Why would one ever use the “in” parameter modifier in C#?

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in was recently introduced to the C# language.

in is actually a ref readonly. Generally speaking, there is only one use case where in can be helpful: high performance apps dealing with lots of large readonly structs.

Assuming you have:

readonly struct VeryLarge
{
    public readonly long Value1;   
    public readonly long Value2;

    public long Compute() { }
    // etc
}

and

void Process(in VeryLarge value) { }

In that case, the VeryLarge struct will be passed by-reference without creating of defensive copies when using this struct in the Process method (e.g. when calling value.Compute()), and the struct immutability is ensured by the compiler.

Note that passing a not-readonly struct with an in modifier will cause the compiler to create a defensive copy when calling struct’s methods and accessing properties in the Process method above, which will negatively affect performance!

There is a really good MSDN blog entry which I recommend to carefully read.

If you would like to get some more historical background of in-introducing, you could read this discussion in the C# language’s GitHub repository.

In general, most developers agree that introducing of in could be seen as a mistake. It’s a rather exotic language feature and can only be useful in high-perf edge cases.

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