textbox 系統控制元件，直接改肯定是沒辦法的，只能自己重寫控制元件，繼承TextBox

而且圓角應該只能完全重寫控制元件

給你個改邊框顏色的程式碼

public class MyTextBox : TextBox

{

public MyTextBox()

{

this.BorderStyle = BorderStyle.FixedSingle;

}

protected override void WndProc(ref Message m)

{

base.WndProc(ref m);

if (m.Msg == 0xf || m.Msg == 0x14 || m.Msg == 0x85)

{

if (this.BorderStyle == BorderStyle.FixedSingle)

{

using (Graphics g = Graphics.FromHwnd(this.Handle))

{

using (Pen pen = new Pen(Color.Red))

{

g.DrawRectangle(pen, 0, 0, this.Width - 1, this.Height - 1);

}

}

}

}

}

}