C# – Flickering in listview with ownerdraw and virtualmode

clistviewwinforms

I'm using listview control with the following parameters set:

        this.listView1.BackColor = System.Drawing.Color.Gainsboro;
        this.listView1.Columns.AddRange(new System.Windows.Forms.ColumnHeader[] {
        this.columnHeader1,
        this.columnHeader2});
        this.listView1.FullRowSelect = true;
        this.listView1.HideSelection = false;
        this.listView1.Location = new System.Drawing.Point(67, 192);
        this.listView1.Name = "listView1";
        this.listView1.Size = new System.Drawing.Size(438, 236);
        this.listView1.TabIndex = 0;
        this.listView1.UseCompatibleStateImageBehavior = false;
        this.listView1.View = System.Windows.Forms.View.Details;
        this.listView1.DrawColumnHeader += new System.Windows.Forms.DrawListViewColumnHeaderEventHandler(this.listView1_DrawColumnHeader);
        this.listView1.RetrieveVirtualItem += new System.Windows.Forms.RetrieveVirtualItemEventHandler(this.listView1_RetrieveVirtualItem);
        this.listView1.DrawSubItem += new System.Windows.Forms.DrawListViewSubItemEventHandler(this.listView1_DrawSubItem);

Two rows are provided with some random text. Ownerdrawing is simple:

    private void listView1_DrawSubItem(object sender, DrawListViewSubItemEventArgs e)
    {
        if (e.ColumnIndex == 0)
        {
            e.DrawBackground();
            e.DrawText();                
        }
        else
            e.DrawDefault = true;
        //Console.WriteLine("{0}\t\tBounds:{1}\tItem:{2}\tSubitem:{3}", (i++).ToString(), e.Bounds.ToString(), e.Item, e.SubItem);
    }

the problem is: when i hover mouse on listview's content, i get flickering of first column. Debugging shows that DrawSubItem is called constantly while the mouse is over it.

Is it bug? How to avoid this behavour?

Best Answer

This is a bug in .NET's ListView and you cannot get around it by double buffering.

On virtual lists, the underlying control generates lots of custom draw events when the mouse is hover over column 0. These custom draw events cause flickering even if you enable DoubleBuffering because they are sent outside of the normal WmPaint msg.

I also seem to remember that this only happens on XP. Vista fixed this one (but introduced others).

You can look in at the code in ObjectListView to see how it solved this problem.

If you want to solve it yourself, you need to delve into the inner plumbing of the ListView control:

override WndProc
intercept the WmPaint msg, and set a flag that is true during the msg
intercept the WmCustomDraw msg, and ignore all msgs that occur outside of a WmPaint event.

Something like this::

protected override void WndProc(ref Message m) {
    switch (m.Msg) {
        case 0x0F: // WM_PAINT
            this.isInWmPaintMsg = true;
            base.WndProc(ref m);
            this.isInWmPaintMsg = false;
            break;
        case 0x204E: // WM_REFLECT_NOTIFY
            NativeMethods.NMHDR nmhdr = (NativeMethods.NMHDR)m.GetLParam(typeof(NativeMethods.NMHDR));
            if (nmhdr.code == -12) { // NM_CUSTOMDRAW
                if (this.isInWmPaintMsg)
                    base.WndProc(ref m);
            } else
                base.WndProc(ref m);
            break;
        default:
            base.WndProc(ref m);
            break;
    }
}

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

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This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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