LINQ to SQL为什么在尝试将新对象添加到数据库时认为我的新对象为空？

Question

I am trying to add a record to my database table using LINQ to SQL and ASP.NET MVC 2.

The snippet in my controller that populates the LINQ object is this:

/* 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 */

The genesisRepository.CreateStream() looks like this:

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

When genesisRepository.CreateStream() gets executed, I get this error:

Updated to more accurate error and stacktrace

    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

When I put a breakpoint into the function, I see that stream is not null. Some strings are null. Required strings are not null (IE: streamName = "name") and StreamID is 0 .

Where am I going wrong?

Edit: How `streamTable` is instanced

I don't think there is a problem with how I instance streamTable but seeing as how I out of ideas and most people here think that it's null, here is the code that instances streamTable .

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; } }
}

And the SqlGenesisRepository is instanced in the controller class like this:

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 */
}

Answer 1

Thanks to everyone for the attention to this issue.

It looks like the incorrect code wasn't in any of the code I posted here. I rephrased my question and posted it here: Is there a secret to using LINQ to SQL to add records when the object has relationships? .

The solution can be found there!

Thanks again.

Answer 2

You don't need to attach the object to the table since the object is new. You just need to InsertOnSubmit.

If you get a null exception after removing that line, your Stream object is likely missing a required field.

Answer 3

Maybe streamTable is null?

Edit

Okay, so based on the stack trace I'm thinking that you may be missing a foreign key constraint. What relationships does a Stream have? Can you create a new Stream in your database using hand-coded SQL, given only the information that you have in this code?

Answer 4

Have you tried explicitly declaring Stream as Genesis.Domain.Entities.Stream ?

As in:

private Table<Genesis.Domain.Entities.Stream> streamTable;


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

Because "Stream" alone could be confusing the compiler, as System.IO.Stream

Also, when you put a breakpoint at

streamTable.InsertOnSubmit(stream);

...to check if streamTable is null, did you check its contents with the debugger? That part where it says "Expanding this item will enumerate ...". It's important because generally it lazy loads, therefore it doesn't go to the DB unless it requires a transaction.