WinForms FAQ - ComboBox

You can subclass ComboBox. In your derived class, make sure you set

  this.DrawMode = System.Windows.Forms.DrawMode.OwnerDrawFixed;
  this.DropDownStyle = ComboBoxStyle.DropDownList;

You also need to handle the DrawItem event to actually implement the drawing. Check out the details in the OwnerDrawnComboBox sample.

This entry was created using the feedback provided by Jay Harlow in the newsgroups.

The enum values can be bound to a combobox as follows:

[C#]
// Setup the binding as follows:
// MyValues is the enum type
comboBox1.DataSource = Enum.GetValues(typeof MyValues);
[VB]
comboBox1.DataSource = Enum.GetValues(Type.GetType(MyValues))

Then in the SelectedValueChanged event for the ComboBox.

[C#]
private void ComboBox1ValueChanged(object sender, EventArgs e)
{
    MyValues v = (MyValues)this.comboBox1.SelectedValue;
}
[VB]
Private  Sub ComboBox1ValueChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim v As MyValues = CType(Me.comboBox1.SelectedValue, MyValues)
End Sub

You can control the height by changing the value of the MaxDropDownItems property of the the Combobox, which is the setting for maximum number of entries that will be displayed by the drop-down list.

Here are some snippets. (Courtesy of Michael Lang)

[C#]
	DataTable list = new DataTable();
	list.Columns.Add(new DataColumn('Display', typeof(string)));
	list.Columns.Add(new DataColumn('Id', typeof(int)));
	list.Rows.Add(list.NewRow());
	list.Rows.Add(list.NewRow());
	list.Rows.Add(list.NewRow());
	list.Rows[0][0] = 'one';
	list.Rows[0][1] = 1;
	list.Rows[1][0] = 'two';
	list.Rows[1][1] = 2;
	list.Rows[2][0] = 'three';
	list.Rows[2][1] = 3;
	comboBox1.DataSource = list;
	comboBox1.DisplayMember = 'Display';
	comboBox1.ValueMember = 'Id';

[VB.NET]
	Dim list As New DataTable()
	list.Columns.Add(New DataColumn('Display', GetType(String)))
	list.Columns.Add(New DataColumn('Id', GetType(Integer)))
	list.Rows.Add(list.NewRow())
	list.Rows.Add(list.NewRow())
	list.Rows.Add(list.NewRow())
	list.Rows(0)(0) = 'one' ’
	list.Rows(0)(1) = 1 ’
	list.Rows(1)(0) = 'two' ’
	list.Rows(1)(1) = 2 ’
	list.Rows(2)(0) = 'three' ’
	list.Rows(2)(1) = 3 ’
	comboBox1.DataSource = list
	comboBox1.DisplayMember = 'Display'
	comboBox1.ValueMember = 'Id'

You can avoid the combobox dropping by overriding its WndProc method and ignoring the WM_LBUTTONDOWN and WM_LBUTTONDBLCLK.

[C#]
 public class MyComboBox : ComboBox
 {
	protected override void WndProc(ref System.Windows.Forms.Message m)
	{
		 if(m.Msg == 0x201  //WM_LBUTTONDOWN
		    || m.Msg == 0x203)  //WM_LBUTTONDBLCLK
  			 return;
		base.WndProc(ref m);
	}
 }

[VB.NET]
Public Class MyComboBox
	Inherits ComboBox
   
	Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
		If m.Msg = &H201 OrElse m.Msg = &H203 Then ’WM_LBUTTONDOWN  or WM_LBUTTONDBLCLK
			Return
		End If

		MyBase.WndProc(m)
	End Sub ’WndProc
End Class ’MyComboBox

Set the combobox’s DropDownStyle property to DropDownList .

You can iterate through the list to find the longest text extent using MeasureString, and then use this as the combobox width adding a fudge factor for the dropdown button.

	System.Drawing.Graphics g = comboBox1.CreateGraphics();
	float maxWidth = 0f;
	foreach(object o in comboBox1.Items)
	{
		float w = g.MeasureString(o.ToString(), comboBox1.Font).Width;
		if(w > maxWidth)
			maxWidth = w;
	}
	g.Dispose();
	comboBox1.Width = (int) maxWidth + 20; // 20 is to take care of button width

Michael Gold discusses this problem in How to create a ComboBox button in a toolbar in .NET on C# Corner.