无法通过连接编译c#linq查询

Question

下面是一些在执行某些linq连接时无法编译的c#代码的示例.有谁知道为什么这不编译？

错误是

无法从查询中推断出类型参数

(在我的真实代码中Fetch()返回一个IQueryable<T>)

using System.Collections.Generic;
using System.Linq;

namespace LinqJoin
{
    public class DataRepository<T>
    {
        public IList<T> Fetch()
        {
            return new List<T>();
        }
    }

    internal class SSOUser
    {
        public int Id { get; set; }
    }

    internal class UserRole
    {
        public int SSOUserId { get; set; }
        public int RoleId { get; set; }
    }

    internal class Role
    {
        public int RoleId { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var users = new DataRepository<SSOUser>().Fetch();
            var userroles = new DataRepository<UserRole>().Fetch();
            var roles = new DataRepository<Role>().Fetch();

            var result = from u in users
                   join ur in userroles on u.Id equals ur.SSOUserId
                   join r in roles on r.RoleId equals ur.RoleId
                   select u;

        //var x1 = users.Join(userroles, u => u.Id, ur => ur.SSOUserId, (u, ur) => new { User = u, UserRole = ur}).Join(roles, x => x.UserRole.RoleId, r => r.RoleId, res => res.User);
        }
    }
}

Answer 1

这种联接是错误的方式:

join r in roles on r.RoleId equals ur.RoleId

它应该是:

join r in roles on ur.RoleId equals r.RoleId

你介绍的范围变量总是要对右边的右手边equals.通常情况下,编译器很高兴告诉您在错误消息中,请注意......