Why does the c# compiler emit Activator.CreateInstance when calling new in with a generic type with a new() constraint?

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I suspect it’s a JITting problem. Currently, the JIT reuses the same generated code for all reference type arguments – so a List<string>‘s vtable points to the same machine code as that of List<Stream>. That wouldn’t work if each new T() call had to be resolved in the JITted code.

Just a guess, but it makes a certain amount of sense.

One interesting little point: in neither case does the parameterless constructor of a value type get called, if there is one (which is vanishingly rare). See my recent blog post for details. I don’t know whether there’s any way of forcing it in expression trees.

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