How to prevent manual input into a ComboBox in C#

Just set your combo as a DropDownList:

this.comboBoxType.DropDownStyle = ComboBoxStyle.DropDownList;

I believe you want to set the DropDownStyle to DropDownList.

this.comboBoxType.DropDownStyle = 
    System.Windows.Forms.ComboBoxStyle.DropDownList;

Alternatively, you can do this from the WinForms designer by selecting the control, going to the Properties Window, and changing the "DropDownStyle" property to "DropDownList".

You can suppress handling of the key press by adding e.Handled = true to the control's KeyPress event:

private void Combo1_KeyPress(object sender, KeyPressEventArgs e)
{
    e.Handled = true;
}