Checking Whether Class Implements Generic Interface

Question

I'm using the following generic function to determine whether a class implements a specified interface:

private static bool HasFieldType<TEntity, TInterface>()
{
    return typeof(TInterface).IsAssignableFrom(typeof(TEntity));
}

This works fine for the majority of the time.

However, I now have an interface which has a generic parameter:

public interface IStatusField<TEnum> where TEnum : System.Enum
{
    TEnum Status { get; set; }
}

And this causes the HasFieldType function to break with an unexpected use of unbound generic name error.

Ideally, I want to call the function like:

if (HasFieldType<TEntity, IStatusField<>>()) 
{
    // builder is an EntityTypeBuilder instance
    builder.Property("Status")
        .HasMaxLength(255)
        .HasConversion(new EnumToStringConverter<>());
}

But this won't work as I'm not specifying the generic type for both the IStatusField<> or the EnumToStringConverter<> .

Is there any way around this?

UPDATE

This code forms part of a generic base IEntityTypeConfiguration class as follows:

public abstract class EntityTypeConfiguration<TPrimaryKey, TEntity> : IEntityTypeConfiguration<TEntity> where TEntity : Entity<TPrimaryKey>
{
    public void Configure(EntityTypeBuilder<TEntity> builder)
    {
        builder.HasKey(e => e.Id);

        builder.Property(e => e.Id)
            .IsRequired()
            .HasMaxLength(13)
            .HasValueGenerator<PrimaryKeyValueGenerator>();

        // Apply the generic interface properties
        builder.ApplyInterfaceFields<TPrimaryKey, TEntity>();

        // Apply any additional configuration
        this.OnConfigure(builder);
    }

    protected abstract void OnConfigure(EntityTypeBuilder<TEntity> builder);
}

// In an extension class, I have
public static void ApplyInterfaceFields<TPrimaryKey, TEntity>(this EntityTypeBuilder<TEntity> builder) where TEntity : Entity<TPrimaryKey>
{
    // Check other implementations (removed for brevity)

    // IStatusField implementation
    if (HasFieldType<TEntity, IStatusField<>())
    {
        builder.Property("Status")
            .HasMaxLength(255)
            .HasConversion(new EnumToStringConverter<>());
    }

}

At the point of checking for IStatusField implementation, I know nothing about the generic type specified. I think this may be the bigger problem...

Answer 1

Ok, so I've managed to punch my way around the problem.

It needs a bit of tidying and some error checking but in the crudest form:

private static bool HasFieldType<TEntity>(Type interfaceType)
{
    var interfaces = typeof(TEntity).GetTypeInfo().ImplementedInterfaces;

    // Iterate through the interfaces
    foreach (var intf in interfaces)
    {
        // Compare interface names
        if (intf.FullName.Contains(interfaceType.FullName))
        {
            return intf.IsAssignableFrom(typeof(TEntity));
        }
    }

    return false;
}

Which now enables this to work:

// IStatusField implementation
if (HasFieldType<TEntity>(typeof(IStatusField<>)))
{
    builder.Property("Status")
        .HasMaxLength(255)
        .HasConversion<string>();
}

I can just use the built-in automatic string-to-enum conversions from EF to do the grunt work.

Answer 2

Instead of trying to wrestle with resolving generic type arguments from nothing, you might consider approaching it from the opposite direction, by getting a list of interfaces implemented by TEntity , filtering it to search for an IStatusField . Once you've located the field, you can get its' generic type arguments and pass those to your EnumToStringConverter :

var statusField = typeof(TEntity)
    .GetInterfaces()
    .FirstOrDefault(x => x.Name.StartsWith("IStatusField"));

Value given TEntity : IStatusField<ConsoleColor> :

statusField.GenericTypeArguments = [ typeof(System.Color) ]

From there though you're not done; you must still construct an instance of the generic type EnumToStringConverter<System.Color> . This is rather simple and outlined here .

Edit: I realized that because you'd be invoking a constructor, it's not quite the same. Here's how you'd accomplish this:

var statusField = typeof(TEntity)
            .GetInterfaces()
            .FirstOrDefault(x => x.Name.StartsWith("IStatusField"));

        if (statusField != null)
        {

            var enumType = statusField.GenericTypeArguments[0]; // get the IStatusField<T> value

            // get the default constructor after supplying generic type arg
            var converterType = typeof(EnumToStringConverter<>)
                .MakeGenericType(enumType)
                .GetConstructors()[0];

            // invoke the constructor. Note the null (optional) param
            dynamic converter = converterType.Invoke(new Object[1]);

                builder.Property("Status")
                    .HasMaxLength(255)
                    .HasConversion(converter);
        }