在LINQ和实体框架（方法）中执行左/右外连接的最佳方式是什么？

Question

我在这方面做了很多搜索，并没有真正找到我正在寻找的东西。我喜欢添加扩展方法，那么如何使用扩展方法来做到这一点？

Answer 1

写一些扩展方法，使得它好和易于：

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 })); 
}