谢谢大家的有趣帖子!

我修改了代码，因为在我的情况下我需要

个性化的连接谓词 一个个性化的联合，独特的比较

这是我修改过的代码(不好意思，用VB写的)

    Module MyExtensions
        <Extension()>
        Friend Function FullOuterJoin(Of TA, TB, TResult)(ByVal a As IEnumerable(Of TA), ByVal b As IEnumerable(Of TB), ByVal joinPredicate As Func(Of TA, TB, Boolean), ByVal projection As Func(Of TA, TB, TResult), ByVal comparer As IEqualityComparer(Of TResult)) As IEnumerable(Of TResult)
            Dim joinL =
                From xa In a
                From xb In b.Where(Function(x) joinPredicate(xa, x)).DefaultIfEmpty()
                Select projection(xa, xb)
            Dim joinR =
                From xb In b
                From xa In a.Where(Function(x) joinPredicate(x, xb)).DefaultIfEmpty()
                Select projection(xa, xb)
            Return joinL.Union(joinR, comparer)
        End Function
    End Module

    Dim fullOuterJoin = lefts.FullOuterJoin(
        rights,
        Function(left, right) left.Code = right.Code And (left.Amount [...] Or left.Description.Contains [...]),
        Function(left, right) New CompareResult(left, right),
        New MyEqualityComparer
    )

    Public Class MyEqualityComparer
        Implements IEqualityComparer(Of CompareResult)

        Private Function GetMsg(obj As CompareResult) As String
            Dim msg As String = ""
            msg &= obj.Code & "_"
            [...]
            Return msg
        End Function

        Public Overloads Function Equals(x As CompareResult, y As CompareResult) As Boolean Implements IEqualityComparer(Of CompareResult).Equals
            Return Me.GetMsg(x) = Me.GetMsg(y)
        End Function

        Public Overloads Function GetHashCode(obj As CompareResult) As Integer Implements IEqualityComparer(Of CompareResult).GetHashCode
            Return Me.GetMsg(obj).GetHashCode
        End Function
    End Class

我不知道这是否适用于所有情况，但从逻辑上讲，这似乎是正确的。其思想是取一个左外连接和一个右外连接，然后取结果的并集。

var firstNames = new[]
{
    new { ID = 1, Name = "John" },
    new { ID = 2, Name = "Sue" },
};
var lastNames = new[]
{
    new { ID = 1, Name = "Doe" },
    new { ID = 3, Name = "Smith" },
};
var leftOuterJoin =
    from first in firstNames
    join last in lastNames on first.ID equals last.ID into temp
    from last in temp.DefaultIfEmpty()
    select new
    {
        first.ID,
        FirstName = first.Name,
        LastName = last?.Name,
    };
var rightOuterJoin =
    from last in lastNames
    join first in firstNames on last.ID equals first.ID into temp
    from first in temp.DefaultIfEmpty()
    select new
    {
        last.ID,
        FirstName = first?.Name,
        LastName = last.Name,
    };
var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin);

这就像写的一样，因为它是在LINQ to Objects中。如果LINQ to SQL或其他，查询处理器可能不支持安全导航或其他操作。你必须使用条件操作符有条件地获取值。

也就是说,

var leftOuterJoin =
    from first in firstNames
    join last in lastNames on first.ID equals last.ID into temp
    from last in temp.DefaultIfEmpty()
    select new
    {
        first.ID,
        FirstName = first.Name,
        LastName = last != null ? last.Name : default,
    };

下面是一个扩展方法:

public static IEnumerable<KeyValuePair<TLeft, TRight>> FullOuterJoin<TLeft, TRight>(this IEnumerable<TLeft> leftItems, Func<TLeft, object> leftIdSelector, IEnumerable<TRight> rightItems, Func<TRight, object> rightIdSelector)
{
    var leftOuterJoin = from left in leftItems
        join right in rightItems on leftIdSelector(left) equals rightIdSelector(right) into temp
        from right in temp.DefaultIfEmpty()
        select new { left, right };

    var rightOuterJoin = from right in rightItems
        join left in leftItems on rightIdSelector(right) equals leftIdSelector(left) into temp
        from left in temp.DefaultIfEmpty()
        select new { left, right };

    var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin);

    return fullOuterJoin.Select(x => new KeyValuePair<TLeft, TRight>(x.left, x.right));
}

在两个输入上执行内存流枚举，并为每一行调用选择器。如果在当前迭代中没有相关性，则选择器参数之一将为空。

例子:

   var result = left.FullOuterJoin(
         right, 
         x=>left.Key, 
         x=>right.Key, 
         (l,r) => new { LeftKey = l?.Key, RightKey=r?.Key });

