Why are you doing this in the TextChanged event?
All you are doing is trying to reload the list of sources from...the list of sources...
Move your code, and remove the WHERE clause, and it'll work:
private void textBox1_TextChanged(object sender, EventArgs e)
{
}
private void Form1_Load(object sender, EventArgs e)
{
DbConnection.Connect();
AutoCompleteStringCollection col = new AutoCompleteStringCollection();
string ss = "select name from ss";
SqlCommand cc = new SqlCommand(ss, DbConnection.mCon);
SqlDataReader dr = null;
dr = cc.ExecuteReader();
while (dr.Read())
{
col.Add(dr["name"].ToString());
}
dr.Close();
textBox1.AutoCompleteMode = AutoCompleteMode.Suggest;
textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;
textBox1.AutoCompleteCustomSource = col;
}