Question

I want to invoke GetClientSite on a .net control. For this purpose I am trying to cast a control (in example Windows.Forms.Form) to IOleObject which returns null.

What should I do to get IOleObject?

using System;
using System.Runtime.InteropServices;
using System.Security;
using System.Windows.Forms;

namespace Test001
{
    public class Form001 : Form
    {
        public Form001()
        {
            InitializeComponent();
        }

        private void InitializeComponent()
        {
            this.SuspendLayout();
            this.Name = "Form001";
            this.Text = "Form001";
            this.Load += new System.EventHandler(this.Form001_Load);
            this.ResumeLayout(false);
        }

        private void Form001_Load(object sender, EventArgs e)
        {
            IOleObject obj = (IOleObject) this;
            //IOleClientSite site = obj.GetClientSite();
        }
    }

    [ComImport, Guid("00000112-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), SuppressUnmanagedCodeSecurity]
    public interface IOleObject
    {
        [PreserveSig]
        int SetClientSite([In, MarshalAs(UnmanagedType.Interface)] IOleClientSite pClientSite);
        IOleClientSite GetClientSite();
        [PreserveSig]
        int SetHostNames([In, MarshalAs(UnmanagedType.LPWStr)] string szContainerApp, [In, MarshalAs(UnmanagedType.LPWStr)] string szContainerObj);
        [PreserveSig]
        int Close(int dwSaveOption);
        [PreserveSig]
        int SetMoniker([In, MarshalAs(UnmanagedType.U4)] int dwWhichMoniker, [In, MarshalAs(UnmanagedType.Interface)] object pmk);
        [PreserveSig]
        int GetMoniker([In, MarshalAs(UnmanagedType.U4)] int dwAssign, [In, MarshalAs(UnmanagedType.U4)] int dwWhichMoniker, [MarshalAs(UnmanagedType.Interface)] out object moniker);
        [PreserveSig]
        int InitFromData([In, MarshalAs(UnmanagedType.Interface)] IDataObject pDataObject, int fCreation, [In, MarshalAs(UnmanagedType.U4)] int dwReserved);
        [PreserveSig]
        int GetClipboardData([In, MarshalAs(UnmanagedType.U4)] int dwReserved, out IDataObject data);
        [PreserveSig]
        int DoVerb(int iVerb, [In] IntPtr lpmsg, [In, MarshalAs(UnmanagedType.Interface)] IOleClientSite pActiveSite, int lindex, IntPtr hwndParent, [In] object lprcPosRect);
        [PreserveSig]
        int EnumVerbs(out object e);
        [PreserveSig]
        int OleUpdate();
        [PreserveSig]
        int IsUpToDate();
        [PreserveSig]
        int GetUserClassID([In, Out] ref Guid pClsid);
        [PreserveSig]
        int GetUserType([In, MarshalAs(UnmanagedType.U4)] int dwFormOfType, [MarshalAs(UnmanagedType.LPWStr)] out string userType);
        [PreserveSig]
        int SetExtent([In, MarshalAs(UnmanagedType.U4)] int dwDrawAspect, [In] object pSizel);
        [PreserveSig]
        int GetExtent([In, MarshalAs(UnmanagedType.U4)] int dwDrawAspect, [Out] object pSizel);
        [PreserveSig]
        int Advise(object pAdvSink, out int cookie);
        [PreserveSig]
        int Unadvise([In, MarshalAs(UnmanagedType.U4)] int dwConnection);
        [PreserveSig]
        int EnumAdvise(out object e);
        [PreserveSig]
        int GetMiscStatus([In, MarshalAs(UnmanagedType.U4)] int dwAspect, out int misc);
        [PreserveSig]
        int SetColorScheme([In] object pLogpal);
    }

    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("00000118-0000-0000-C000-000000000046")]
    public interface IOleClientSite
    {
        [PreserveSig]
        int SaveObject();
        [PreserveSig]
        int GetMoniker([In, MarshalAs(UnmanagedType.U4)] int dwAssign, [In, MarshalAs(UnmanagedType.U4)] int dwWhichMoniker, [MarshalAs(UnmanagedType.Interface)] out object moniker);
        [PreserveSig]
        int GetContainer(out object container);
        [PreserveSig]
        int ShowObject();
        [PreserveSig]
        int OnShowWindow(int fShow);
        [PreserveSig]
        int RequestNewObjectLayout();
    }
}

Answer 1

The IOleObject interface is a nested interface inside the internal UnsafeNativeMethods class in the System.Windows.Forms assembly. The System.Windows.Forms.Control implements it internally, and explicitly.

Creating another interface with the same name and guid will not make it “the same interface” on a managed level.

This line

IOleObject obj = (IOleObject) this;

represents managed .net casting, and has nothing to do with COM. This cast will work only with the exact same interface from the winforms assembly, which isn’t public.

You could try to use reflection via the InterfaceMapping structure to get the method (but note that this is not recommendable):

Type thisType = this.GetType();

Type oleInterface = thisType.GetInterface("IOleObject");

MethodInfo getSiteMethod = oleInterface.GetMethod("GetClientSite");

//InterfaceMapping is used to get more complex interface scenarios
InterfaceMapping map = thisType.GetInterfaceMap(oleInterface);

//at which index is the explicit implementation
int index = Array.IndexOf(map.InterfaceMethods, getSiteMethod);
MethodInfo actualExplicitMethod = map.TargetMethods[index];

//late-bound call (slow)
object o = actualExplicitMethod.Invoke(this, new object[] { });

Now, first, you get an internal type wrapped in System.Object which cannot be cast to your interface since the original interface is internal, so you get to have more fun with reflection as long as you intend to use that object.

Second, I’ve tried it, the technique works, but in your specific scenario this method called on a windows Form throws an exception – "Top-level Windows Forms control cannot be exposed as an ActiveX control.".