Does `&as;` Always Offer Performance Gains with Nullable Types?

Performance Pitfalls with "&as"; and Nullable Types

Enhancing performance through concise syntax often comes to mind when utilizing the "&as;" operator. However, as demonstrated by a recent benchmark, this operator may not always deliver its intended optimization for nullable types.

The benchmark reveals unexpected performance disparity between "&as;," "&is;" followed by casting, and even LINQ solutions. To the astonishment of many, the legacy "&is;" approach outperforms "&as;", with a 20-fold difference. This raises the question: why such a slowdown with "&as;"?

The underlying reason lies in the code generated by the JIT compiler. For "&is;," a quick type check and unboxing suffices. "&as;," on the other hand, invokes a CLR helper function for value conversion due to the mismatch between boxed integer and Nullable representations. This additional overhead, involving potentially hundreds of instructions, incurs a significant performance hit.

While the LINQ solution expectedly lags behind due to iterators, its sluggishness relative to "&as;" is puzzling. A possible culprit could be ngen.exe optimizations.

In conclusion, while "&as;" offers syntactical convenience, its performance trade-offs with nullable types must be carefully considered in performance-sensitive scenarios.

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