C#

[DllImport("user32.dll", EntryPoint = "GetClassNameA")]
        public static extern int GetClassName(IntPtr hwnd, StringBuilder lpClassName, int nMaxCount);
        /*delegate to handle EnumChildWindows*/
        public delegate int EnumProc(IntPtr hWnd, ref IntPtr lParam);
        [DllImport("user32.dll")]
        public static extern int EnumChildWindows(IntPtr hWndParent, EnumProc lpEnumFunc, ref  IntPtr lParam);
        [DllImport("user32.dll", EntryPoint = "RegisterWindowMessageA")]
        public static extern int RegisterWindowMessage(string lpString);
        [DllImport("user32.dll", EntryPoint = "SendMessageTimeoutA")]
        public static extern int SendMessageTimeout(IntPtr hwnd, int msg, int wParam, int lParam, int fuFlags, int uTimeout, out int lpdwResult);
        [DllImport("OLEACC.dll")]
        public static extern int ObjectFromLresult(int lResult, ref Guid riid, int wParam, ref IHTMLDocument2 ppvObject);
        public const int SMTO_ABORTIFHUNG = 0x2;
        public Guid IID_IHTMLDocument = new Guid("626FC520-A41E-11CF-A731-00A0C9082637");

int i = SendMessageTimeout(hWnd, lngMsg, 0, 0, SMTO_ABORTIFHUNG, 1000, out lRes);
                        if (lRes != 0 && i !=0)

If the function succeeds, the return value is nonzero.

using System;
using System.Runtime.InteropServices;

internal static class NativeMethods
{
    public const int SMTO_NORMAL = 0x0000;
    public const int SMTO_BLOCK = 0x0001;
    public const int SMTO_ABORTIFHUNG = 0x0002;
    public const int SMTO_NOTIMEOUTIFNOTHUNG = 0x0008;
    public const int SMTO_ERRORONEXIT = 0x0020;

    // http://msdn.microsoft.com/en-us/library/windows/desktop/ms644952(v=vs.85).aspx
    /*
LRESULT WINAPI SendMessageTimeout(
__in       HWND hWnd,
__in       UINT Msg,
__in       WPARAM wParam,
__in       LPARAM lParam,
__in       UINT fuFlags,
__in       UINT uTimeout,
__out_opt  PDWORD_PTR lpdwResult
); */
    [DllImport("User32.dll", SetLastError = true)]
    public static extern IntPtr SendMessageTimeout(
        IntPtr hWnd,
        int Msg, // could use uint
        IntPtr wParam,
        IntPtr lParam,
        int fuFlags, // could use uint
        int uTimeout, // could use uint
        out IntPtr lpdwResult);
}

The value of this parameter depends on the message that is specified.

private void updateTreeview()
        {
            //add letters sort nodes
            for (int i = System.Convert.ToInt32('A'); i <= System.Convert.ToInt32('Z'); i++)
            {
                treeView1.Nodes.Add(Convert.ToString((char)(i)), Convert.ToString((char)(i))); //set key and text
            }

            //add customers
            TreeNode childnode = null;
            string sContactsName = null;
            foreach (DataRow rowContacts in dtContacts.Rows)
            {
                sContactsName = string.Concat(rowContacts["LastName"].ToString(), ", ", rowContacts["FirstName"].ToString());

                if (!(System.Convert.IsDBNull(rowContacts["MiddleInitial"].ToString())))
                {
                    sContactsName = string.Concat(sContactsName, " ", rowContacts["MiddleInitial"].ToString(), ".");
                }
                //---------------------Key lines here --------------
                childnode = new TreeNode(sContactsName, 0, 1);
                childnode.Name = sContactsName; //set text as key
                childnode.Tag = rowContacts["ContactID"].ToString();
                //--------------------------------------------------

                string sortnode = sContactsName.Substring(0, 1).ToUpper(); //gets alphabet sort node by first letter
                treeView1.Nodes[sortnode].Nodes.Add(childnode);
            }

            //Update TreeView
            treeView1.Visible = true;
            treeView1.Enabled = true;
            treeView1.ExpandAll();
        }

private void treeView1_NodeMouseClick(object sender, TreeNodeMouseClickEventArgs e)
{
    string nodeName = e.Node.Name;

    if (nodeName.Length == 1)
    {
        return; //is alphabet sort node, exit
    }

    //see if child form already exists and show it
    foreach (Form f in this.MdiChildren)
    {
        if (f.Name == nodeName)
        {
            f.Activate();
            return;
        }
    }


    string id = e.Node.Tag.ToString();

    displayContact(nodeName, id);
}