`Is 'is null' or '== null' Better for Checking Null in C#?`

Pattern Matching vs. Equality Comparison with Null

In C# 7, a new pattern-matching operator, "is," was introduced as an alternative to the equality operator, "==". While they may appear to behave similarly when checking for null, there are some subtle differences.

Semantic Differences:

• For Null: Using "is null" and "== null" have the same effect.
• Pattern Matching: When used with a literal or constant value other than null, "is" checks for type compatibility and pattern matching. For example, "if (x is 1)" checks if "x" is of type int and has a value of 1.

Implementation Details:

• "is null": Compiles to a call to System.Object.Equals(object, object).
• "== null": Compiles to a call to the ceq instruction, which performs an equality check.

Performance:

In earlier versions of the Roslyn compiler, "is null" incurred a slight performance penalty compared to "== null" due to the additional type and pattern matching checks. However, in newer versions, the behavior has been optimized and both operators perform comparably when there is no overloaded equality operator.

Usage Recommendations:

• Use "is null" when you want to perform both null and type checking.
• Use "== null" for simple equality checks, especially if performance is a concern.
• Consider using "is" with pattern matching for more complex scenarios, such as verifying the type and value of a constant or literal.

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