AutoComplete TextBox Control

Check out the AutoCompleteSource, AutoCompleteCustomSource and AutoCompleteMode properties.

textBox1.AutoCompleteMode = AutoCompleteMode.Suggest;
textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
AutoCompleteStringCollection col = new AutoCompleteStringCollection();
col.Add("Foo");
col.Add("Bar");
textBox1.AutoCompleteCustomSource = col;

Note that the designer allows you to do that without writing any code...

There are two ways to accomplish this textbox effect:

Either using the graphic user interface (GUI); or with code

Using the Graphic User Interface:
Go to: "Properties" Tab; then set the following properties:

However; the best way is to create this by code. See example below.

AutoCompleteStringCollection sourceName = new AutoCompleteStringCollection();

foreach (string name in listNames)
{    
    sourceName.Add(name);
}

txtName.AutoCompleteCustomSource = sourceName;
txtName.AutoCompleteMode = AutoCompleteMode.Suggest;
txtName.AutoCompleteSource = AutoCompleteSource.CustomSource;

This might not be the best way to do things, but should work:

 this.textBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
 this.textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;

private void textBox1_TextChanged(object sender, EventArgs e)
{
    TextBox t = sender as TextBox;
    if (t != null)
    {
        //say you want to do a search when user types 3 or more chars
        if (t.Text.Length >= 3)
        {
            //SuggestStrings will have the logic to return array of strings either from cache/db
            string[] arr = SuggestStrings(t.Text);

            AutoCompleteStringCollection collection = new AutoCompleteStringCollection();
            collection.AddRange(arr);

            this.textBox1.AutoCompleteCustomSource = collection;
        }
    }
}