该类型或方法具有3个通用参数，但提供了2个通用参数

Question

I have the following working code:

Context context = new Context(_options);

Expression<Func<Context.Person, Context.Address>> e1 = x => x.Address;
Expression<Func<Context.Address, Context.Country>> e2 = x => x.Country;

IIncludableQueryable<Context.Person, Context.Address> a = context.Persons.Include(e1);
IIncludableQueryable<Context.Person, Context.Country> b = a.ThenInclude(e2);

List<Context.Person> result = context.Persons.Include(e1).ThenInclude(e2).ToList();

Where Include and ThenInclude are Entity Framework Core extension methods.

In my code I will need to use generic types so I am using reflection:

IQueryable<Context.Person> persons = context.Persons;

MethodInfo include = typeof(EntityFrameworkQueryableExtensions).GetMethods().First(x => x.Name == "Include" && x.GetParameters().Select(y => y.ParameterType.GetGenericTypeDefinition()).SequenceEqual(new[] { typeof(IQueryable<>), typeof(Expression<>) }));

MethodInfo thenInclude = typeof(EntityFrameworkQueryableExtensions).GetMethods().First(x => x.Name == "ThenInclude" && x.GetParameters().Select(y => y.ParameterType.GetGenericTypeDefinition()).SequenceEqual(new[] { typeof(IIncludableQueryable<,>), typeof(Expression<>) }));

Expression<Func<Context.Person, Context.Address>> l1 = x => x.Address;

Expression<Func<Context.Address, Context.Country>> l2 = x => x.Country;


try {

  MethodInfo includeInfo = include.MakeGenericMethod(typeof(Context.Person), l1.ReturnType);

  IIncludableQueryable<Context.Person, Context.Address> r1 = (IIncludableQueryable<Context.Person, Context.Address>)includeInfo.Invoke(null, new Object[] { persons, l1 });

  MethodInfo thenIncludeInfo = thenInclude.MakeGenericMethod(typeof(Context.Address), l2.ReturnType);

  IIncludableQueryable<Context.Address, Context.Country> r2 = (IIncludableQueryable<Context.Address, Context.Country>)thenIncludeInfo.Invoke(null, new Object[] { r1, l2 });

  var r = r2.AsQueryable();

} catch (Exception ex) { }

But on this code line:

MethodInfo thenIncludeInfo = thenInclude.MakeGenericMethod(typeof(Context.Address), l2.ReturnType);

I get the following error:

The type or method has 3 generic parameter(s), but 2 generic argument(s) were provided. A generic argument must be provided for each generic parameter.

I can understand the error by looking at ThenInclude definition but I am not sure how to solve it ...

Answer 1

As the message suggests, ThenInclude expects 3 type parameters: TEntity, TPreviousProperty, TProperty . From your code, it would seem this would work:

MethodInfo thenIncludeInfo = thenInclude.MakeGenericMethod(
    typeof(Context.Person), typeof(Context.Address), l2.ReturnType);