SqlDataAdapter sda = new SqlDataAdapter("SELECT TEST1 FROM TestTable", "Data Source=.;Initial Catalog=SSO;User ID=sa;Password=******");
DataTable dt = new DataTable();
sda.Fill(dt);
var source1 = new AutoCompleteStringCollection();
for (int i = 0; i < dt.Rows.Count; i++)
{
Source1.Add(dt.Rows[i][0].ToString());
}
textBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
textBox1.AutoCompleteCustomSource = source1;