HowTo: get the actual type name behind System.__ComObject with Visual C# or VB.NET

Developers accessing Application.Selection and Application.ActiveSheet in Excel or Explorer.Selection.Item(i), Folder.Items(j), Inspector.CurrentItem in Outlook do know the notorious “System.__ComObject” string. It indicates that the actual type of the COM object is unknown.

In HOW TO: Check the Type of a COM Object (System.__ComObject) with Visual C# .NET, Microsoft suggests this approach:

C#:

private void ButtonClick(object sender, System.EventArgs e)
{
    object o = oExcel.Selection;
 
    //Display a message about the selection type in Excel.
    if ((o as Excel.Range) != null)
    {
        textBox1.Text = "Selection is Excel.Range";
    }
    else if ((o as Excel.Rectangle) != null)
    {
        textBox1.Text = "Selection is Excel.Rectangle";
    }
    else if ((o as Excel.ChartObject) != null)
    {
        textBox1.Text = "Selection is Excel.ChartObject";
    }
    else if ((o as Excel.ChartArea) != null)
    {
        textBox1.Text = "Selection is Excel.ChartArea";
    }
    // ... There are many additional Selection types you could check for if needed ...
    else
    {
        textBox1.Text = "Selection is Unknown";
    }
}

This code is simple if only you know what object types are selectable in the Excel UI. If you are not familiar with the Excel object model, you would prefer to have the type name of the selected object in a debugging message. Do you remember how they do this in the old good VBA:

VBA:

MsgBox("The selection object type is " & TypeName(Selection))

In another article named HOWTO: Know the actual type behind a System.__ComObject type returned by an .Object property the proposed solution is to call Microsoft.VisualBasic.Information.TypeName(); this requires referencing Microsoft.VisualBasic.dll.

The solution below gets the actual type name of such an object in three steps:

1. Cast the object to the IDispatch type.
2. Get the ITypeInfo interface via IDispatch.GetTypeInfo().
3. Get the type name using ITypeInfo.GetDocumentation().

Below is the code; I posted it previously on Microsoft forums. Be accurate; release every COM object you create in your code!

C#:

using System;
using System.Runtime.InteropServices;
using ComTypes = System.Runtime.InteropServices.ComTypes;
 
namespace ComUtils
{
    public class ComHelper
    {
        /// <summary>
        /// Returns a string value representing the type name of the specified COM object.
        /// </summary>
        /// <param name="comObj">A COM object the type name of which to return.</param>
        /// <returns>A string containing the type name.</returns>
        public static string GetTypeName(object comObj)
        {
 
            if (comObj == null)
                return String.Empty;
 
            if (!Marshal.IsComObject(comObj))
                //The specified object is not a COM object
                return String.Empty;
 
            IDispatch dispatch = comObj as IDispatch;
            if (dispatch == null)
                //The specified COM object doesn't support getting type information
                return String.Empty;
 
            ComTypes.ITypeInfo typeInfo = null;
            try
            {
                try
                {
                    // obtain the ITypeInfo interface from the object
                    dispatch.GetTypeInfo(0, 0, out typeInfo);
                }
                catch (Exception ex)
                {
                    //Cannot get the ITypeInfo interface for the specified COM object
                    return String.Empty;
                }
 
                string typeName = "";
                string documentation, helpFile;
                int helpContext = -1;
 
                try
                {
                    //retrieves the documentation string for the specified type description 
                    typeInfo.GetDocumentation(-1, out typeName, out documentation,
						out helpContext, out helpFile);
                }
                catch (Exception ex)
                {
                    // Cannot extract ITypeInfo information
                    return String.Empty;
                }
                return typeName;
            }
            catch (Exception ex)
            {
                // Unexpected error
                return String.Empty;
            }
            finally
            {
                if (typeInfo != null) Marshal.ReleaseComObject(typeInfo);
            }
        }
    }
 
    /// <summary>
    /// Exposes objects, methods and properties to programming tools and other
	/// applications that support Automation.
    /// </summary>
    [ComImport()]
    [Guid("00020400-0000-0000-C000-000000000046")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    interface IDispatch
    {
        [PreserveSig]
        int GetTypeInfoCount(out int Count);
 
        [PreserveSig]
        int GetTypeInfo(
            [MarshalAs(UnmanagedType.U4)] int iTInfo,
            [MarshalAs(UnmanagedType.U4)] int lcid,
            out ComTypes.ITypeInfo typeInfo);
 
        [PreserveSig]
        int GetIDsOfNames(
            ref Guid riid,
            [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPWStr)]
			string[] rgsNames,
            int cNames,
            int lcid,
            [MarshalAs(UnmanagedType.LPArray)] int[] rgDispId);
 
        [PreserveSig]
        int Invoke(
            int dispIdMember,
            ref Guid riid,
            uint lcid,
            ushort wFlags,
            ref ComTypes.DISPPARAMS pDispParams,
            out object pVarResult,
            ref ComTypes.EXCEPINFO pExcepInfo,
            IntPtr[] pArgErr);
    }
}

And this is how you get the type name of the Excel selection:
C#:

object selection = ExcelApp.Selection;
            if (selection != null)
            {
                string typeName = ComHelper.GetTypeName(selection);
                Debug.Print(typeName);
                Marshal.ReleaseComObject(selection);
            }

Good luck!

Available downloads:

This sample COM Add-in for Excel was developed using Add-in Express for Office and .net:

C# sample COM add-in
VB.NET sample COM add-in