Performance of C# method polymorphism with generics
- by zildjohn01
I noticed in C#, unlike C++, you can combine virtual and generic methods. For example:
using System.Diagnostics;
class Base {
public virtual void Concrete() {Debug.WriteLine("base concrete");}
public virtual void Generic<T>() {Debug.WriteLine("base generic");}
}
class Derived : Base {
public override void Concrete() {Debug.WriteLine("derived concrete");}
public override void Generic<T>() {Debug.WriteLine("derived generic");}
}
class App {
static void Main() {
Base x = new Derived();
x.Concrete();
x.Generic<PerformanceCounter>();
}
}
Given that any number of versions of Generic<T> could be instantiated, it doesn't look like the standard vtbl approach could be used to resolve method calls, and in fact it's not. Here's the generated code:
x.Concrete();
mov ecx,dword ptr [ebp-8]
mov eax,dword ptr [ecx]
call dword ptr [eax+38h]
x.Generic<PerformanceCounter>();
push 989A38h
mov ecx,dword ptr [ebp-8]
mov edx,989914h
call 76A874F1
mov dword ptr [ebp-4],eax
mov ecx,dword ptr [ebp-8]
call dword ptr [ebp-4]
The extra code appears to be looking up a dynamic vtbl according to the generic parameters, and then calling into it. Has anyone written about the specifics of this implementation? How well does it perform compared to the non-generic case?
