How Do Dynamic Variables in C# Impact Performance?

Performance impact of C# dynamic variables

Dynamic variables in C# can impact performance, raising concerns about the extent of recompilation they trigger and its performance impact.

Compiler behavior for dynamic variables

To be clear, the compiler will not recompile the entire method containing dynamic variables. Instead, the compiler generates a dynamic call site object for each dynamic expression encountered in the code.

Dynamic call site object

These dynamic call site objects contain information about the operation being performed. For example, if we have the expression d1.Foo(), the compiler creates a dynamic call site object that represents a call to the Foo method on the object referenced by d1.

Dynamic Language Runtime (DLR)

When executing a dynamic expression, the DLR asks if the object involved has any special capabilities, such as if it is a COM object or an Iron Python object. If not, the DLR treats it as a normal C# object.

C# compiler call

For ordinary C# objects, the DLR uses a lightweight version of the C# compiler to analyze the object's type and the method being called. The compiler builds an expression tree representing the operation and passes it back to the DLR.

Caching and IL generation

The DLR caches expression trees to perform the same operation on objects of the same type in the future. If a cache hit occurs, the DLR compiles the cached expression tree into IL and calls the generated delegate directly, bypassing compiler analysis.

Performance impact

This process introduces overhead for every dynamic expression encountered. The overhead is most noticeable when dynamic operations are performed frequently, especially when dealing with objects of different types.

Benchmark results

The sample code provided in the question illustrates the significant performance hit caused by dynamic variables. Using dynamic variables in a simple for loop may slow down execution by two orders of magnitude compared to using int variables.

Conclusion

While dynamic variables provide flexibility, it is important to be aware of their performance implications. For scenarios where code performance is critical, it is recommended to avoid excessive use of dynamic variables or to consider using statically typed alternatives.

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