What is the difference between “x is null” and “x == null”?

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Update: The Roslyn compiler has been updated to make the behavior of the two operators the same when there is no overloaded equality operator. Please see the code in the current compiler results (M1 and M2 in the code) that shows what happens when there is no overloaded equality comparer. They both now have the better-performing == behavior. If there is an overloaded equality comparer, the code still differs.

See for older versions of the Roslyn compiler the below analysis.

For null there isn’t a difference with what we are used to with C# 6. However, things become interesting when you change null to another constant.

Take this for example:

Test(1);

public void Test(object o)
{
    if (o is 1) Console.WriteLine("a");
    else Console.WriteLine("b");
}

The test yields a. If you compare that to o == (object)1 what you would have written normally, it does make a lot of difference. is takes in consideration the type on the other side of the comparison. That is cool!

I think the == null vs. is null constant pattern is just something that is very familiar ‘by accident’, where the syntax of the is operator and the equals operator yield the same result.

As svick commented, is null calls System.Object::Equals(object, object) where == calls ceq.

IL for is:

IL_0000: ldarg.1              // Load argument 1 onto the stack
IL_0001: ldnull               // Push a null reference on the stack
IL_0002: call bool [mscorlib]System.Object::Equals(object, object) // Call method indicated on the stack with arguments
IL_0007: ret                  // Return from method, possibly with a value

IL for ==:

IL_0000: ldarg.1              // Load argument 1 onto the stack
IL_0001: ldnull               // Push a null reference on the stack
IL_0002: ceq                  // Push 1 (of type int32) if value1 equals value2, else push 0
IL_0004: ret                  // Return from method, possibly with a value

Since we are talking about null, there is no difference since this only makes a difference on instances. This could change when you have overloaded the equality operator.

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