Dim cmd As New SqlCommand("Select col_name  FROM college ", cn)
    If cn.State = ConnectionState.Closed Then cn.Open()
    Dim ds As New DataSet
    Dim sqda As New SqlDataAdapter(cmd)
    sqda.Fill(ds, "college")

    Dim col As New AutoCompleteStringCollection
    Dim i As Integer
    For i = 0 To ds.Tables(0).Rows.Count - 1
        col.Add(ds.Tables(0).Rows(i)("col_name").ToString())
    Next
    TextBox1.AutoCompleteSource = AutoCompleteSource.CustomSource
    TextBox1.AutoCompleteCustomSource = col
    TextBox1.AutoCompleteMode = AutoCompleteMode.Suggest

cmd = New SqlCommand("insert into college (col_name) values (N'" & TextBox1.Text.Trim & "')", cn)