Windows Forms FAQ : Windows Forms ComboBox : Syncfusion



	Questions and answers in this FAQ have been collected from newsgroup posts, various mailing lists and the employees of Syncfusion.

20. Windows Forms ComboBox

WinForms FAQ Home

20.1	How do I bind the values of an enum to a ComboBox?

20.2	How can I programmatically prevent the combobox from dropping?

20.3	How can I adjust the height of the of my ComboBox dropdown?

20.4	How do I implement an owner drawn combobox?

20.5	How do I add a ComboBox button to a toolbar?

20.6	How do I set the width of a combobox to fit the entries in its list?

20.7	How can I programmatically create a new list for my ComboBox dropdown?

20.8	How can I turn off editing in the textbox portion of a ComboBox, restricting the user to selecting only those options in the drop list?

20.1 How do I bind the values of an enum to a ComboBox?

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

20.2 How can I programmatically prevent the combobox from dropping?

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

20.3 How can I adjust the height of the of my ComboBox dropdown?

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.

20.4 How do I implement an owner drawn 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.

20.5 How do I add a ComboBox button to a toolbar?

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

20.6 How do I set the width of a combobox to fit the entries in its list?

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

20.7 How can I programmatically create a new list for my ComboBox dropdown?

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[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(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"

20.8 How can I turn off editing in the textbox portion of a ComboBox, restricting the user to selecting only those options in the drop list?

Set the combobox's DropDownStyle property to DropDownList .