dev treelist控件，显示3角符，或显示+ -符的控制

解决方案1：

  在 program.cs中把 下面一句暂时注解，你就会发现所有的 treelist会自动显示 + -号;

     如果放出来，就显示3角符

        DevExpress.LookAndFeel.UserLookAndFeel.Default.SetSkinStyle("DevExpress Style");

解决方案2：

   自己画+-号

private void treeList1_CustomDrawNodeButton(object sender, DevExpress.XtraTreeList.CustomDrawNodeButtonEventArgs e)
        {
            Brush backBrush = new System.Drawing.Drawing2D.LinearGradientBrush(e.Bounds, Color.White, Color.Black,
    System.Drawing.Drawing2D.LinearGradientMode.ForwardDiagonal);
            e.Graphics.FillRectangle(backBrush, e.Bounds);
            // painting 3D borders
            ControlPaint.DrawBorder3D(e.Graphics, e.Bounds, Border3DStyle.Flat);

            // determining the character to display
            string displayCharacter = e.Expanded ? "-" : "+";
            // formatting the output character
            StringFormat outCharacterFormat = new StringFormat();
            outCharacterFormat.Alignment = StringAlignment.Center;
            outCharacterFormat.LineAlignment = StringAlignment.Center;

            // painting the character
            e.Graphics.DrawString(displayCharacter, new Font("Verdana", 8),
              new SolidBrush(Color.White), e.Bounds, outCharacterFormat);

            // prohibiting default painting
            e.Handled = true;

        }