Efcore: Map enum to string set default enum value via HasDefaultValueSql

[wangpengxpy]

First，mapping enum to tinyint is fine，code show as below
``` C#
public class Blog
{
public int Id { get; set; }
public string Name { get; set; }
public Color Color { get; set; }
public List Posts { get; set; }
}

public enum Color : byte
{
    Black = 1,
    Red,
    Blue
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
     modelBuilder.Entity<Blog>(b => 
     {
          b.Property(p => p.Color).HasDefaultValueSql("1");
     });
}

but map to string can not work，code show as below
``` C#
    public class Blog
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public Color Color { get; set; }
        public List<Post> Posts { get; set; }
    }

    public enum Color
    {
        Black = 1,
        Red,
        Blue
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
         modelBuilder.Entity<Blog>(b => 
         {
              b.Property(p => p.Color).HasColumnType("VARCHAR(20)").HasDefaultValueSql("Black");
        });
    }

The follow exception：

Microsoft.Data.SqlClient.SqlException: "The name" Black "is not allowed in this context. Valid expressions include constants, constant expressions, and variables (in some contexts). Column names are not allowed.

More details：

EF Core version: 3.1.1
Database provider: (Microsoft.EntityFrameworkCore.SqlServer)
Target framework: (.NET Core 3.1)
Operating system: Windows 10
IDE: (Visual Studio 2019 16.4.4)

[ajcvickers]

@wangpengxpy HasDefaultValueSql requires the raw SQL literal expression. Try .HasDefaultValueSql("'Black'"), or use .HasDefaultValue("Black") which will create the SQL literal from the given string.

[wangpengxpy]

@ajcvickers thanks your reply
use HasDefaultValueSql("'Black'") works，but try HasDefaultValue("Black") before and will throw an exception：System.InvalidOperationException:“Cannot set default value 'Black' of type 'System.String' on property 'Color' of type 'Model.Color' in entity type 'Blog'.” ,so I think using HasDefaultValue support is the best way in this case.Does it need improvement?

[ajcvickers]

@wangpengxpy Thanks for the additional info.

Team: I can reproduce the issue with HasDefaultValue("Black"). Stack trace:

"C:\Program Files\dotnet\dotnet.exe" C:/Stuff/MyRepros/AllTogetherNow/Local/bin/Debug/netcoreapp3.1/Local.dll
Unhandled exception. System.InvalidOperationException: Cannot set default value 'Black' of type 'System.String' on property 'Color' of type 'Color' in entity type 'Blog'.
   at Microsoft.EntityFrameworkCore.RelationalPropertyExtensions.ConvertDefaultValue(IProperty property, Object value)
   at Microsoft.EntityFrameworkCore.RelationalPropertyExtensions.SetDefaultValue(IMutableProperty property, Object value)
   at Microsoft.EntityFrameworkCore.RelationalPropertyBuilderExtensions.HasDefaultValue(PropertyBuilder propertyBuilder, Object value)
   at Microsoft.EntityFrameworkCore.RelationalPropertyBuilderExtensions.HasDefaultValue[TProperty](PropertyBuilder`1 propertyBuilder, Object value)
   at SomeDbContext.<>c.<OnModelCreating>b__0_0(EntityTypeBuilder`1 b) in C:\Stuff\MyRepros\AllTogetherNow\Local\Local.cs:line 49
   at Microsoft.EntityFrameworkCore.ModelBuilder.Entity[TEntity](Action`1 buildAction)
   at SomeDbContext.OnModelCreating(ModelBuilder modelBuilder) in C:\Stuff\MyRepros\AllTogetherNow\Local\Local.cs:line 47
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelCustomizer.Customize(ModelBuilder modelBuilder, DbContext context)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.CreateModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Infrastructure.ModelSource.GetModel(DbContext context, IConventionSetBuilder conventionSetBuilder)
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.CreateModel()
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.get_Model()
   at Microsoft.EntityFrameworkCore.Infrastructure.EntityFrameworkServicesBuilder.<>c.<TryAddCoreServices>b__7_3(IServiceProvider p)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.DbContext.get_DbContextDependencies()
   at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.DbContext.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.InfrastructureExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Infrastructure.AccessorExtensions.GetService[TService](IInfrastructure`1 accessor)
   at Microsoft.EntityFrameworkCore.Infrastructure.DatabaseFacade.get_Dependencies()
   at Microsoft.EntityFrameworkCore.Infrastructure.DatabaseFacade.EnsureDeleted()
   at ThreeOne.Main() in C:\Stuff\MyRepros\AllTogetherNow\Local\Local.cs:line 22

[wangpengxpy]

@ajcvickers，thanks for reopening，I hope this problem can be improved

[ajcvickers]

@wangpengxpy The team pointed out to me that my suggestion to use .HasDefaultValue("Black") was incorrect. Because the property is an enum in the model, the enum value should be used here. For example, .HasDefaultValue(Color.Black). EF then does the conversion to string based on the mapping.