Override DrawItem of ComboBox

问题

I changed the highlight color of various of the controls, and I am planning to make more changes. So I though is better to create my own controls and reuse them instead of making the changed for each and every one of them.

I created a new user control, and inherited from System.Windows.Forms.ComboBox. The problem is I cannot find a way to override onDraw like I would for onClick.

So how I would go and override it? Here is the code I used for each control onDraw event

public void comboMasterUsers_DrawItem(object sender, DrawItemEventArgs e)
    {
        e.DrawBackground();

        Graphics g = e.Graphics;
        Brush brush = ((e.State & DrawItemState.Selected) == DrawItemState.Selected) ?
                      Brushes.LightSeaGreen : new SolidBrush(e.BackColor);

        g.FillRectangle(brush, e.Bounds);
        e.Graphics.DrawString(comboMasterUsers.Items[e.Index].ToString(), e.Font,
                 new SolidBrush(e.ForeColor), e.Bounds, StringFormat.GenericDefault);

        e.DrawFocusRectangle();
    }

Thanks!

回答1:

Here you go:

public class myCombo : ComboBox
{
    // expose properties as needed
    public Color SelectedBackColor{ get; set; }

    // constructor
    public myCombo()
    {
        DrawItem += new DrawItemEventHandler(DrawCustomMenuItem);
        DrawMode = System.Windows.Forms.DrawMode.OwnerDrawFixed;
        SelectedBackColor= Color.LightSeaGreen;
    }

    protected  void DrawCustomMenuItem(object sender, DrawItemEventArgs e)
    {
        e.DrawBackground();
         // a dropdownlist may initially have no item selected, so skip the highlighting:
        if (e.Index >= 0) 
        {  
          Graphics g = e.Graphics;
          Brush brush = ((e.State & DrawItemState.Selected) == DrawItemState.Selected) ?
                         new SolidBrush(SelectedBackColor) : new SolidBrush(e.BackColor);
          Brush tBrush = new SolidBrush(e.ForeColor);

          g.FillRectangle(brush, e.Bounds);
          e.Graphics.DrawString(this.Items[e.Index].ToString(), e.Font,
                     tBrush, e.Bounds, StringFormat.GenericDefault);
          brush.Dispose();
          tBrush.Dispose();
        }
        e.DrawFocusRectangle();
    }
}

You may consider exposing more Properties as you expand your customziation, so you can change them for each instance when you want to..

Also don't forget to dispose GDI objects you create, like brushes and pens!

Edit: Just noticed that BackColor would hide the original property. Changed it to SelectedBackColor, which actually says what it is!

Edit 2: As Simon noted in the comments, there is a HasFlag method and so as of .Net 4.0 one can also write:

      Brush brush = ((e.State.HasFlag(DrawItemState.Selected) ?

which is a little clearer and shorter.

Edit 3: Actually for drawing onto controls the use of TextRenderer.DrawText is recommended over graphics.DrawString..

回答2:

Thanks for this helpful post. I made one small enhancement that others may find useful.

When the ComboBox has its DisplayMember property set to access a specific property of the displayed items, ToString() may not give the expected text. The fix for this is to use:

    GetItemText(Items[e.Index])

to retrieve the required text in the call to DrawString().

来源：https://stackoverflow.com/questions/24475812/override-drawitem-of-combobox

标签

inheritance

combobox

controls

ondraw