Requires an IComparer for the correlation type, uses the Comparer.Default if not provided. Requires that 'OrderBy' is applied to the input enumerables /// <summary> /// Performs a full outer join on two <see cref="IEnumerable{T}" />. /// </summary> /// <typeparam name="TLeft"></typeparam> /// <typeparam name="TValue"></typeparam> /// <typeparam name="TRight"></typeparam> /// <typeparam name="TResult"></typeparam> /// <param name="left"></param> /// <param name="right"></param> /// <param name="leftKeySelector"></param> /// <param name="rightKeySelector"></param> /// <param name="selector">Expression defining result type</param> /// <param name="keyComparer">A comparer if there is no default for the type</param> /// <returns></returns> [System.Diagnostics.DebuggerStepThrough] public static IEnumerable<TResult> FullOuterJoin<TLeft, TRight, TValue, TResult>( this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TValue> leftKeySelector, Func<TRight, TValue> rightKeySelector, Func<TLeft, TRight, TResult> selector, IComparer<TValue> keyComparer = null) where TLeft: class where TRight: class where TValue : IComparable { keyComparer = keyComparer ?? Comparer<TValue>.Default; using (var enumLeft = left.OrderBy(leftKeySelector).GetEnumerator()) using (var enumRight = right.OrderBy(rightKeySelector).GetEnumerator()) { var hasLeft = enumLeft.MoveNext(); var hasRight = enumRight.MoveNext(); while (hasLeft || hasRight) { var currentLeft = enumLeft.Current; var valueLeft = hasLeft ? leftKeySelector(currentLeft) : default(TValue); var currentRight = enumRight.Current; var valueRight = hasRight ? rightKeySelector(currentRight) : default(TValue); int compare = !hasLeft ? 1 : !hasRight ? -1 : keyComparer.Compare(valueLeft, valueRight); switch (compare) { case 0: // The selector matches. An inner join is achieved yield return selector(currentLeft, currentRight); hasLeft = enumLeft.MoveNext(); hasRight = enumRight.MoveNext(); break; case -1: yield return selector(currentLeft, default(TRight)); hasLeft = enumLeft.MoveNext(); break; case 1: yield return selector(default(TLeft), currentRight); hasRight = enumRight.MoveNext(); break; } } } }

我猜@sehe的方法更强大，但在我更好地理解它之前，我发现自己跳过了@MichaelSander的扩展。我修改了它，以匹配这里描述的内置Enumerable.Join()方法的语法和返回类型。我在@JeffMercado的解决方案下为@cadrell0的注释添加了“distinct”后缀。

public static class MyExtensions {

    public static IEnumerable<TResult> FullJoinDistinct<TLeft, TRight, TKey, TResult> (
        this IEnumerable<TLeft> leftItems, 
        IEnumerable<TRight> rightItems, 
        Func<TLeft, TKey> leftKeySelector, 
        Func<TRight, TKey> rightKeySelector,
        Func<TLeft, TRight, TResult> resultSelector
    ) {

        var leftJoin = 
            from left in leftItems
            join right in rightItems 
              on leftKeySelector(left) equals rightKeySelector(right) into temp
            from right in temp.DefaultIfEmpty()
            select resultSelector(left, right);

        var rightJoin = 
            from right in rightItems
            join left in leftItems 
              on rightKeySelector(right) equals leftKeySelector(left) into temp
            from left in temp.DefaultIfEmpty()
            select resultSelector(left, right);

        return leftJoin.Union(rightJoin);
    }

}

在这个例子中，你可以这样使用它:

var test = 
    firstNames
    .FullJoinDistinct(
        lastNames,
        f=> f.ID,
        j=> j.ID,
        (f,j)=> new {
            ID = f == null ? j.ID : f.ID, 
            leftName = f == null ? null : f.Name,
            rightName = j == null ? null : j.Name
        }
    );

在未来，随着我了解更多，我有一种感觉，我会迁移到@sehe的逻辑，因为它很受欢迎。但即使这样，我也必须小心，因为我觉得如果可行的话，至少有一个重载与现有的“.Join()”方法的语法匹配是很重要的，原因有两个:

方法的一致性有助于节省时间、避免错误和避免意外行为。 如果将来有一个开箱即用的“. fulljoin()”方法，我想它会尽量保持当前存在的“. join()”方法的语法。如果是这样，那么如果您想迁移到它，您可以简单地重命名函数，而不需要更改参数或担心不同的返回类型破坏您的代码。

对于泛型、扩展、Func语句和其他特性，我还是新手，所以当然欢迎反馈。

编辑:我没花很长时间就意识到我的代码有问题。我在LINQPad中做了一个. dump()，并查看返回类型。它只是IEnumerable，所以我试着匹配它。但是当我实际上在我的扩展上做了. where()或. select()时，我得到了一个错误:“系统集合。IEnumerable'不包含'Select'和…"的定义。因此，最后我能够匹配. join()的输入语法，但不能匹配返回行为。

编辑:将“TResult”添加到函数的返回类型中。在阅读微软的文章时错过了这一点，当然这是有道理的。有了这个修复，现在看来返回行为是符合我的目标毕竟。

正如您所发现的，Linq没有“外部连接”结构。您所能得到的最接近的是使用您所声明的查询的左外连接。为此，你可以添加任何没有在join中表示的姓氏列表元素:

outerJoin = outerJoin.Concat(lastNames.Select(l=>new
                            {
                                id = l.ID,
                                firstname = String.Empty,
                                surname = l.Name
                            }).Where(l=>!outerJoin.Any(o=>o.id == l.id)));