Request: Support Custom Key Types (that can be converted to int)

[ardalis]

I'd like to be able to use a custom key type that can be assigned freely to/from an int (or another allowable type) as my entity ID type. Currently I get the following exception when I try to add a migration.

Exception message:
The property 'Cart.Id' could not be mapped, because it is of type 'CartKey' which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
Stack trace:
System.InvalidOperationException: The property 'Cart.Id' could not be mapped, because it is of type 'CartKey' which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.PropertyMappingValidationConvention.Apply(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.ImmediateConventionScope.OnModelBuilt(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Conventions.Internal.ConventionDispatcher.OnModelBuilt(InternalModelBuilder modelBuilder)
   at Microsoft.EntityFrameworkCore.Metadata.Internal.Model.Validate()
   at Microsoft.EntityFrameworkCore.Metadata.Internal.InternalModelBuilder.Validate()
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.<>c__DisplayClass5_0.<GetModel>b__0(Object k)
   at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder, IModelValidator validator)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__7_1(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, ServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(IServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.<>c__DisplayClass22_0.<RealizeService>b__0(ServiceProvider provider)
   at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType)

Steps to reproduce

Create an entity with a custom typed ID property. Try to add a migration.

// Entity
    public class Cart
    {
        public CartKey Id { get; set; }
        //public Customer Customer { get; set; }
        //public List<CartItem> Items { get; set; }
    }

// Key Type
    [ImmutableObject(true)]
    public struct CartKey : IComparable<CartKey>
    {
        public CartKey(int value) { Value = value; }

        [Pure] public int Value { get; }
        [Pure] public bool Equals(CartKey other) => Value == other.Value;

        [Pure]
        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null, obj)) return false;
            if (Equals(this, obj)) return true;
            return obj.GetType() == GetType() && Equals((CartKey)obj);
        }

        [Pure]
        public override int GetHashCode()
        {
            unchecked
            {
                return (base.GetHashCode() * 397) ^ Value;
            }
        }

        [Pure] public static bool operator ==(CartKey left, CartKey right) => Equals(left, right);
        [Pure] public static bool operator !=(CartKey left, CartKey right) => !Equals(left, right);
        [Pure] public int CompareTo(CartKey other) => Equals(this, other) ? 0 : Value.CompareTo(other.Value);
        [Pure] public static bool operator <(CartKey left, CartKey right) => Comparer<CartKey>.Default.Compare(left, right) < 0;
        [Pure] public static bool operator >(CartKey left, CartKey right) => Comparer<CartKey>.Default.Compare(left, right) > 0;
        [Pure] public static bool operator <=(CartKey left, CartKey right) => Comparer<CartKey>.Default.Compare(left, right) <= 0;
        [Pure] public static bool operator >=(CartKey left, CartKey right) => Comparer<CartKey>.Default.Compare(left, right) >= 0;
        [Pure] public override string ToString() => $"{nameof(Value)}: {Value}";
        [Pure] public static implicit operator int(CartKey value) { return value.Value;  }
    }

Further technical details

EF Core version: 2.1
Database Provider: (e.g. Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10
IDE: Visual Studio 2017 15.5.6

[ardalis]

Related to issue #10994 which demonstrates how I tried to work around this issue by using an int backing field (but this, too, didn't work).

[ajcvickers]

Also duplicate of #242.