How Can I Create a Custom AutoComplete Feature in C# TextBoxes that Supports Non-Prefix Matching?

Custom AutoComplete for C# TextBoxes

When implementing autocomplete functionality in C# textboxes, it's often desirable to allow users to complete entries regardless of the order in which they type. For instance, if an entry exists with the format "[001] Last, First Middle", it should appear in autocomplete results when the user types "John" for the first name.

Limitations of Default AutoComplete

The default AutoComplete functionality in C# only supports prefix matching. This means that users must type the beginning of the entry to trigger autocomplete.

Overriding AutoComplete with Custom Event Handling

To overcome this limitation, you can implement a custom autocomplete function by overriding the OnTextChanged event. This allows you to handle the text input and dynamically display relevant autocomplete results.

Example Implementation

The following example demonstrates how to create a rudimentary autocomplete function using a ListBox:

1. Create a ListBox and set its visibility to false by default.
2. Use the OnTextChanged event of the TextBox to monitor the user's input.
3. In the OnTextChanged event handler, iterate through the autocomplete source and add matching items to the ListBox.
4. Set the visibility of the ListBox to true to display the autocomplete results.
5. Use the SelectedIndexChanged event of the ListBox to select an item and populate the TextBox with the selected text.

Custom Function

Here's an example of how to implement the custom autocomplete function described above:

private void textBox1_TextChanged(object sender, System.EventArgs e)
{
    listBox1.Items.Clear();
    if (textBox1.Text.Length == 0)
    {
        hideResults();
        return;
    }

    foreach (String s in textBox1.AutoCompleteCustomSource)
    {
        if (s.Contains(textBox1.Text))
        {
            Console.WriteLine("Found text in: " + s);
            listBox1.Items.Add(s);
            listBox1.Visible = true;
        }
    }
}

Enhancements

This basic implementation can be enhanced by adding functionality such as appending text to the TextBox, capturing additional keyboard commands, and implementing filtering based on more complex criteria.

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