Silverlight组合框绑定到数据

Question

我是Silverlight，C＃，visualstudio初学者，我正在寻找解决方案。我想将combobox绑定到SQL server表中的数据。表中有ID和Name。另外当组合框将成功绑定我想，在应用程序中从组合框中选择一些东西并用“OK”按钮插入到Sql server表中作为值int（我正在使用sql过程）。

插入：

public ObservableCollection<ComboboxPW> ReadComboboxPW(int ID_pw)
        {
            ObservableCollection<ComboboxPW> result = new ObservableCollection<ComboboxPW>();
            string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
            SqlDataReader dr;
            SqlConnection conn = new SqlConnection(nwConn);

            try
            {
                SqlCommand cmd = new SqlCommand();
                cmd.CommandType = CommandType.StoredProcedure;

                cmd.Connection = conn;
                cmd.CommandText = "COMBOBOX_PW";
                cmd.Parameters.AddWithValue("ID_pw", ID_pw);
                conn.Open();
                dr = cmd.ExecuteReader();
                while (dr.Read())
                {

                   ComboboxPW lista = new ComboboxPW();
                   lista.PW = dr.GetString(0);               
                   lista.IDPW = dr.GetString(0);
                   result.Add(lista);

                }
                dr.Close();
                return result;

            }
            catch (SqlException e)
            {
                ComboboxPW lista = new ComboboxPW();
                lista.PW = e.Message;                        
                result.Add(lista);
                return result;

            }
            finally
            {
                conn.Close();

            };
        }

MY CLASS：

public class ComboboxPW
    {
        public string PW { get; set; }
        public string IDPW { get; set; }

    }

XAML：

<ComboBox x:Name="comboBoxPW" HorizontalAlignment="Left" Margin="40,117,0,0" VerticalAlignment="Top" Width="366"   
               SelectedItem="{Binding ReadComboboxPW}" DisplayMemberPath="PW"  SelectedValuePath="IDPW" SelectionChanged="comboBoxPW_SelectionChanged"  />

BUTTON OK：

    private void OKButton_Click(object sender, RoutedEventArgs e)
        {
              this.DialogResult = true;
              ServiceReference1.Service1Client webService = new ServiceReference1.Service1Client();
webService.InsertPWAsync(comboBoxPW.SelectedValue); 

以上是错误，低于图片：

SQL中的程序：

 PROCEDURE [dbo].[COMBOBOX_PW] 
    @ID_pw int
AS
BEGIN
        SET NOCOUNT ON;

    Select 
         Nazwa_PW, ID_PW
            FROM  [dbo].[DOSTEPNE_PW]
            where    (ID_PW = @ID_pw or @ID_pw = 0)

END

public int InsertPW(int PW)
    {
        string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
        SqlConnection conn = new SqlConnection(nwConn);

        try
        {
            SqlCommand cmd = new SqlCommand();
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Connection = conn;
            cmd.CommandText = "Zapisz_PW";
            cmd.Parameters.AddWithValue("PW", PW);
            conn.Open();
            int result = cmd.ExecuteNonQuery();
            conn.Close();
            if (result > 0)
            {
                return result;
            }
            else
            {
                return 0;
            }
        }
        catch (SqlException e)
        {
            int result = 0;
            return result;
        }
        finally
        {
            conn.Close();
        };

插入sql表：

 public int InsertPW(int PW)
        {
            string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
            SqlConnection conn = new SqlConnection(nwConn);

            try
            {
                SqlCommand cmd = new SqlCommand();
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Connection = conn;
                cmd.CommandText = "Zapisz_PW";
                cmd.Parameters.AddWithValue("PW", PW);
                conn.Open();
                int result = cmd.ExecuteNonQuery();
                conn.Close();
                if (result > 0)
                {
                    return result;
                }
                else
                {
                    return 0;
                }
            }
            catch (SqlException e)
            {
                int result = 0;
                return result;
            }
            finally
            {
                conn.Close();
            };