在LINQ和Entity Framework（方法）中執行左/右外部聯接的最佳方法是什么？

Question

我對此進行了大量搜索，但並沒有真正找到我想要的東西。 我喜歡添加擴展方法，那么如何使用擴展方法呢？

Answer 1

編寫一些擴展方法，使其變得簡單易用：

public class JoinResult<TL, TR>
{
    public TL Left { get; set; }
    public TR Right { get; set; }
}

public static IQueryable<JoinResult<TL, TR>> LeftJoin<TL, TR, TKey>(this IQueryable<TL> left, IQueryable<TR> right, Expression<Func<TL, TKey>> leftKeySelector, Expression<Func<TR, TKey>> rightKeySelector)
{
    return left
        .GroupJoin(right, leftKeySelector, rightKeySelector, (l, rightGroup) => new { l, rightGroup = rightGroup.DefaultIfEmpty() })
        .SelectMany(z => z.rightGroup.Select(r => new JoinResult<TL, TR> { Left = z.l, Right = r }));
}

public static IQueryable<JoinResult<TL, TR>> RightJoin<TL, TR, TKey>(this IQueryable<TL> left, IQueryable<TR> right, Expression<Func<TL, TKey>> leftKeySelector, Expression<Func<TR, TKey>> rightKeySelector)
{
    return right
        .GroupJoin(left, rightKeySelector, leftKeySelector, (r, leftGroup) => new { leftGroup = leftGroup.DefaultIfEmpty(), r })
        .SelectMany(z => z.leftGroup.Select(l => new JoinResult<TL, TR> { Left = l, Right = z.r }));
}