C# Null Checks: When to Use `x is null` vs. `x == null`?

Null Reference Comparisons in C#: "x is null" vs. "x == null"

In C# 7, the introduction of the pattern matching operator is has raised questions about its usage compared to the traditional == null expression. While these expressions may seem interchangeable when dealing with null, there are key differences to consider when comparing reference types.

Type Consideration

Unlike == null, which compares object references for equality, x is null checks for the type of x. If x is a reference type, x is null returns true only if x is null. In contrast, x == null returns true for both null and reference types with values other than null.

class Foo {}

Foo foo = null;

Console.WriteLine(foo is null); // True
Console.WriteLine(foo == null); // True

Performance Implications

Internaly, x is null calls System.Object.Equals(object, object), while x == null calls ceq (a comparison method for reference types). For null references, both expressions have the same performance since they both return true. However, for non-null references, x is null can be significantly slower due to its additional type checking.

Overloaded Equality Operators

When an equality operator (such as == or Equals) is overloaded for a specific type, x is null no longer checks for type equality and instead delegates to the overloaded operator. On the other hand, x == null always checks for reference equality.

class Foo
{
    public static bool operator ==(Foo left, Foo right) => ...
    public static bool operator !=(Foo left, Foo right) => ...
}

Foo foo1 = new Foo();
Foo foo2 = null;

Console.WriteLine(foo1 is null); // False (checks type only)
Console.WriteLine(foo1 == null); // False (delegates to overloaded operator)
Console.WriteLine(foo2 is null); // True (checks type only)
Console.WriteLine(foo2 == null); // True (reference comparison)

When to Use One over the Other

In general:

• Use x == null if you're concerned about performance or if the equality operator is overloaded.
• Use x is null if you need to check for type equality (e.g., within a pattern matching expression).

Ultimately, the choice between x is null and x == null depends on the specific requirements of your code.

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