Change ComboBox Border Color in Windows Forms

You can inherit from ComboBox and override WndProc and handle WM_PAINT message and draw border for your combo box:

using System;
using System.Drawing;
using System.Windows.Forms;

public class FlatCombo : ComboBox
{
    private const int WM_PAINT = 0xF;
    private int buttonWidth = SystemInformation.HorizontalScrollBarArrowWidth;
    Color borderColor = Color.Blue;
    public Color BorderColor
    {
        get { return borderColor; }
        set { borderColor = value; Invalidate(); }
    }
    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        if (m.Msg == WM_PAINT && DropDownStyle != ComboBoxStyle.Simple)
        {
            using (var g = Graphics.FromHwnd(Handle))
            {
                using (var p = new Pen(BorderColor))
                {
                    g.DrawRectangle(p, 0, 0, Width - 1, Height - 1);

                    var d = FlatStyle == FlatStyle.Popup ? 1 : 0;
                    g.DrawLine(p, Width - buttonWidth - d,
                        0, Width - buttonWidth - d, Height);
                }
            }
        }
    }
}

Note:

• In the above example I used fore color for border, you can add a BorderColor property or use another color.
• If you don’t like the left border of dropdown button, you can comment that DrawLine method.
• You need to draw line when the control is RightToLeft from (0, buttonWidth) to (Height, buttonWidth)
• To learn more about how to render a flat combo box, you can take a look at source code of internal ComboBox.FlatComboAdapter class of .Net Framework.

Flat ComboBox

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