`x is null` vs. `x == null` in C#: When Should You Use Which?

x is null vs x == null

In C#, comparing an object to null using == and is has traditionally been similar. However, C# 7 introduced a new operator, is null, which raises the question: are there any advantages to using this over the conventional == null?

Semantics

Contrary to popular belief, the semantics of x == null and x is null are not identical. While they both compare an object to null, their behavior differs when comparing to constants other than null.

Advantages of is null

One advantage of is null is its ability to consider the object's type during comparison. For instance:

Test(1);

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

In this example, the is operator allows the comparison of o to the constant 1, taking into account their respective types.

When to Use is null

Typically, x is null is tercih edilir when dealing with constants other than null, especially when considering types. For comparisons involving only null, both == null and is null serve the same purpose.

When to Use == null

== null remains a reliable choice for comparisons involving null. When there is no need to consider object types, == null is commonly used.

Update

The Roslyn compiler has been modified to align the behavior of == and is null in scenarios where there is no overloaded equality operator. As a result, they both now optimize to the more efficient == behavior. However, if an overloaded equality operator is present, the behavior of is null and == null differs.

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