Windows Form Combobox Auto complete suggestion on click returns null / blank

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HI All,

I have a Combo Box (CB) which is being populated from a list through CB.Datasource

The CB is populated but when any user clicks on the suggested auto complete the flashes quickly CB.Text then sets the CB.Text to a null value or empty.

If tab and arrow keys are used on the selected value field in the CB it works perfectly. Is there a way or solution anyone I can fix it so on clicking the suggested autocomplete the CB.Text fills with the selected value.

C#

private List<string> items;

Its set to query Active Directory for usernames and add them to the list through this method.

C#

private void findAllUsers()
       {
           items = new List<string>();

           PrincipalContext corp = new PrincipalContext(ContextType.Domain);
           UserPrincipal uName = new UserPrincipal(corp);
           PrincipalSearcher srch = new PrincipalSearcher(uName);

           foreach (var found in srch.FindAll())
           {
               UserPrincipal foundUser = found as UserPrincipal;
               try
               {
                   if (foundUser != null)
                   {
                       items.Add(foundUser.SamAccountName);
                   }

               }

               catch (Exception)
               {

               }
           }
       }

Items are then loaded through a background worker on form load.

C#

public void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
     {
         findAllUsers();
     }

C#

private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    comboBox1.DataSource = items;
    comboBox1.SelectedItem = null;

}

Posted 22-Jan-16 2:49am

FMG Tech

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BillWoodruff · Answer 1 · 2016-01-23T12:02:00

First: understand that a ComboBox is composed of a TextBox and a List (internally, these are both Type 'ComboBoxChildNativeWindow which inherits from 'NativeWindow).

The standard behavior of a ComboBox when the user clicks in the TextBox is not to trigger a Selection Event, it is to set the focus ... place the edit/insertion cursor ... in the TextBox. You should have a very good reason to change that standard behavior because: that's what users are socialized through experience to expect.

Note: what you say about the 'Tab Key puzzles me: to my knowledge the Tab Key does not do anything unique with a ComboBox.

Most problems like this are the result of not setting the ComboBox.AutoCompleteMode to 'AutoComplete.SuggestAppend, or 'AutoComplete.Append. And, not setting the 'AutoCompleteSource to AutoCompleteSource.Custom.

Here's how it works: Put a ComboBox, 'comboBox1, on a main Form in a WinForm Project, put a TextBox, 'textBox1 on the Form.

C#

string[] data = new string[]
{
    "Absecon","Abstracta","Abundantia","Academia","Acadiau",
    "Acamas","Ackerman","Ackley","Ackworth","Acomita","Aconcagua",
    "Acton","Acushnet","Acworth","Ada","Ada","Adair","Adairs","Adair",
    "Adak","Adalberta","Adamkrafft","Adams"};
}

private void Form1_Load(object sender, EventArgs e)
{
    comboBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
    comboBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
    AutoCompleteStringCollection aCompl = new AutoCompleteStringCollection();
    aCompl.AddRange(data);
    comboBox1.Items.Clear();
    comboBox1.DataSource = data;
    comboBox1.SelectedIndexChanged += comboBox1_SelectedIndexChanged;
    comboBox1.AutoCompleteCustomSource = aCompl;
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs eventArgs)
{
    textBox1.Text = comboBox1.SelectedItem.ToString();
}

There are three types of user-action that will cause the 'SelectedIndexChanged Event to occur:

1. the user starts typing, an auto-complete entry appears in the text field, the user hits the 'Return key

2. the user drops down the Item selector List and clicks a choice from that List

3. the user starts typing, an auto-complete entry appears, the user uses the arrow-keys to scroll down in the Item selector List, and when the user hits the 'Return key, the current high-lighted Item in the selector List is selected