下面是使用方法语法的一个相当容易理解的版本:

IEnumerable<JoinPair> outerLeft =
    lefts.SelectMany(l => 
        rights.Where(r => l.Key == r.Key)
              .DefaultIfEmpty(new Item())
              .Select(r => new JoinPair { LeftId = l.Id, RightId = r.Id }));

下面是一个例子，如果你需要连接2个以上的表:

from d in context.dc_tpatient_bookingd
join bookingm in context.dc_tpatient_bookingm 
     on d.bookingid equals bookingm.bookingid into bookingmGroup
from m in bookingmGroup.DefaultIfEmpty()
join patient in dc_tpatient
     on m.prid equals patient.prid into patientGroup
from p in patientGroup.DefaultIfEmpty()

裁判:https://stackoverflow.com/a/17142392/2343

下面是一个版本的扩展方法解决方案，使用IQueryable代替IEnumerable

public class OuterJoinResult<TLeft, TRight>
{
    public TLeft LeftValue { get; set; }
    public TRight RightValue { get; set; }
}

public static IQueryable<TResult> LeftOuterJoin<TLeft, TRight, TKey, TResult>(this IQueryable<TLeft> left, IQueryable<TRight> right, Expression<Func<TLeft, TKey>> leftKey, Expression<Func<TRight, TKey>> rightKey, Expression<Func<OuterJoinResult<TLeft, TRight>, TResult>> result)
{
    return left.GroupJoin(right, leftKey, rightKey, (l, r) => new { l, r })
          .SelectMany(o => o.r.DefaultIfEmpty(), (l, r) => new OuterJoinResult<TLeft, TRight> { LeftValue = l.l, RightValue = r })
          .Select(result);
}

这是一种SQL语法，与用于内连接和左外连接的LINQ语法相比。 左外连接:

http://www.ozkary.com/2011/07/linq-to-entity-inner-and-left-joins.html

下面的示例在产品和类别之间进行组连接。这本质上是左连接。即使类别表为空，into表达式也会返回数据。要访问类别表的属性，现在必须通过在catList.DefaultIfEmpty()语句中添加from cl从可枚举结果中进行选择。

class Program
{
    List<Employee> listOfEmp = new List<Employee>();
    List<Department> listOfDepart = new List<Department>();

    public Program()
    {
        listOfDepart = new List<Department>(){
            new Department { Id = 1, DeptName = "DEV" },
            new Department { Id = 2, DeptName = "QA" },
            new Department { Id = 3, DeptName = "BUILD" },
            new Department { Id = 4, DeptName = "SIT" }
        };


        listOfEmp = new List<Employee>(){
            new Employee { Empid = 1, Name = "Manikandan",DepartmentId=1 },
            new Employee { Empid = 2, Name = "Manoj" ,DepartmentId=1},
            new Employee { Empid = 3, Name = "Yokesh" ,DepartmentId=0},
            new Employee { Empid = 3, Name = "Purusotham",DepartmentId=0}
        };

    }
    static void Main(string[] args)
    {
        Program ob = new Program();
        ob.LeftJoin();
        Console.ReadLine();
    }

    private void LeftJoin()
    {
        listOfEmp.GroupJoin(listOfDepart.DefaultIfEmpty(), x => x.DepartmentId, y => y.Id, (x, y) => new { EmpId = x.Empid, EmpName = x.Name, Dpt = y.FirstOrDefault() != null ? y.FirstOrDefault().DeptName : null }).ToList().ForEach
            (z =>
            {
                Console.WriteLine("Empid:{0} EmpName:{1} Dept:{2}", z.EmpId, z.EmpName, z.Dpt);
            });
    }
}

class Employee
{
    public int Empid { get; set; }
    public string Name { get; set; }
    public int DepartmentId { get; set; }
}

class Department
{
    public int Id { get; set; }
    public string DeptName { get; set; }
}

输出

下面是使用方法语法的一个相当容易理解的版本:

IEnumerable<JoinPair> outerLeft =
    lefts.SelectMany(l => 
        rights.Where(r => l.Key == r.Key)
              .DefaultIfEmpty(new Item())
              .Select(r => new JoinPair { LeftId = l.Id, RightId = r.Id }));