How can I dynamically change auto complete entries in a C# combobox or textbox?
- by Sam Hopkins
I have a combobox in C# and I want to use auto complete suggestions with it, however I want to be able to change the auto complete entries as the user types, because the possible valid entries are far too numerous to populate the AutoCompleteStringCollection at startup.
As an example, suppose I'm letting the user type in a name. I have a list of possible first names ("Joe", "John") and a list of surnames ("Bloggs", "Smith"), but if I have a thousand of each, then that would be a million possible strings - too many to put in the auto complete entries. So initially I want to have just the first names as suggestions ("Joe", "John") , and then once the user has typed the first name, ("Joe"), I want to remove the existing auto complete entries and replace them with a new set consisting of the chosen first name followed by the possible surnames ("Joe Bloggs", "Joe Smith"). In order to do this, I tried the following code:
void InitializeComboBox()
{
ComboName.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
ComboName.AutoCompleteSource = AutoCompleteSource.CustomSource;
ComboName.AutoCompleteCustomSource = new AutoCompleteStringCollection();
ComboName.TextChanged += new EventHandler( ComboName_TextChanged );
}
void ComboName_TextChanged( object sender, EventArgs e )
{
string text = this.ComboName.Text;
string[] suggestions = GetNameSuggestions( text );
this.ComboQuery.AutoCompleteCustomSource.Clear();
this.ComboQuery.AutoCompleteCustomSource.AddRange( suggestions );
}
However, this does not work properly. It seems that the call to Clear() causes the auto complete mechanism to "turn off" until the next character appears in the combo box, but of course when the next character appears the above code calls Clear() again, so the user never actually sees the auto complete functionality. It also causes the entire contents of the combo box to become selected, so between every keypress you have to deselect the existing text, which makes it unusable. If I remove the call to Clear() then the auto complete works, but it seems that then the AddRange() call has no effect, because the new suggestions that I add do not appear in the auto complete dropdown.
I have been searching for a solution to this, and seen various things suggested, but I cannot get any of them to work - either the auto complete functionality appears disabled, or new strings do not appear. Here is a list of things I have tried:
Calling BeginUpdate() before changing the strings and EndUpdate() afterwards.
Calling Remove() on all the existing strings instead of Clear().
Clearing the text from the combobox while I update the strings, and adding it back afterwards.
Setting the AutoCompleteMode to "None" while I change the strings, and setting it back to "SuggestAppend" afterwards.
Hooking the TextUpdate or KeyPress event instead of TextChanged.
Replacing the existing AutoCompleteCustomSource with a new AutoCompleteStringCollection each time.
None of these helped, even in various combinations. Spence suggested that I try overriding the ComboBox function that gets the list of strings to use in auto complete. Using a reflector I found a couple of methods in the ComboBox class that look promising - GetStringsForAutoComplete() and SetAutoComplete(), but they are both private so I can't access them from a derived class. I couldn't take that any further.
I tried replacing the ComboBox with a TextBox, because the auto complete interface is the same, and I found that the behaviour is slightly different. With the TextBox it appears to work better, in that the Append part of the auto complete works properly, but the Suggest part doesn't - the suggestion box briefly flashes to life but then immediately disappears.
So I thought "Okay, I'll
