Does the 'as' Operator Always Offer Performance Advantages over 'is' for Nullable Types in C#?

Performance Surprise with the "as" Operator and Nullable Types

In chapter 4 of C# in Depth, nullable types and the "as" operator are being discussed. Specifically, the expectation is that using "as" for type checking can improve performance over traditional "is" checks and casting, as it simplifies the process to a single type check and value check. However, a surprising result has been observed.

Performance Benchmark Test

To assess performance, a benchmark test was conducted, which involved summing integers within an object array that included numerous null references and string references. The test measured the execution time of the following code snippets:

• C# 1-style code with "is" and casting
• Code using the "as" operator
• LINQ solution

To the astonishment of the researcher, the C# 1 code outperformed both the "as" code and the LINQ code by a significant margin.

Analysis of Results

The performance discrepancy is attributed to the following factors:

JIT Compiler Optimization for "is" and Casting:
The "is" operator test and the subsequent cast can be optimized by the JIT compiler, resulting in highly efficient machine code that executes in a minimal number of instructions. This optimization is possible because the boxed value type can be directly unboxed into a variable of the same type without any value conversions or copying.

Complexity of Converting to Nullable:
Casting to int? using "as" requires a more complex conversion process because the value representation of the boxed integer differs from the memory layout of Nullable. This necessitates the involvement of a general-purpose unboxing helper function called JIT_Unbox_Nullable, which introduces additional overhead due to its generic nature and type checking.

Unexpected Behavior of LINQ:
The OfType() extension method in LINQ also utilizes the "is" operator and the JIT_Unbox() helper function. However, its performance was comparable to the "as" cast to Nullable, despite the expectation that it should be less expensive. This discrepancy might be attributed to potential issues caused by ngen.exe.

Conclusion

While the "as" operator provides a convenient way to perform type checking and nullable value handling, its performance characteristics under specific scenarios may not always be as expected. The optimized code generated for "is" and casting in C# 1 remains significantly faster in cases involving numerous unboxing operations, highlighting the importance of considering performance implications when selecting coding techniques.

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