Why is C#'s 'as' Operator Slower Than Traditional Casting in This Nullable Integer Summation Test?

Performance Surprise with "as" and Nullable Types

Background

The "as" operator in C# allows for dynamic type checking and safe casting. This can potentially improve performance over the traditional "is" operator followed by a cast. However, unexpected performance results were observed in a recent test case.

Test App and Results

A test app was developed to sum integers from an object array containing null references, string references, and boxed integers. The app utilized three approaches: "Cast" (C# 1 equivalent), "As" (using the "as" operator), and "LINQ" (using the OfType() extension method). Surprisingly, the "Cast" approach was significantly faster than the "As" approach, which in turn was slightly slower than the "LINQ" approach.

Analysis

The JIT compiler can generate highly efficient code for the "Cast" approach because it utilizes in-memory value conversion. The object can only be unboxed to a variable with the same type as the boxed value, which simplifies the casting process.

The "As" approach requires a conversion to Nullable as the value representation of the boxed integer is not compatible with the memory layout of Nullable. This is done through a helper function, JIT_Unbox_Nullable, which introduces additional overhead.

The "LINQ" approach employs the "is" operator and a generic cast through the helper function JIT_Unbox(). While it performs slightly better than the "As" approach, it is still slower due to ngen.exe optimizations.

Conclusion

The "as" operator may not always provide performance benefits over the "Cast" approach, especially in performance-sensitive situations. While the "LINQ" approach is a viable alternative, it may also encounter performance limitations. Therefore, it is important to consider the specific context and weigh the trade-offs before choosing the optimal approach.

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