`x is null` vs. `x == null` in C# 7: When Should You Use Which?

Distinguishing "x is null" from "x == null" in C# 7

C# 7 introduces the "x is null" operator as an alternative to "x == null". While the new operator may seem like a mere syntactic variation, there are subtle differences between the two that impact when and how they should be used.

Are the Semantics Different?

No. The semantics of "x is null" and "x == null" are identical for null comparisons. However, the "x is null" operator considers the type when comparing to constants. This can result in different behavior compared to "x == null" when comparing to custom types that overload the equality operator.

Performance Implications

In previous Roslyn compiler versions, "x == null" was faster than "x is null". However, optimizations in the latest Roslyn compiler have made the two operators equally efficient when there is no overloaded equality operator.

Deciding Which Operator to Use

The choice between "x is null" and "x == null" depends on the specific scenario:

• For null comparisons, both operators can be used interchangeably.
• For constant comparisons, "x is null" should be preferred if you want to consider the type of the constant.
• For comparisons involving custom types with overloaded equality operators, "x is null" and "x == null" will behave differently. In such cases, choose the operator that aligns with the intended semantics.

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