How to handle nullreference exception in "NM_CUSTOMDRAW" notification message?

Question

Hello,
I have a windows forms application having a user control . The user control has a Treeview named as TriView. TriView's drawmode is set to OwnerDrawAll.
The form is visible all the time. When I run my application and switch between the other controls and TriView 3-4-5 times, I get a NullReferenceEXception while populating TriView

   TriView.Nodes.Add(oTreeNode)  

None of the fields are null here. Still I get the exception.

These are the details of exception:

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Windows.Forms.TreeNode.get_Handle()
at System.Windows.Forms.TreeNode.get_RowBounds()
at System.Windows.Forms.TreeView.CustomDraw(Message& m)
at System.Windows.Forms.TreeView.WmNotify(Message& m)
at System.Windows.Forms.TreeView.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Similar question is asked here: nullreference-exception-in-treenode-get-handle-in-ownerdrawn-treeview

Author says she has solved the issue by handling the null reference exception in "NM_CUSTOMDRAW" notification message but didn't tell how she did it. I neither get any source for the same through google search. I don't know how to handle messages.

Can any one help me by showing some code to handle the null reference exception in "NM_CUSTOMDRAW" notification message.

Answer 1

NM_CUSTOMDRAW is handled in WM_NOFITY

If your TreeView control is inherited from standard TreeView, use WM_REFLECT + WM_NOFITY
Something like this =>

public class TreeViewCustom : TreeView  
{  
    private const int NM_FIRST = 0;  
    private const int NM_CLICK = NM_FIRST - 2;  
    private const int NM_CUSTOMDRAW = NM_FIRST - 12;  
    private const int WM_REFLECT = 0x2000;  
    private const int WM_NOFITY = 0x004E;  

    [StructLayout(LayoutKind.Sequential)]  
    private struct NMHDR  
    {  
        public IntPtr hwndFrom;  
        public IntPtr idFrom;  
        public int code;  
    }  

    [StructLayout(LayoutKind.Sequential)]  
    private struct NMCUSTOMDRAW  
    {  
        public NMHDR hdr;  
        public int dwDrawStage;  
        public IntPtr hdc;  
        public RECT rc;  
        public IntPtr dwItemSpec;  
        public uint uItemState;  
        public IntPtr lItemlParam;  
    }  

    [StructLayout(LayoutKind.Sequential)]  
    private class NMTVCUSTOMDRAW  
    {  
        public NMCUSTOMDRAW nmcd;  
        public int clrText;  
        public int clrTextBk;  
        public int iLevel;  
    }  

    [FlagsAttribute]  
    internal enum CDRF  
    {  
        CDRF_DODEFAULT = 0x00000000,  
        CDRF_NEWFONT = 0x00000002,  
        CDRF_SKIPDEFAULT = 0x00000004,  
        CDRF_DOERASE = 0x00000008,  
        CDRF_SKIPPOSTPAINT = 0x00000100,  
        CDRF_NOTIFYPOSTPAINT = 0x00000010,  
        CDRF_NOTIFYITEMDRAW = 0x00000020,  
        CDRF_NOTIFYSUBITEMDRAW = 0x00000020,  
        CDRF_NOTIFYPOSTERASE = 0x00000040  
    }  

    [FlagsAttribute]  
    internal enum CDDS  
    {  
        CDDS_PREPAINT = 0x00000001,  
        CDDS_POSTPAINT = 0x00000002,  
        CDDS_PREERASE = 0x00000003,  
        CDDS_POSTERASE = 0x00000004,  
        CDDS_ITEM = 0x00010000,  
        CDDS_ITEMPREPAINT = (CDDS_ITEM | CDDS_PREPAINT),  
        CDDS_ITEMPOSTPAINT = (CDDS_ITEM | CDDS_POSTPAINT),  
        CDDS_ITEMPREERASE = (CDDS_ITEM | CDDS_PREERASE),  
        CDDS_ITEMPOSTERASE = (CDDS_ITEM | CDDS_POSTERASE),  
        CDDS_SUBITEM = 0x00020000  
    }  

    [StructLayout(LayoutKind.Sequential)]  
    public struct RECT  
    {  
        public int left;  
        public int top;  
        public int right;  
        public int bottom;  
    }  

    public TreeViewCustom()  
    {  
        DrawMode = TreeViewDrawMode.OwnerDrawAll;  
        // Code...  
    }  

    protected override void WndProc(ref Message m)  
    {  
        if (m.Msg == WM_REFLECT + WM_NOFITY)  
        {  
            var pnmhdr = (NMHDR)m.GetLParam(typeof(NMHDR));  
            if (pnmhdr.code == NM_CUSTOMDRAW)  
            {  
                try  
                {  
                		// Test code...  
                    var pnmtv = (NMTVCUSTOMDRAW)m.GetLParam(typeof(NMTVCUSTOMDRAW));  
                    switch (pnmtv.nmcd.dwDrawStage)  
                    {  
                        case (int)CDDS.CDDS_PREPAINT:  
                            {  
                                m.Result = new IntPtr((int)CDRF.CDRF_NOTIFYITEMDRAW);  
                            }  
                            break;  
                        case (int)CDDS.CDDS_ITEMPREPAINT:  
                            {  
                                pnmtv.clrText = ColorTranslator.ToWin32(System.Drawing.Color.Red);  
                                //pnmtv.clrTextBk = ColorTranslator.ToWin32(System.Drawing.Color.Lime);  
                                Marshal.StructureToPtr(pnmtv, m.LParam, false);  
                                m.Result = new IntPtr((int)(CDRF.CDRF_NEWFONT | CDRF.CDRF_NOTIFYPOSTPAINT));  
                            }  
                            break;  
                        case (int)CDDS.CDDS_ITEMPOSTPAINT:  
                            {  
                                // Code...  
                            }  
                            break;  
                    }  

                }  
                catch (NullReferenceException)  
                {  
                    // Code...  
                }                      
            }  
            return;  
        }             
        else  
            base.WndProc(ref m);  
    }  
}