How Can I Implement Partial String Matching in C# AutoComplete TextBox Functionality?

C# AutoComplete: Partial Search Functionality in TextBox

When implementing autocomplete features in C#, developers often encounter a limitation where the entered string must match the beginning of the item in the autocomplete list. In other words, the current implementation only supports prefix searches. However, if you need this functionality to be more comprehensive and allow for partial matches, such as searching by first name instead of requiring the prefix number, the default autocomplete behavior becomes insufficient.

To overcome this limitation, it is possible to create a custom autocomplete function by overriding the OnTextChanged event of the TextBox control. This grants control over filtering and displaying the suggested items based on the user's input.

For instance, a ListBox can be added immediately below the TextBox, initially hidden, and set to display matching items as the user types. The OnTextChanged event of the TextBox and the SelectedIndexChanged event of the ListBox can be used to display and select the appropriate autocomplete results.

Below is a basic example of how this custom implementation might be achieved:

public Form1()
{
    InitializeComponent();

    acsc = new AutoCompleteStringCollection();
    textBox1.AutoCompleteCustomSource = acsc;
    textBox1.AutoCompleteMode = AutoCompleteMode.None;
    textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
}

private void button1_Click(object sender, EventArgs e)
{
    acsc.Add("[001] some kind of item");
    acsc.Add("[002] some other item");
    acsc.Add("[003] an orange");
    acsc.Add("[004] i like pickles");
}

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;
        }
    }
}

void listBox1_SelectedIndexChanged(object sender, System.EventArgs e)
{
    textBox1.Text = listBox1.Items[listBox1.SelectedIndex].ToString();
    hideResults();
}

void listBox1_LostFocus(object sender, System.EventArgs e)
{
    hideResults();
}

void hideResults()
{
    listBox1.Visible = false;
}

This code demonstrates a rudimentary custom autocomplete function. With minimal effort, additional features could be implemented, such as dynamically adjusting the TextBox text and handling various keyboard inputs. By overriding the default autocomplete behavior, you gain greater flexibility in designing custom solutions that meet your specific requirements.

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