Is 'as' Always Faster Than 'is' with Nullable Types in C#?

Performance surprise with ""as"" and nullable types

This article investigates the performance difference between using the ""as"" operator and a traditional ""is"" check followed by a cast when working with nullable types.

Initially, it was assumed that the ""as"" operator would be more efficient as it seems to simplify the dynamic type checking and value check into a single step. However, benchmark results revealed that the traditional ""is"" check is significantly faster, even outperforming the LINQ solution.

The primary reason for this discrepancy lies in the implementation of nullable type casting. Unlike value conversions between primitive types, converting a boxed value to a nullable type requires a more complex process, involving unboxing and possible enum type checks. This process is handled by the CLR helper function JIT_Unbox_Nullable, which generates more machine code instructions and consumes substantial resources.

It is noteworthy that the JIT compiler optimizes code for scenarios where the object's type is known and consistent with the target type. In the case of the traditional ""is"" check, the optimizer can generate more efficient code that directly unboxes the value with minimal overhead.

While the ""as"" operator simplifies code readability, its use in performance-sensitive situations should be carefully considered. It is recommended to use the traditional ""is"" check and cast combination for better performance, especially when working with nullable types.

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