Why does Int32.Equals(Int16) return true where the reverse doesn't? [duplicate]

Question

The .Net Equals() returns different results, though we are comparing the same values. Can someone explain me why that is the case?

class Program
{
    static void Main(string[] args)
    {
        Int16 a = 1;
        Int32 b = 1;

        var test1 = b.Equals(a);    //true
        var test2 = a.Equals(b);    //false
    }
}

Has it got to do anything with the range of the types we are comparing against?

Answer 1

Int32 has an Equals(Int32) overload and Int16 can be implicity converted to an equivalent Int32. With this overload, it's now comparing two 32-bit integers, checks for value equality, and naturally returns true.

Int16 has its own Equals(Int16) method overload, but there is no implicit conversion from an Int32 to an Int16 (because you can have values that are out of range for a 16-bit integer). Thus the type system ignores this overload and reverts to the Equals(Object) overload. Its documentation reports:

true if obj is an instance of Int16 and equals the value of this instance; otherwise, false.

But, the value we're passing in, while it "equals the value of this instance" (1 == 1) it's not an instance of Int16 as it's an Int32.

---

The equivalent code for the b.Equals(a) that you have would look like this:

Int16 a = 1;
Int32 b = 1;

Int32 a_As_32Bit = a; //implicit conversion from 16-bit to 32-bit

var test1 = b.Equals(a_As_32Bit); //calls Int32.Equals(Int32)

Now it's clear we're comparing both numbers as 32-bit integers.

The equivalent code for the a.Equals(b) would look this:

Int16 a = 1;
Int32 b = 1;

object b_As_Object = b; //treats our 16-bit integer as a System.Object

var test2 = a.Equals(b_As_Object); //calls Int16.Equals(Object)

Now it's clear we're calling a different equality method. Internally, that equality method is doing more or less this:

Int16 a = 1;
Int32 b = 1;

object b_As_Object = b;

bool test2;
if (b_As_Object is Int16) //but it's not, it's an Int32
{
    test2 = ((Int16)b_As_Object) == a;
}
else
{
    test2 = false; //and this is where your confusing result is returned
}

Answer 2

You should use equality operator (==) because Equals() methods are not supposed to return true for objects of different types. Also there is no type in your code inherited from both short and int. change to this returns true:

var test2 = a == b.Id;    //true