为ASP.NET core Web API应用程序实现服务并获得持久错误。

1. 即使我只实现接口而不向方法添加任何额外代码，错误仍然会发生。
2. 我对所有签名进行了检查，两次检查，甚至三次检查，以确保没有错别字。
在查看其他SO线程后，
3. 尝试将GetUserInvite函数设置为IEnumerable。还是没有运气。

接口:

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using API.Data.Dtos.InviteDto;
using API.Domain;

namespace API.Services
{
    public interface IInviteService
    {
        Task<UserInvite> GetUserInvite(Guid sourceUserId, Guid invitedUserId);
        Task<IEnumerable<InviteDto>> GetUserInvites(string predicate, Guid userId);
        Task<User> GetUserWithInvites(Guid userId);
    }
}

服务:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using API.Data;
using API.Data.Dtos.InviteDto;
using API.Domain;
using Microsoft.EntityFrameworkCore;

namespace API.Services
{
    public class InviteService : IInviteService
    {
        private readonly DataContext _context;
        public InviteService(DataContext context)
        {
            _context = context;
        }

        public async Task<UserInvite> GetUserInvite(Guid sourceUserId, Guid invitedUserId)
        {
            return await _context.Invites.FindAsync(sourceUserId, invitedUserId);
        }

        public async Task<User> GetUserWithInvites(Guid userId)
        {
            return await _context.Users.Include(x => x.InvitedUsers).FirstOrDefaultAsync(x => x.Id == userId);
        }

        public async Task<IEnumerable<InviteDto>> GetUserInvites(string predicate, Guid userId)
        {
            var users = _context.Users.OrderBy(u => u.UserName).AsQueryable();
            var invites = _context.Invites.AsQueryable();

            if (predicate == "invited")
            {
                invites = invites.Where(invite => invite.SourceUserId == userId);
                users = invites.Select(invite => invite.InvitedUser);
            }

            if (predicate == "invitedBy")
            {
                invites = invites.Where(invite => invite.InvitedUserId == userId);
                users = invites.Select(invite => invite.SourceUser);
            }

            return await users.Select(user => new InviteDto
            {
                Username = user.UserName,
                City = user.City
            }).ToListAsync();
        }

    }
}