1     /// <summary>
 2     ///     Expression表达式扩展操作类
 3     /// 调用方法：repository.GetAll().AsExpandable().Where(predicate)
 4     /// </summary>
 5     public static class ExpressionExtensions
 6     {
 7         /// <summary>
 8         ///     以特定的条件运行组合两个Expression表达式
 9         /// </summary>
10         /// <typeparam name="T">表达式的主实体类型</typeparam>
11         /// <param name="first">第一个Expression表达式</param>
12         /// <param name="second">要组合的Expression表达式</param>
13         /// <param name="merge">组合条件运算方式</param>
14         /// <returns>组合后的表达式</returns>
15         public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second,
16             Func<Expression, Expression, Expression> merge)
17         {
18             var map =
19                 first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
20             var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
21             return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
22         }
23 
24         /// <summary>
25         ///     以 Expression.AndAlso 组合两个Expression表达式
26         /// </summary>
27         /// <typeparam name="T">表达式的主实体类型</typeparam>
28         /// <param name="first">第一个Expression表达式</param>
29         /// <param name="second">要组合的Expression表达式</param>
30         /// <returns>组合后的表达式</returns>
31         public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first,
32             Expression<Func<T, bool>> second)
33         {
34             return first.Compose(second, Expression.AndAlso);
35         }
36 
37         /// <summary>
38         ///     以 Expression.OrElse 组合两个Expression表达式
39         /// </summary>
40         /// <typeparam name="T">表达式的主实体类型</typeparam>
41         /// <param name="first">第一个Expression表达式</param>
42         /// <param name="second">要组合的Expression表达式</param>
43         /// <returns>组合后的表达式</returns>
44         public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first,
45             Expression<Func<T, bool>> second)
46         {
47             return first.Compose(second, Expression.OrElse);
48         }
49 
50         private class ParameterRebinder : ExpressionVisitor
51         {
52             private readonly Dictionary<ParameterExpression, ParameterExpression> _map;
53 
54             private ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
55             {
56                 _map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
57             }
58 
59             public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map,
60                 Expression exp)
61             {
62                 return new ParameterRebinder(map).Visit(exp);
63             }
64 
65             protected override Expression VisitParameter(ParameterExpression node)
66             {
67                 ParameterExpression replacement;
68                 if (_map.TryGetValue(node, out replacement))
69                     node = replacement;
70                 return base.VisitParameter(node);
71             }
72         }
73     }