HasKey Throwing InvalidOperationException -- Is this a bug in Entity Framework Code First?

Question

Okay, I've been staring at the screen for a couple of hours here and have no idea why I am getting this error. I've used Code First on a number of other projects and have had no problem with this before...

Here is the error:

System.InvalidOperationException was unhandled by user code
  Message=The properties expression 'sci => sci.ShoppingCartItemId' is not valid. The expression should represent a property: C#: 't => t.MyProperty'  VB.Net: 'Function(t) t.MyProperty'. When specifying multiple properties use an anonymous type: C#: 't => new { t.MyProperty1, t.MyProperty2 }'  VB.Net: 'Function(t) New From { t.MyProperty1, t.MyProperty2 }'.
  Source=EntityFramework
  StackTrace:
       at System.Data.Entity.ModelConfiguration.Utilities.ExpressionExtensions.GetSimplePropertyAccessList(LambdaExpression propertyAccessExpression)
       at System.Data.Entity.ModelConfiguration.EntityTypeConfiguration`1.HasKey[TKey](Expression`1 keyExpression)
       at BillingPlatform.DataLayer.BillingDb.OnModelCreating(DbModelBuilder modelBuilder) in [somepath]\BillingDb.cs:line 57
       at System.Data.Entity.Internal.LazyInternalContext.CreateModelBuilder()
       at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
       at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
  InnerException: 

Here is the code that's throwing the error. The first line:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{                       
   modelBuilder.Entity<ShoppingCartItem>().HasKey(sci => sci.ShoppingCartItemId);
   modelBuilder.Entity<Product>().HasKey<Guid>(p => p.ProductId);
   modelBuilder.Entity<DependentItemType>().HasKey<Guid>(dit => dit.DependentItemTypeId);
   modelBuilder.Entity<ProductCategory>().HasKey<Guid>(pc => pc.ProductCategoryId);

   base.OnModelCreating(modelBuilder);
}

Here is just the ShoppingCartItem class as a reference:

namespace BillingPlatform.Libraries
{
    public class ShoppingCartItem
    {
        /// <summary>
        /// The unique identifier of this shopping cart item.
        /// </summary>        
        public Guid ShoppingCartItemId { get; set; }

        public Product Product { get; set; }

        public decimal Price { get; set; }

        public decimal Tax { get; set; }

        public Guid UserId { get; set; }

        public bool InCart { get; set; }

        public string ProductData { get; set; }

        public DependentItemType DependentItemType { get; set; }

        public string DependentItemId { get; set; }
    }
}

Does anyone understand why Entity Framework would throw this error? My lambda expression:

modelBuilder.Entity<ShoppingCartItem>().HasKey(s => s.ShoppingCartItemId);

is super simple. I don't see what could be going wrong... Thank you for any help you can give!

Answer 1

Okay the problem was that my class members were originally just fields. Code First expects properties. After making the code change and rebuilding, I was still getting the same error. But once Visual Studio was forced to push the updated DLLs, everything worked fine.

Answer 2

Not the case here (landed here from a Google search), but might be worth posting that this exact exception can also occur if HasKey uses an anonymous type that defines its own field names:

HasKey(t => new { KeyField1 = t.KeyField1, KeyField2 = t.KeyField2 });

Should be fixed to look like this:

HasKey(t => new { t.KeyField1, t.KeyField2 });