C# - How To Convert Object To IntPtr And Back?
I want to pass an object from managed code to a WinApi function as IntPtr
. It will pass this object back to my callback function in managed code as IntPtr
. It's not a structure, it's an instance of a class.
How do I convert object
to IntPtr
and back ?
Solution 1:
So if I want to pass a list to my callback function through WinApi I use GCHandle
// object to IntPtr (before calling WinApi):
List<string> list1 = new List<string>();
GCHandle handle1 = GCHandle.Alloc(list1);
IntPtr parameter = (IntPtr) handle1;
// call WinAPi and pass the parameter here
// then free the handle when not needed:
handle1.Free();
// back to object (in callback function):
GCHandle handle2 = (GCHandle) parameter;
List<string> list2 = (handle2.Target as List<string>);
list2.Add("hello world");
Thx to David Heffernan
Edit: As noted in the comments, you need to free the handle after use. Also I used casting. It might be wise to use the static methods GCHandle.ToIntPtr(handle1)
and GCHandle.FromIntPtr(parameter)
like here. I haven't verified that.
Solution 2:
While the accepted answer is correct, I wanted to add a little bit to it.
I have grown fond of creating extensions for this so it reads: list1.ToIntPtr()
.
public static class ObjectHandleExtensions
{
public static IntPtr ToIntPtr(this object target)
{
return GCHandle.Alloc(target).ToIntPtr();
}
public static GCHandle ToGcHandle(this object target)
{
return GCHandle.Alloc(target);
}
public static IntPtr ToIntPtr(this GCHandle target)
{
return GCHandle.ToIntPtr(target);
}
}
Also, depending on how much of this you're doing, it might be nice to contain your list in an IDisposable
.
public class GCHandleProvider : IDisposable
{
public GCHandleProvider(object target)
{
Handle = target.ToGcHandle();
}
public IntPtr Pointer => Handle.ToIntPtr();
public GCHandle Handle { get; }
private void ReleaseUnmanagedResources()
{
if (Handle.IsAllocated) Handle.Free();
}
public void Dispose()
{
ReleaseUnmanagedResources();
GC.SuppressFinalize(this);
}
~GCHandleProvider()
{
ReleaseUnmanagedResources();
}
}
And then you might consume it like this:
using (var handleProvider = new GCHandleProvider(myList))
{
var b = EnumChildWindows(hwndParent, CallBack, handleProvider.Pointer);
}