Thanks for contacting Syncfusion support.
We have analyzed your reported query “How to set the round corner and border color in combo dropdown”. In fact, ComboDropDown is our classic control. As a result, we recommend using the SfComboBox control as an alternative to the combodropdown control. Combo box is a text box component that allows user to type a value or choose an option from a list of predefined options. It has several out of the box features such as data binding, filtering, UI customization, accessibility, and more.
For more details, please refer our UG documentation from the following link:
Please refer the following code sample.
//Raises the paint event of the SfComboBox.
this.editMode.Paint += EditMode_Paint;
private void EditMode_Paint(object sender, PaintEventArgs e)
this.editMode.Style.EditorStyle.BorderColor = Color.Red;
//Rounded rectangle corder radius. The radius must be less than 10.
int radius = 2;
editMode.Region = new Region(DrawingUtils.GetRoundedRectangle(this.editMode.ClientRectangle, radius));
e.Graphics.DrawPath(new Pen(Color.Red), DrawingUtils.GetRoundedRectangle(this.editMode.ClientRectangle, radius));
Please let us know if you need any further assistance.