within C#:

• Invocation of a method named in an invocation-expression (§7.6.5.1).
• Invocation of an instance constructor named in an object-creation-expression (§7.6.10.1).
• Invocation of an indexer accessor through an element-access (§7.6.6).
• Invocation of a predefined or user-defined operator referenced in an expression (§7.3.3 and §7.3.4).

Each of these contexts defines the set of candidate function members and the list of arguments in its own unique way, as described in detail in the sections listed above. For example, the set of candidates for a method invocation does not include methods marked override (§7.4), and methods in a base class are not candidates if any method in a derived class is applicable (§7.6.5.1).

When we look at 7.4:

A member lookup of a name N with K type parameters in a type T is processed as follows:

? First, a set of accessible members named N is determined:

• If T is a type parameter, then the set is the union of the sets of
  accessible members named N in each of the types specified as a primary constraint or secondary constraint (§10.1.5) for T, along with the set of accessible members named N in object.

• Otherwise, the set consists of all accessible (§3.5) members named N in T, including inherited members and the accessible membersnamed N in object. If T is a constructed type, the set of members is obtained by substituting type arguments as described in §10.3.2. Members that include an override modifier are excluded from the set.

If you remove override the compiler picks the Execute(string) overload when you cast the item.