Why “Equals” method resolution with generics differs from explicit calls

问题

I have the following example:

namespace ComparisonExample
{
    class Program
    {
        static void Main(string[] args)
        {
            var hello1 = new Hello();
            var hello2 = new Hello();

            // calls Hello.Equals
            var compareExplicitly = hello1.Equals(hello2);

            // calls Object.Equals
            var compareWithGenerics = ObjectsEqual<Hello>(hello1, hello2); 
        }

        private static bool ObjectsEqual<TValue>(TValue value1, TValue value2)
        {
            return value1.Equals(value2);
        }
    }

    class Hello : IEquatable<Hello>
    {
        public bool Equals(Hello other)
        {
            return true; // doesn't matter
        }
    }
}

The question is why in the second "Equals" call I'm redirected to Object.Equals instead of Hello.Equals even though I'm specifying the exact type in generic argument?

回答1:

Because you haven't told the generic method that your object implements IEquatable<T>:

Try now with:

private static bool ObjectsEqual<TValue>(TValue value1, TValue value2) 
               where TValue : IEquatable<TValue> // IMPORTANT!!!
{
    return value1.Equals(value2);
}

In your ObjectsEqual method you have access only to methods/properties/fields of TValue that are defined in the object class plus the methods that are defined in the interfaces/base classes defined in the constraints. No constraints => you have access only to Equals(object), GetHashCode(), GetType(), (and if you have the constraint class: operator==, operator!=.) Of these two are virtual (Equals(object), GetHashCode()), so you'll use the "correct" version, the third isn't normally overwritten (GetType()), so you'll probably use the "correct" version. Only the two operators ==/!= are often overwritten and lo and behold! In your generic method you can't use the "correct" version of the two! :-)

回答2:

Addition from MSDN:

Unbounded Type Parameters.
Type parameters that have no constraints, such as T in public class SampleClass<T> { }, are called unbounded type parameters.
Unbounded type parameters have the following rules:

• The != and == operators cannot be used because there is no guarantee that the concrete type argument will support these operators.
• They can be converted to and from System.Object or explicitly converted to any interface type.
• You can compare to null. If an unbounded parameter is compared to null, the comparison will always return false if the type argument is a value type.

In this case TValue is converted to System.Object and Equals method called.

来源：https://stackoverflow.com/questions/18657753/why-equals-method-resolution-with-generics-differs-from-explicit-calls