Automapper + EF4 + ASP.NET MVC - getting 'context disposed' error (I know why, but how to fix it?)

问题

I have this really basic code in a MVC controller action. It maps an Operation model class to a very basic OperationVM view-model class .

public class OperationVM: Operation 
{
    public CategoryVM CategoryVM { get; set; }
}

I need to load the complete list of categories in order to create a CategoryVM instance.
Here's how I (try to) create a List<OperationVM> to show in the view.

public class OperationsController : Controller {

    private SomeContext context = new SomeContext ();

    public ViewResult Index()
    {
        var ops = context.Operations.Include("blah...").ToList();
        Mapper.CreateMap<Operation, OperationVM>()
            .ForMember(
                dest => dest.CategoryVM, 
                opt => opt.MapFrom(
                    src => CreateCatVM(src.Category, context.Categories)
                    //  trouble here ----------------^^^^^^^
                )
            );
        var opVMs = ops.Select(op => Mapper.Map<Operation, OperationVM>(op))
                       .ToList();

        return View(opVMs);
    }
}

All works great first time I hit the page. The problem is, the mapper object is static. So when calling Mapper.CreateMap(), the instance of the current DbContext is saved in the closure given to CreateMap().

The 2nd time I hit the page, the static map is already in place, still using the reference to the initial, now disposed, DbContext.

The exact error is:

The operation cannot be completed because the DbContext has been disposed.

The question is: How can I make AutoMapper always use the current context instead of the initial one?

Is there a way to use an "instance" of automapper instead of the static Mapper class? If this is possible, is it recommended to re-create the mapping every time? I'm worried about reflection slow-downs.

I read a bit about custom resolvers, but I get a similar problem - How do I get the custom resolver to use the current context?

回答1:

It is possible, but the setup is a bit complicated. I use this in my projects with help of Ninject for dependency injection.

AutoMapper has concept of TypeConverters. Converters provide a way to implement complex operations required to convert certain types in a separate class. If converting Category to CategoryVM requires a database lookup you can implement that logic in custom TypeConverter class similar to this:

using System;
using AutoMapper;

public class CategoryToCategoryVMConverter : 
        TypeConverter<Category, CategoryVM>
{
    public CategoryToCategoryVMConverter(DbContext context)
    {
        this.Context = context;
    }

    private DbContext Context { get; set; }

    protected override CategoryVM ConvertCore(Category source)
    {
        // use this.Context to lookup whatever you need
        return CreateCatVM(source, this.Context.Categories);
    }
}

You then to configure AutoMapper to use your converter:

Mapper.CreateMap<Category, CategoryVM>().ConvertUsing<CategoryToCategoryVMConverter>();

Here comes the tricky part. AutoMapper will need to create a new instance of our converter every time you map values, and it will need to provide DbContext instance for constructor. In my projects I use Ninject for dependency injection, and it is configured to use the same instance of DbContext while processing a request. This way the same instance of DbContext is injected both in your controller and in your AutoMapper converter. The trivial Ninject configuration would look like this:

Bind<DbContext>().To<SomeContext>().InRequestScope();

You can of course use some sort of factory pattern to get instance of DbContext instead of injecting it in constructors.

Let me know if you have any questions.

回答2:

I've found a workaround that's not completely hacky. Basically, I tell AutoMapper to ignore the tricky field and I update it myself.

The updated controller looks like this:

public class OperationsController : Controller {

    private SomeContext context = new SomeContext ();

    public ViewResult Index()
    {
        var ops = context.Operations.Include("blah...").ToList();
        Mapper.CreateMap<Operation, OperationVM>()
            .ForMember(dest => dest.CategoryVM, opt => opt.Ignore());

        var opVMs = ops.Select(
            op => {
                var opVM = Mapper.Map<Operation, OperationVM>(op);
                opVM.CategoryVM = CreateCatVM(op.Category, context.Categories);
                return opVM;
            })
            .ToList();

        return View(opVMs);
    }
}

Still curious how this could be done from within AutoMapper...

回答3:

The answer from @LeffeBrune is perfect. However, I want to have the same behavior, but I don't want to map every property myself. Basically I just wanted to override the "ConstructUsing".

Here is what I came up with.

public static class AutoMapperExtension
{
    public static void ConstructUsingService<TSource, TDestination>(this IMappingExpression<TSource, TDestination> mappingExression, Type typeConverterType)
    {
        mappingExression.ConstructUsing((ResolutionContext ctx) =>
        {
            var constructor = (IConstructorWithService<TSource, TDestination>)ctx.Options.ServiceCtor.Invoke(typeConverterType);
            return constructor.Construct((TSource)ctx.SourceValue);
        });
    }
}

public class CategoryToCategoryVMConstructor : IConstructorWithService<Category, CategoryVM>
{
    private DbContext dbContext;

    public DTOSiteToHBTISiteConverter(DbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public CategoryVM Construct(Category category)
    {
        // Some commands here
        if (category.Id > 0)
        {
            var vmCategory = dbContext.Categories.FirstOrDefault(m => m.Id == category.Id);
            if (vmCategory == null)
            {
                throw new NotAllowedException();
            }

            return vmCategory;
        }

        return new CategoryVM();
    }
}

// Initialization
Mapper.Initialize(cfg =>
{
    cfg.ConstructServicesUsing(type => nInjectKernelForInstance.Get(type));
    cfg.CreateMap<Category, CategoryVM>().ConstructUsingService(typeof(CategoryToCategoryVMConstructor));
};

来源：https://stackoverflow.com/questions/11751313/automapper-ef4-asp-net-mvc-getting-context-disposed-error-i-know-why-b