Overload Resolution with Unconstrained Generic Type Testing

[fitdev]

Consider the following code:

using System;
using System.Diagnostics.CodeAnalysis;

public class Test {
    
    public void M<T>(T a, T b) {
        var _equal = false;
        if(a is IEquatable<T> ae && b is IEquatable<T> be) _equal = ae.Equals(be); // <== Why here object.Equals(object) overload is used instead of IEquatable one despite the cast?
    }
    
}

struct Foo : IEquatable<Foo> {
    
    public int A;

    public bool Equals(Foo other) => A == other.A;

}

Question: Why in ae.Equals(be) an overload for object.Equals(object) is chosen instead of IEquatable<T> one despite the cast? Shouldn't the cast take precedence and effectively "constrain" T to IEquatable<T>?

[PathogenDavid]

Because IEquatable<T> provides the method Equals(T), not Equals(IEquatable<T>).

You should just use b directly instead:

if(a is IEquatable<T> ae) _equal = ae.Equals(b);

And if you expect an oddball scenario where a and b have a common T but don't both implement IEquatable<T> you might extend this to:

if(a is IEquatable<T> ae) _equal = ae.Equals(b);
else if(b is IEquatable<T> be) _equal = be.Equals(a);

[fitdev]

Thanks a lot for a thorough explanation. Silly of me to overlook such a simple solution and not pay close attention to the actual signature of a method.