以前有一段时间被EF LINQ中的动态表达式实现困扰，因为在实际项目中有很多的可变条件搜索，比方说，我要查询一个人，那么搜索条件就可能有按照用户名搜索、按照昵称搜索、按照年龄搜索，那么这些条件组合在一起就会有很多种，但是我们不肯能为每一种单独写一段程序，所以应该把这些可能产生条件抽离出来。
1 /* ===============================================================
2 * 创 建 者：wms
3 * 创建日期：2016/12/5 13:11:22
4 * 功能描述：动态表达式帮助类
5 * ===============================================================*/
6
7 using System;
8 using System.Linq;
9 using System.Linq.Expressions;
10
11 namespace Framework.Core.Linq
12 {
13 ///
14 /// 动态表达式帮助类
15 ///
16 public static class ExpressionHelper
17 {
18 ///
19 /// Creates a predicate that evaluates to true.
20 ///
21 public static Expressionbool>> True() { return param => true; }
22
23 ///
24 /// Creates a predicate that evaluates to false.
25 ///
26 public static Expressionbool>> False() { return param => false; }
27
28 ///
29 /// Creates a predicate expression from the specified lambda expression.
30 ///
31 public static Expressionbool>> Create(Expressionbool>> predicate) { return predicate; }
32
33 ///
34 /// Combines the first predicate with the second using the logical "and".
35 ///
36 public static Expressionbool>> And(this Expressionbool>> first, Expressionbool>> second)
37 {
38 return first.Compose(second, Expression.AndAlso);
39 }
40
41 ///
42 /// Combines the first predicate with the second using the logical "or".
43 ///
44 public static Expressionbool>> Or(this Expressionbool>> first, Expressionbool>> second)
45 {
46 return first.Compose(second, Expression.OrElse);
47 }
48
49 ///
50 /// Negates the predicate.
51 ///
52 public static Expressionbool>> Not(this Expressionbool>> expression)
53 {
54 var negated = Expression.Not(expression.Body);
55 return Expression.Lambdabool>>(negated, expression.Parameters);
56 }
57
58 ///
59 /// Combines the first expression with the second using the specified merge function.
60 ///
61 static Expression Compose(this Expression first, Expression second, Func merge)
62 {
63 // zip parameters (map from parameters of second to parameters of first)
64 var map = first.Parameters
65 .Select((f, i) => new { f, s = second.Parameters[i] })
66 .ToDictionary(p => p.s, p => p.f);
67
68 // replace parameters in the second lambda expression with the parameters in the first
69 var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
70
71 // create a merged lambda expression with parameters from the first expression
72 return Expression.Lambda(merge(first.Body, secondBody), first.Parameters);
73 }
74 }
75 }
1 using System.Collections.Generic;
2 using System.Linq.Expressions;
3
4 namespace Framework.Core.Linq
5 {
6 ///
7 /// ParameterRebinder
8 ///
9 public class ParameterRebinder : ExpressionVisitor
10 {
11 ///
12 /// The ParameterExpression map
13 ///
14 readonly Dictionary map;
15
16 ///
17 /// Initializes a new instance of the class.
18 ///
19 /// The map.
20 ParameterRebinder(Dictionary map)
21 {
22 this.map = map ?? new Dictionary();
23 }
24
25 ///
26 /// Replaces the parameters.
27 ///
28 /// The map.
29 /// The exp.
30 /// Expression
31 public static Expression ReplaceParameters(Dictionary map, Expression exp)
32 {
33 return new ParameterRebinder(map).Visit(exp);
34 }
35
36 ///
37 /// Visits the parameter.
38 ///
39 /// The p.
40 /// Expression
41 protected override Expression VisitParameter(ParameterExpression p)
42 {
43 ParameterExpression replacement;
44
45 if (map.TryGetValue(p, out replacement))
46 {
47 p = replacement;
48 }
49
50 return base.VisitParameter(p);
51 }
52 }
53 }
1 public ActionResult Index(TestUser user)
2 {
3 Expressionbool>> predicate = Framework.Core.Linq.ExpressionHelper.True();
4 if (!string.IsNullOrEmpty(user.UserName))
5 {
6 predicate = predicate.And(m => m.UserName.Contains(user.UserName));
7 }
8 if (user.Age != null)
9 {
10 predicate = predicate.And(m => m.Age == user.Age);
11 }
12 if (!string.IsNullOrEmpty(user.NickName))
13 {
14 predicate = predicate.And(m => m.NickName.Contains(user.NickName));
15 }
16 TestContext dbContext = new TestContext();
17
18 var list = dbContext.TestUser.Where(predicate).ToList();
19
20 return View(list);
21 }
