Get Memory Address of .NET Object (C#)

Question

I am trying to track down a bug in the mono runtime where a variable appears to be allocated to one valid object, and then is reassigned later to a bogus object, specifically

//early in code I allocate, fine
var o = new object(); // valid allocation
// later in code this is called, not fine
lock(o) // <- is triggering bug due to "o" now referencing a nonsense memory location.

I would like to know when the reference to "o" becomes nonsense, and to do this am looking for a way to determine the address of "o" at various timepoints within the C# code. I know is similar to other questions with answers "don't do that there is a GC", but the GC doesn't work so I need a workaround.

Does anyone know how I can determine the address of a mono object in C#? Am fine to link in unmanaged code or whatever. (Any other clues to ways to diagnose the main issue appreciated to).

Answer 1

Turns out this is not possible in .NET directly, but can be accomplished by altering the mono runtime code. To create a C# method that can read the memory address, make the following changes to the mono source code:

Alter gc-internal.h to add

gpointer    ves_icall_System_GCHandle_GetAddrOfObject (MonoObject *obj) MONO_INTERNAL;

Alter gc.c to add:

gpointer    ves_icall_System_GCHandle_GetAddrOfObject (MonoObject *obj) {
    return (char*)obj;
}

Alter GCHandle.cs to add:

MethodImplAttribute(MethodImplOptions.InternalCall)]
private extern static IntPtr GetAddrOfObject(object obj);

public static IntPtr AddrOfObject(object o)
{
    IntPtr res = GetAddrOfObject(o);
    return res;
}

Alter icall-def.h to add

ICALL(GCH_6, "GetAddrOfObject", ves_icall_System_GCHandle_GetAddrOfObject)

Note that these must be in order, so add it above the GetAddrOfPinnedObject line Rebuild

Finally, call it from C#

for (int i = 0; i < 100; i++) {
    object o = new object ();
    var ptr = GCHandle.AddrOfObject (o);
    Console.WriteLine ("Address: " + ptr.ToInt64().ToString ("x"));
}