Web Forms :: Display In Textbox From Database Using DropDown List

the following content I created a table (Workers) with two columns(username,userid). Now i have Dropdownlist and textbox. Dropdownlist for Userid, Textbox for Username. If i click on the userid in dropdownlist, then the particular username should get displayed on the textbox. For this i have tried codings.

private void DropDownList()
{
OleDbConnection databaseConnection = new OleDbConnection(ConfigurationManager.ConnectionStrings[1].ConnectionString);
databaseConnection.Open();
OleDbDataAdapter dataAdapter = new OleDbDataAdapter("Select userid from workers", databaseConnection);
DataTable dataTable = new DataTable();
dataAdapter.Fill(dataTable);
DropDownListMaterials.DataSource = dataTable;
DropDownListMaterials.DataTextField = "userid";
DropDownListMaterials.DataValueField = "userid";
DropDownListMaterials.DataBind();
}
protected void DropDownListMaterials_SelectedIndexChanged(object sender, EventArgs e)
{
OleDbConnection databaseConnection = new OleDbConnection(ConfigurationManager.ConnectionStrings[1].ConnectionString);
databaseConnection.Open();
OleDbCommand MyCommand = new OleDbCommand("select username from gridview where userid=('" + DropDownListMaterials.SelectedItem + "')", databaseConnection);
TextBoxUsername.Text =
}
private void DropDownList()