问为什么当我尝试将新对象添加到数据库时，LINQ to SQL会认为它是空的？

EN

/* other code removed */
        if (ModelState.IsValid)
        {
            var stream = new Genesis.Domain.Entities.Stream();

            // Handle stream
            // Is this stream new?
            if (form.StreamID == 0)
            {
                // create new stream
                stream.StreamUrl = form.StreamUrl;
                stream.StreamName = form.StreamName;
                stream.StreamBody = form.StreamBody;
                stream.StreamTitle = form.StreamTitle;
                stream.StreamKeywords = form.StreamKeywords;
                stream.StreamDescription = form.StreamDescription;

                form.StreamID = genesisRepository.CreateStream(stream); // CreateStream() returns ID as long
            }
/* other code removed */

public partial class SqlGenesisRepository : IGenesisRepository
{
    public long CreateStream(Stream stream)
    {
        streamTable.InsertOnSubmit(stream);
        streamTable.Context.SubmitChanges();
        return stream.StreamID;
    }
}

    Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 13:         public long CreateStream(Stream stream)
Line 14:         {
Line 15:             streamTable.InsertOnSubmit(stream);
Line 16:             streamTable.Context.SubmitChanges();
Line 17:             return stream.StreamID;


Source File: C:\path\to\SqlGenesisRepositoryStreamPartial.cs    Line: 15 

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   System.Data.Linq.Mapping.EntitySetDefSourceAccessor`2.GetValue(T instance) +18
   System.Data.Linq.Mapping.MetaAccessor`2.GetBoxedValue(Object instance) +47
   System.Data.Linq.StandardTrackedObject.HasDeferredLoader(MetaDataMember deferredMember) +106
   System.Data.Linq.StandardTrackedObject.get_HasDeferredLoaders() +107
   System.Data.Linq.StandardChangeTracker.Track(MetaType mt, Object obj, Dictionary`2 visited, Boolean recurse, Int32 level) +175
   System.Data.Linq.StandardChangeTracker.Track(Object obj, Boolean recurse) +83
   System.Data.Linq.StandardChangeTracker.Track(Object obj) +12
   System.Data.Linq.Table`1.InsertOnSubmit(TEntity entity) +183
   Genesis.Domain.Concrete.SqlGenesisRepository.CreateStream(Stream stream) in C:\Documents and Settings\bquakkelaar\Desktop\dropstuff\asp.net mvc\Genesis.0.02\Genesis.Domain\Concrete\SqlGenesisRepositoryStreamPartial.cs:15
   Genesis_0_02.Controllers.AdminStreamController.StreamEdit(StreamEditModel form) in C:\Documents and Settings\bquakkelaar\Desktop\dropstuff\asp.net mvc\Genesis.0.02\Genesis.0.02\Controllers\AdminStreamController.cs:107
   lambda_method(Closure , ControllerBase , Object[] ) +108
   System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +51
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +409
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +52
   System.Web.Mvc.<>c__DisplayClassd.<InvokeActionMethodWithFilters>b__a() +127

public partial class SqlGenesisRepository : IGenesisRepository
{
    private Table<Stream> streamTable;

    public SqlGenesisRepository(string connectionString)
    {
        streamTable = (new DataContext(connectionString)).GetTable<Stream>();
    }

    public IQueryable<Stream> Streams { get { return streamTable; } }
}

public class AdminStreamController : Controller
{
    private IGenesisRepository genesisRepository;

    public AdminStreamController()
    {
        //genesisRepository = new FakeGenesisRepository();
        genesisRepository = new SqlGenesisRepository(ConfigurationManager.ConnectionStrings["genesis"].ConnectionString);
    }
    /* rest of code removed for brevity */
}