Editable History ComboBox

• Download demo project - 21 KB

Introduction

The Compact Framework does not provide the option for an editable combobox, but there is an easy way to work around this limitation. The following article will explain how to create an editable combobox that remembers up to 10 previous entries (Like the address bar in Internet Explorer).

Using the code

To make a combobox editable, you simply overlay a text box on a combobox.  To make the textbox and combobox fit together seamlessly you can just add code to each combobox like so:

int dropDownButtonWidth = 14;
txtTest1.Bounds = cboTest1.Bounds;
txtTest1.Width -= dropDownButtonWidth;

Now, to complete the behavior of the editable combobox, you just need to make sure the textbox gets updated with the currently selected item from the combobox.  Refer to the code below:

private void cboTest1_SelectedIndexChanged(object sender, System.EventArgs e)
{
    txtTest1.Text = cboTest1.Text;
}

This is all you need to do to get the behavior of an editable combobox.  If you would like to create a history combobox, you will need an XML file to persist the combobox entries. The code to load the past entries is pretty straightforward.

// Load the combobox entries from the XML file.
xdoc.Load(@"\Path to comboboxes.xml");
XmlElement root = xdoc.DocumentElement;
XmlNodeList nodeList = root.ChildNodes;
ComboBox cboHistory;

for(int j=0; j<nodeList.Item(i).ChildNodes.Count; ++j)
{
    cboHistory.Items.Add(nodeList.Item(i).ChildNodes.Item(j).InnerText);    
}

The code to save new entries is a little more involved but not too bad.

int maxEntriesToStore = 10;
XmlTextWriter tw = new XmlTextWriter(@"\Path to comboboxes.xml", 
                                     System.Text.Encoding.UTF8);
tw.WriteStartDocument();
tw.WriteStartElement("comboboxes");
tw.WriteStartElement("combobox");
tw.WriteStartAttribute("name", string.Empty);
tw.WriteString("cboTest1");
tw.WriteEndAttribute();

// Write the item from the text box first.
if(txtTest1.Text.Length != 0)
{
    tw.WriteStartElement("entry");
    tw.WriteString(txtTest1.Text);
    tw.WriteEndElement();
    maxEntriesToStore -= 1;
}

// Write the rest of the entries (up to 10).
for(int i=0; i < cboTest1.Items.Count && i < maxEntriesToStore; ++i)
{
    if(txtTest1.Text != cboTest1.Items[i].ToString())
    {
        tw.WriteStartElement("entry");
        tw.WriteString(cboTest1.Items[i].ToString());
        tw.WriteEndElement();
    }
}

tw.WriteEndElement();
tw.WriteEndElement();
tw.Flush();
tw.Close();

The demo project contains code to handle 3 comboboxes and is arranged a little differently than the sample code above. Enjoy!