Direct comparisons of C# Value types

Question

I have read the following statement regarding to the comparison of C# value types in C# in Depth, Second Edition several times.

page 77,

When a type parameter is unconstrained (no constraints are applied to it), you can use == and != operators, but only to compare a value of that type with null. You can’t compare two values of type T with each other.

...

When a type parameter is constrained to be a value type, == and != can’t be used with it at all.

If I understand (I don't think so) it correctly, it basically tells me that you cannot use == or != to compare two value types. Why why why?

It will be better if a simple example can be given for this case. Can someone give me a little idea what the above paragraph tries to convey?

Answer 1

It simply means this when constraining to a value type (second paragraph)

static bool TryToCompare<T>(T first, T second) where T : struct
{
    return first == second; // not legal
    return first.Equals(second); // legal
}

Without the value-type constraint on the generic, it also says this (first paragraph)

static bool TryToCompare<T>(T first, T second) 
{
    return first == second; // not legal
    return first == null; // legal
    return first.Equals(second); // legal
}

If you constrain T to a reference type, you can get away with using ==

static bool TryToCompare<T>(T first, T second) where T : class
{
    return first == second; // legal
    return first == null; // legal
    return first.Equals(second); // legal
}