problem in login page using window applcations

private void button4_Click(object sender, EventArgs e)
{

    try
    {
        con = new SqlConnection("Data Source=.;Initial Catalog=DSIIDC2;Integrated Security=True");
        cmd = new SqlCommand("select * from login where username='" + txtusername.Text + "' and password='" + txtpassword.Text + "' and role='admin01'", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable datatab = new DataTable();
        da.Fill(datatab);
        if ("admin" == txtusername.Text && "dsiidc" == txtpassword.Text && "admin01" == "admin01")
        {
            //Session["userone"] = TextBox1.Text;

            frm2.Show();


        }
        else if (("admin" == txtusername.Text && "admin" == txtpassword.Text && "admin02" == "admin02"))
        {

            ad.Show();
            da.Dispose();

        }
        else
        {
            txtpassword.Text = "";
            txtpassword.Text = "";


        }

    }

    catch (Exception ex)
    {
        MessageBox.Show("Please Enter User name and password");
    }


}