Display AutoComplete TextBox Validation message in Windows Forms Message Box using C# and VB.Net

Change the TextChanged event with below.

C#

private void txtCustomers_TextChanged(object sender, EventArgs e)
{
    DataRow[] rows = this.GetData().Select("Name='" + txtCustomers.Text.Trim() + "'");
    if (rows.Length == 0)
    {
        MessageBox.Show("Please select from List only.");
    }
}

VB.Net

Private Sub txtCustomers_TextChanged(ByVal sender As Object, ByVal e As EventArgs) Handles txtCustomers.TextChanged
    Dim rows As DataRow() = Me.GetData().Select("Name='" & txtCustomers.Text.Trim() & "'")
    If rows.Length = 0 Then
        MessageBox.Show("Please select from List only.")
    End If
End Sub