除非泛型类型被明确设置，否则不会调用Nullable 的扩展方法(Extension method on Nullable not called unless generic type is set explicitly)

 在调查另一个问题时：“ 为什么没有Nullable <T> .Equals（T值）方法？ ”我在Nullable<T>上做了一个扩展方法，揭示了泛型Equals<T>(T) 
 
public static class NullableExtensions
{
    public static bool Equals<T>(this T? left, T right) where T : struct, IEquatable<T>
    {
        if (!left.HasValue)
            return false;

        return right.Equals(left.Value);
    }
}
 
 但是，这样调用它： 
 
double? d = null;
d.Equals(0.0);
 
 使用boxing调用基本Equals(object) ，就像你在IL中看到的那样： 
 
IL_0000: nop
IL_0001: ldloca.s d
IL_0003: initobj valuetype [mscorlib]System.Nullable`1<float64>
IL_0009: ldloca.s d
IL_000b: ldc.r8 0.0
IL_0014: box [mscorlib]System.Double
IL_0019: constrained. valuetype [mscorlib]System.Nullable`1<float64>
IL_001f: callvirt instance bool [mscorlib]System.Object::Equals(object)
 
 如果我将该调用更改为明确声明泛型类型： 
 
d.Equals<double>(0.0);
 
 它调用我的扩展方法： 
 
IL_0000: nop
IL_0001: ldloca.s d
IL_0003: initobj valuetype [mscorlib]System.Nullable`1<float64>
IL_0009: ldloc.0
IL_000a: ldc.r8 0.0
IL_0013: call bool ConsoleApplication8.NullableExtensions::Equals<float64>(valuetype [mscorlib]System.Nullable`1<!!0>, !!0)
 
 为什么编译器不会在Equals(object)方法上选择扩展方法？ 
 
 是因为我刚刚选择了Equals<T>(T)的差方法名称，它实际上并不是Equals的真正覆盖，也不是继承查找的一部分？ 

While investigating another question: "Why there is no Nullable<T>.Equals(T value) method?" I made an extension method on Nullable<T> that exposed a generic Equals<T>(T) method:
 
public static class NullableExtensions
{
    public static bool Equals<T>(this T? left, T right) where T : struct, IEquatable<T>
    {
        if (!left.HasValue)
            return false;

        return right.Equals(left.Value);
    }
}
 
However, calling it like so:
 
double? d = null;
d.Equals(0.0);
 
Calls into the base Equals(object) with boxing, as you can see in the IL:
 
IL_0000: nop
IL_0001: ldloca.s d
IL_0003: initobj valuetype [mscorlib]System.Nullable`1<float64>
IL_0009: ldloca.s d
IL_000b: ldc.r8 0.0
IL_0014: box [mscorlib]System.Double
IL_0019: constrained. valuetype [mscorlib]System.Nullable`1<float64>
IL_001f: callvirt instance bool [mscorlib]System.Object::Equals(object)
 
If I change the call to explicitly state the generic type:
 
d.Equals<double>(0.0);
 
It calls my extension method:
 
IL_0000: nop
IL_0001: ldloca.s d
IL_0003: initobj valuetype [mscorlib]System.Nullable`1<float64>
IL_0009: ldloc.0
IL_000a: ldc.r8 0.0
IL_0013: call bool ConsoleApplication8.NullableExtensions::Equals<float64>(valuetype [mscorlib]System.Nullable`1<!!0>, !!0)
 
Why does the compiler not choose the extension method over the Equals(object) method? 
 
Is it because I've just chosen a poor method name of Equals<T>(T) where it isn't actually a true override of Equals and not part of the inheritance look-up?

原文：https://stackoverflow.com/questions/18736146