Why does Int32.Equals (Int16) return true if not otherwise?

Question

.NET Equals () returns different results, although we are comparing the same values. Can someone explain to me why this is so?

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 } } 

Do we need to do something with the range of types we are comparing?

Answer 1

Int32 has an Equals(Int32) overload, and Int16 can be implicitly converted to an Int32 equivalent. With this overload, he now compares two 32-bit integers, checks for equality of values, and naturally returns true .

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

true if obj is an Int16 instance and is equal to the value of this example; otherwise false.

But the value that we pass, while it is "equal to the value of this instance" ( 1 == 1 ), it is not an instance of Int16 , because it is Int32 .

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The equivalent code for b.Equals(a) that you will have is as follows:

 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 that we are comparing both numbers as 32-bit integers.

The equivalent code for a.Equals(b) would look like 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 that we are calling another method of equality. Internally, this equality method does more or less the following:

 Int16 a = 1; Int32 b = 1; object b_As_Object = b; bool test2; if (b_As_Object is Int16) //but it not, it 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 the equality operator ( == ), because the Equals() methods should not return true for objects of different types. Also in your code there is no type inherited from short and int . changing to this returns true:

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