使用Equals比较Enum和const int时未出现编译错误

Question

C#: Code parts:

class ConstValues
{
   public const int NULL=1;
}

class Example
{
   private enum FormatFunction
   {
      Date,
      Unknown
   }

   ...

   FormatFunction returnValue = fn();

   ...

Now I have two scenarios.

When I used this way to compare my return code to a value

if (!returnValue.Equals(ConstValues.NULL))
{
   ...

I get no compile time error (and the code does not work as intended because this is a bug of mine that I missed out).

But when I change to

if (returnValue != ConstValues.NULL)
{
   ...

I get a compile time error and discover the mistake I did.

I understand that the underlying structure of an enum is int, but I would rather get a compile time error even when using Equals.

Why do the first way pass and the second don't?

Answer 1

It is because default virtual Equals method is receiving object instead of strong typed value like in second example. It is boxed into object and will check types only in runtime.

Why it is made so? The reasoning is pretty funny - for possibility to compare cats and dogs if they walk on all fours. Compare two completely different objects by some almost identical properties.

Thing is, when you can - work with strong typed objects. This will prevent bad things from happening in compile time.

Answer 2

!= is a language convention so this is C# specific. Calling this operator is early bound , in other words, it will occur at compile time.

Equals is a framework convention, .NET in this case, and that is bound at runtime.

When you call != , the decision is made by the C# compiler during compilation so you get an error. When you call Equals the decision is made by the framework at runtime. Since your enum is not of type object, it will be turned into an object (boxing) and then the runtime will check if your type has overridden the Equals method, since you have not, it will use the default implementation.

---

Equals for Reference type

If the instance is a reference type, the default implementation of Equals checks if one object reference is the same as the other object reference. If they are the same reference, it returns true. Otherwise it returns false.

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Equals for Value type

If the instance is a value type, then it will test for value equality. This is your case. It will check if the enum value you have is equal to the constant value. No error will be shown or thrown: it either equals or it does not.