C# implicit conversions and == operator

Question

Some code for context:

class a
{

}

class b
{
    public a a{get;set;}
    public static implicit operator a(b b)
    {
        return b.a;
    }
}

  a a=null;
  b b=null;
  a = b;

  //compiler: cannot apply operator '==' to operands of type tralala...
  bool c = a == b;

Is it possible to use == operator on different type instances, where one can implicitly convert to another? What did i miss?

Edit:
If types must be the same calling ==, then why

int a=1;
double b=1;
bool c=a==b;

works?

Answer 1

Use this

 bool c = a.Equals(b);

Answer 2

http://msdn.microsoft.com/en-us/library/8edha89s.aspx

In each case, one parameter must be the same type as the class or struct that declares the operator (...)

Answer 3

The implicit operator only works for assignment.

You want to overload the equality (==) operator, as such:

class a
{
    public static bool operator ==(a x, b y)
    {
        return x == y.a;
    }

    public static bool operator !=(a x, b y)
    {
        return !(x == y);
    }
}

class b
{
    public a a{get;set;}
    public static implicit operator a(b b)
    {
        return b.a;
    }
}

This should then allow you to compare two objects of type a and b as suggested in your post.

var x = new a();
var b = new b();
bool c = (x == y); // will compile fine

Note:

I recommmend simply overriding the GetHashCode and Equals method, as the compiler warns, but as you seem to want to supress them, you can do that as follows.

Change your class declaration of a to:

#pragma warning disable 0660, 0661
class a
#pragma warning restore 0660, 0661
{
    // ...
}

Answer 4

I would imagine that you need to actually override the == operator for the types you are interested in. Whether the compile/runtime will still complain even if the types are implicity convertable is something you'll have to experiment with.

public static bool operator ==(a a, b b)
    {
        //Need this check or we can't do obj == null in our Equals implementation
        if (((Object)a) == null)
        {
            return false;
        }
        else
        {
            return a.Equals(b);
        }
    }

Alternatively just use Equals implementations like ole6ka suggests and ensure that the implementation does the type casting you need.

Answer 5

Is it possible to use == operator on different type instances, where one can implicitly convert to another?

Yes.

What did i miss?

Here's the relevant portion of the specification. You missed the highlighted word.

The predefined reference type equality operators require [that] both operands are reference-type values or the literal null. Furthermore, a standard implicit conversion exists from the type of either operand to the type of the other operand.

A user-defined conversion is by definition not a standard conversion. These are reference types. Therefore, the predefined reference type equality operator is not a candidate.

If types must be the same calling ==, then why [double == int] works?

Your supposition that the types must be the same is incorrect. There is a standard implicit conversion from int to double and there is an equality operator that takes two doubles, so this works.

I think you also missed this bit:

It is a compile-time error to use the predefined reference type equality operators to compare two references that are known to be different at compile-time. For example, if the compile-time types of the operands are two class types A and B, and if neither A nor B derives from the other, then it would be impossible for the two operands to reference the same object. Thus, the operation is considered a compile-time error.