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

It's a great question that was unfortunately learned the hard way (through WinDbg rather than reflector or ILDasm). A colleague of mine (Herman Chan) was kind enough to pass on his findings – in summary, be very careful of the number of instantiations you have per request on generic types marked with new().

We were fully aware of the costs associated with late-bound instantiation [3] when building our ORM solution using reflection, and measures were taken to compensate for it; but we didn't have a good sense that Generic types required the same level of attention!

The 2nd link provides a nice summary [2] – note the distinction between CreateInstance() and CreateInstance<T>():

Running 10000000 iterations of creation test.
Direct Call	00:00:00.5320932
Delegate Wrapper	00:00:00.8127212
Generic New	00:00:20.2164442
Activator.CreateInstance	00:00:43.3707797

References:

[1] –
http://stackoverflow.com/questions/367577/why-does-the-c-compiler-emit-activator-createinstance-when-calling-new-in-with-a
[2] –
http://blogs.msdn.com/zelmalki/archive/2008/12/13/creating-objects-using-generic-new-constraint.aspx
[3] -
http://hyperthink.net/blog/PermaLink,guid,0d573ba5-1228-419f-bd69-065f53fc64a8.aspx http://aspnetresources.com/blog/dynamic_insstantiation_perf.aspx