扩展方法，类似于使用join语法的左连接

public static class LinQExtensions
{
    public static IEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
        this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, 
        Func<TOuter, TKey> outerKeySelector, 
        Func<TInner, TKey> innerKeySelector, 
        Func<TOuter, TInner, TResult> resultSelector)
    {
        return outer.GroupJoin(
            inner, 
            outerKeySelector, 
            innerKeySelector,
            (outerElement, innerElements) => resultSelector(outerElement, innerElements.FirstOrDefault()));
    }
}

我刚刚在。net内核中编写了它，它看起来像预期的那样工作。

小测试:

        var Ids = new List<int> { 1, 2, 3, 4};
        var items = new List<Tuple<int, string>>
        {
            new Tuple<int, string>(1,"a"),
            new Tuple<int, string>(2,"b"),
            new Tuple<int, string>(4,"d"),
            new Tuple<int, string>(5,"e"),
        };

        var result = Ids.LeftJoin(
            items,
            id => id,
            item => item.Item1,
            (id, item) => item ?? new Tuple<int, string>(id, "not found"));

        result.ToList()
        Count = 4
        [0]: {(1, a)}
        [1]: {(2, b)}
        [2]: {(3, not found)}
        [3]: {(4, d)}

看一下这个例子。 这个查询应该工作:

var leftFinal = from left in lefts
                join right in rights on left equals right.Left into leftRights
                from leftRight in leftRights.DefaultIfEmpty()
                select new { LeftId = left.Id, RightId = left.Key==leftRight.Key ? leftRight.Id : 0 };

扩展方法，类似于使用join语法的左连接

public static class LinQExtensions
{
    public static IEnumerable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
        this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, 
        Func<TOuter, TKey> outerKeySelector, 
        Func<TInner, TKey> innerKeySelector, 
        Func<TOuter, TInner, TResult> resultSelector)
    {
        return outer.GroupJoin(
            inner, 
            outerKeySelector, 
            innerKeySelector,
            (outerElement, innerElements) => resultSelector(outerElement, innerElements.FirstOrDefault()));
    }
}

我刚刚在。net内核中编写了它，它看起来像预期的那样工作。

小测试:

        var Ids = new List<int> { 1, 2, 3, 4};
        var items = new List<Tuple<int, string>>
        {
            new Tuple<int, string>(1,"a"),
            new Tuple<int, string>(2,"b"),
            new Tuple<int, string>(4,"d"),
            new Tuple<int, string>(5,"e"),
        };

        var result = Ids.LeftJoin(
            items,
            id => id,
            item => item.Item1,
            (id, item) => item ?? new Tuple<int, string>(id, "not found"));

        result.ToList()
        Count = 4
        [0]: {(1, a)}
        [1]: {(2, b)}
        [2]: {(3, not found)}
        [3]: {(4, d)}

如“Perform left outer joins”中所述:

var q =
    from c in categories
    join pt in products on c.Category equals pt.Category into ps_jointable
    from p in ps_jointable.DefaultIfEmpty()
    select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };

我想补充的是，如果您获得MoreLinq扩展，现在支持同质和异构左连接

http://morelinq.github.io/2.8/ref/api/html/Overload_MoreLinq_MoreEnumerable_LeftJoin.htm

例子:

//Pretend a ClientCompany object and an Employee object both have a ClientCompanyID key on them

return DataContext.ClientCompany
    .LeftJoin(DataContext.Employees,                         //Table being joined
        company => company.ClientCompanyID,                  //First key
        employee => employee.ClientCompanyID,                //Second Key
        company => new {company, employee = (Employee)null}, //Result selector when there isn't a match
        (company, employee) => new { company, employee });   //Result selector when there is a match

编辑:

回想起来，这可能会起作用，但它将IQueryable转换为IEnumerable，因为morelinq不会将查询转换为SQL。

您可以使用如下所述的GroupJoin: https://stackoverflow.com/a/24273804/4251433

这将确保它仍然是一个IQueryable，以防您以后需要对它进行进一步的逻辑操作。

通过扩展方法的左外连接的实现如下所示

public static IEnumerable<Result> LeftJoin<TOuter, TInner, TKey, Result>(
  this IEnumerable<TOuter> outer, IEnumerable<TInner> inner
  , Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
  , Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
  {
    if (outer == null)
      throw new ArgumentException("outer");

    if (inner == null)
      throw new ArgumentException("inner");

    if (outerKeySelector == null)
      throw new ArgumentException("outerKeySelector");

    if (innerKeySelector == null)
      throw new ArgumentException("innerKeySelector");

    if (resultSelector == null)
      throw new ArgumentException("resultSelector");

    return LeftJoinImpl(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer ?? EqualityComparer<TKey>.Default);
  }

  static IEnumerable<Result> LeftJoinImpl<TOuter, TInner, TKey, Result>(
      IEnumerable<TOuter> outer, IEnumerable<TInner> inner
      , Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector
      , Func<TOuter, TInner, Result> resultSelector, IEqualityComparer<TKey> comparer)
  {
    var innerLookup = inner.ToLookup(innerKeySelector, comparer);

    foreach (var outerElment in outer)
    {
      var outerKey = outerKeySelector(outerElment);
      var innerElements = innerLookup[outerKey];

      if (innerElements.Any())
        foreach (var innerElement in innerElements)
          yield return resultSelector(outerElment, innerElement);
      else
        yield return resultSelector(outerElment, default(TInner));
     }
   }

然后，resultselector必须处理空元素。外汇。

   static void Main(string[] args)
   {
     var inner = new[] { Tuple.Create(1, "1"), Tuple.Create(2, "2"), Tuple.Create(3, "3") };
     var outer = new[] { Tuple.Create(1, "11"), Tuple.Create(2, "22") };

     var res = outer.LeftJoin(inner, item => item.Item1, item => item.Item1, (it1, it2) =>
     new { Key = it1.Item1, V1 = it1.Item2, V2 = it2 != null ? it2.Item2 : default(string) });

     foreach (var item in res)
       Console.WriteLine(string.Format("{0}, {1}, {2}", item.Key, item.V1, item.V2));
   }