i want use SQL on Web service but where is my fault?

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C#

I can't read parameter "zyd" and actually "b" . How can I read this? I cant find any problem. But I think have logical problem. My SQL table--> KullaniciAdi | Sifre
a | b
NULL | NULL

static string zyd;
[WebMethod]
public void deneme()
{
    string b="a";
    string KullaniciAdi = "a";
    string connect = "Data Source=BERKAY-PC\\SQLEXPRESS;Initial Catalog=Hastahane;Integrated Security=True;";
    SqlConnection deneme = new SqlConnection(connect);
    SqlCommand komut = new SqlCommand("SELECT * FROM Giris where KullaniciAdi='" + KullaniciAdi + "'order by KullaniciAdi desc", deneme);

    try
    {
        deneme.Open();
        SqlDataReader oku = komut.ExecuteReader();
        while (oku.Read())
        {
            b = oku["Sifre"].ToString();
        }
    }
    finally
    {
        deneme.Close();
        komut.Dispose();
    }
    zyd=b;
}