Question

How does the default implementation for GetHashCode() work? And does it handle structures, classes, arrays, etc. efficiently and well enough?

I am trying to decide in what cases I should pack my own and in what cases I can safely rely on the default implementation to do well. I don’t want to reinvent the wheel, if at all possible.

Answer 1

namespace System {     public class Object {         [MethodImpl(MethodImplOptions.InternalCall)]         internal static extern int InternalGetHashCode(object obj);          public virtual int GetHashCode() {             return InternalGetHashCode(this);         }     } } 

InternalGetHashCode is mapped to an ObjectNative::GetHashCode function in the CLR, which looks like this:

FCIMPL1(INT32, ObjectNative::GetHashCode, Object* obj) {       CONTRACTL       {           THROWS;           DISABLED(GC_NOTRIGGER);           INJECT_FAULT(FCThrow(kOutOfMemoryException););           MODE_COOPERATIVE;           SO_TOLERANT;       }       CONTRACTL_END;        VALIDATEOBJECTREF(obj);        DWORD idx = 0;        if (obj == 0)           return 0;        OBJECTREF objRef(obj);        HELPER_METHOD_FRAME_BEGIN_RET_1(objRef);        // Set up a frame        idx = GetHashCodeEx(OBJECTREFToObject(objRef));        HELPER_METHOD_FRAME_END();        return idx;   }   FCIMPLEND 

The full implementation of GetHashCodeEx is fairly large, so it’s easier to just link to the C++ source code.