Updating error using (ling to sql)

问题

I have two tables, binded by foreign key CarrierID:

Carrier[CarrierID,CarrierName] 
CarrierID = 1, CarrierName = DHL
CarrierID = 2, CarrierName = Fedex
...
Vendor[VendorID, VendorName, CarrierID]  
VendorID = 1, VendorName =D-link , CarrierID=1  
VendorID = 2, VendorName = Netbes , CarrierID= 2

When i try to update the carriername in vendorview. Im getting this error "ForeignKeyReferenceAlreadyHasValueException"

I have class called editvendor

public static void editvendor(Vendor vendor)
        {

            using (MyinvoiceDataDataContext connv = new MyinvoiceDataDataContext())
            {

                Vendor editven = (from s in connv.Vendors
                                 where s.VendorID == vendor.VendorID
                                 select s).FirstOrDefault();

                editven.VendorAddress = editven.VendorAddress;
                editven.VendorBalance = editven.VendorBalance;
                editven.VendorContactName = editven.VendorContactName;
                editven.VendorEmail = editven.VendorEmail;
                editven.VendorFax = editven.VendorFax;
                editven.VendorName = editven.VendorName;
                editven.VendorPaymentTerms = editven.VendorPaymentTerms;
                editven.VendorPhone = editven.VendorPhone;
                editven.VendorRemark = editven.VendorRemark;
                editven.VendorTax = editven.VendorTax;
                editven.VendorWebsite = editven.VendorWebsite;
                editven.CarrierID = editven.Carrier.CarrierID;
                connv.SubmitChanges();

            }
        }
        #endregion

In the windows form(edit vendor form) i use this:

 private void Window_Loaded(object sender, RoutedEventArgs e)
        {

            MyinvoiceDataDataContext contecta = new MyinvoiceDataDataContext();
            var tb = from s in contecta.Carriers
                     select new { s.CarrierID, s.CarrierName };
            comVendorCarrier.ItemsSource = tb;
            comVendorCarrier.DisplayMemberPath = "CarrierName";
            comVendorCarrier.SelectedValuePath = "CarrierID";

            ViewVendor vendview = new ViewVendor();
            txtVendorName.Text = vendor.VendorName.Trim();
            txtVendorBalance.Text = vendor.VendorBalance.Trim();
            txtVendorContactName.Text = vendor.VendorContactName.Trim();
            txtVendorPhone.Text = vendor.VendorPhone.Trim();
            txtVendorFax.Text = vendor.VendorTax.Trim();
            txtVendorEmail.Text = vendor.VendorEmail.Trim();
            txtVendorWebsite.Text = vendor.VendorWebsite.Trim();
            txtVendorRemarks.Text = vendor.VendorRemark.Trim();
            txtVendorAddress.Text = vendor.VendorAddress.Trim();
            txtVendorTax.Text = vendor.VendorTax.Trim();
            comVendorCarrier.Text = vendor.Carrier.CarrierName.Trim();
            ComVendorPaymentTerms.Text = vendor.VendorPaymentTerms.Trim();
        }

This code i use to save the edited vendor

 private void EditVendorSavebtn_Click_1(object sender, RoutedEventArgs e)
        {
            int SelectedCarrierId = int.Parse(comVendorCarrier.SelectedValue.ToString());
            if (txtVendorName.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's Name");

            if (txtVendorBalance.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's balance");

            if (txtVendorContactName.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's contact name");

            if (txtVendorPhone.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's phone number");

            if (txtVendorFax.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's fax number");

            if (txtVendorEmail.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's E-maill");

            if (txtVendorWebsite.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's Website");

            if (txtVendorAddress.Text.Trim() == "")
                MessageBox.Show("Please fill vendor's address");

            else
            {

                vendor.VendorName = txtVendorName.Text.Trim();
                vendor.VendorBalance = txtVendorBalance.Text.Trim();
                vendor.VendorContactName = txtVendorContactName.Text.Trim();
                vendor.VendorPhone = txtVendorPhone.Text.Trim();
                vendor.VendorFax = txtVendorFax.Text.Trim();
                vendor.VendorEmail = txtVendorEmail.Text.Trim();
                vendor.VendorWebsite = txtVendorWebsite.Text.Trim();
                vendor.VendorRemark = txtVendorRemarks.Text.Trim();
                vendor.VendorAddress = txtVendorAddress.Text.Trim();
                vendor.VendorPaymentTerms = ComVendorPaymentTerms.Text;
                vendor.CarrierID = SelectedCarrierId;
                vendor.VendorTax = txtVendorFax.Text.Trim();
                vendorinfo.editvendor(vendor);
                MessageBox.Show(vendor.VendorName + " succesfully edited.");
                Window_Loaded(null, null);
            }

回答1:

I expect it is this combination:

// somewhere
comVendorCarrier.Text = vendor.Carrier.CarrierName.Trim();
...
// somewhere else
vendor.CarrierID = SelectedCarrierId;

One is using the object-oriented approach, one is using the id-based approach - however, if both are loaded and are incompatible, problems. I wonder if you should use:

vendor.Carrier = null;
vendor.CarrierID = SelectedCarrierId;

then there is exactly one definition of which carrier to use.

Alternatively, handle the carrier separately, for example, instead of:

comVendorCarrier.Text = vendor.Carrier.CarrierName.Trim();

use:

var carrier  = context.Carriers.Single(x => x.Id = vendor.CarrierId);
comVendorCarrier.Text = carrier.CarrierName.Trim();

which then never loads vendor.Carrier as an object.

来源：https://stackoverflow.com/questions/9885568/updating-error-using-ling-to-sql