C# Signature:

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool EnumChildWindows(IntPtr hwndParent, EnumWindowsProc lpEnumFunc, IntPtr lParam);

public delegate bool Win32Callback(IntPtr hwnd, IntPtr lParam);
[DllImport("user32.Dll")]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern IntPtr EnumChildWindows(IntPtr parentHandle, Win32Callback callback,IntPtr lParam);

VB Signature:

<DllImport("user32.dll", CharSet:=CharSet.Auto)> _
Private Shared Function EnumChildWindows(ByVal hWndParent As System.IntPtr, _
   ByVal lpEnumFunc As EnumWindowsProc, ByVal lParam As Integer) As Boolean
    ' Leave function empty    
End Function

User-Defined Types:

EnumWindowsProc

Notes:

The corresponding unmanaged return type is a 4-byte Win32 'BOOL', so mark the method with the MarshalAsAttribute(UnmanagedType.Bool).

Tips & Tricks:

http://www.vbaccelerator.com/home/NET/Code/Libraries/Windows/Enumerating_Windows/article.asp has a nice article

Sample Code:

    [DllImport("user32")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool EnumChildWindows(IntPtr window, EnumWindowProc callback, IntPtr i);

    /// <summary>
    /// Returns a list of child windows
    /// </summary>
    /// <param name="parent">Parent of the windows to return</param>
    /// <returns>List of child windows</returns>
    public static List<IntPtr> GetChildWindows(IntPtr parent)
    {
    List<IntPtr> result = new List<IntPtr>();
    GCHandle listHandle = GCHandle.Alloc(result);
    try
    {
        EnumWindowProc childProc = new EnumWindowProc(EnumWindow);
        EnumChildWindows(parent, childProc, GCHandle.ToIntPtr(listHandle));
    }
    finally
    {
        if (listHandle.IsAllocated)
        listHandle.Free();
    }
    return result;
    }

    /// <summary>
    /// Callback method to be used when enumerating windows.
    /// </summary>
    /// <param name="handle">Handle of the next window</param>
    /// <param name="pointer">Pointer to a GCHandle that holds a reference to the list to fill</param>
    /// <returns>True to continue the enumeration, false to bail</returns>
    private static bool EnumWindow(IntPtr handle, IntPtr pointer)
    {
    GCHandle gch = GCHandle.FromIntPtr(pointer);
    List<IntPtr> list = gch.Target as List<IntPtr>;
    if (list == null)
    {
        throw new InvalidCastException("GCHandle Target could not be cast as List<IntPtr>");
    }
    list.Add(handle);
    //  You can modify this to check to see if you want to cancel the operation, then return a null here
    return true;
    }

    /// <summary>
    /// Delegate for the EnumChildWindows method
    /// </summary>
    /// <param name="hWnd">Window handle</param>
    /// <param name="parameter">Caller-defined variable; we use it for a pointer to our list</param>
    /// <returns>True to continue enumerating, false to bail.</returns>
    public delegate bool EnumWindowProc(IntPtr hWnd, IntPtr parameter);

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