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表达式树的说明与运用

说明: 在日常代码开发中很多的地方都用到了Lambda表达式进行过滤操作，很多优秀的ORM也是使用表达式来进行数据的查询。但是对于一些复杂的过滤单纯的使用Lambda已经不能够解决问题了那么就需要表达式树来进行条件的一个拼接。

下面介绍一个本人写的一个工具类有助于项目中更好的使用:

  1   public static class ExpressionTreeTools
  2     {
  3         /// <summary>
  4         /// 相当于&&操作
  5         /// ——just0ne
  6         /// </summary>
  7         /// <param name="thisFilter">已生成的过滤条件</param>
  8         /// <param name="otherFilter">未生成的过滤条件</param>
  9         /// <returns>新的过滤</returns>
 10         public static Expression And(this Expression thisFilter, Expression otherFilter)
 11         {
 12             return Expression.AndAlso(thisFilter, otherFilter);
 13         }
 14         /// <summary>
 15         /// 相当于||操作
 16         /// ——just0ne
 17         /// </summary>
 18         /// <param name="thisFilter">已生成的过滤条件</param>
 19         /// <param name="otherFilter">未生成的过滤条件</param>
 20         /// <returns>新的过滤</returns>
 21         public static Expression Or(this Expression thisFilter, Expression otherFilter)
 22         {
 23             return Expression.OrElse(thisFilter, otherFilter);
 24         }
 25         /// <summary>
 26         /// 相当于==操作
 27         /// ——just0ne
 28         /// </summary>
 29         /// <param name="thisParameterExpression">查询对象</param>
 30         /// <param name="propertieName">属性名称</param>
 31         /// <param name="propertieValue">属性值</param>
 32         /// <returns>新的过滤</returns>
 33         public static Expression GotoEqual(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 34         {
 35             return Expression.Equal(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue));
 36         }
 37         /// <summary>
 38         /// 相当于>=操作
 39         /// ——just0ne
 40         /// </summary>
 41         /// <param name="thisParameterExpression">查询对象</param>
 42         /// <param name="propertieName">属性名称</param>
 43         /// <param name="propertieValue">属性值</param>
 44         /// <returns>新的过滤</returns>
 45         public static Expression GotoGreaterThanOrEqual<T>(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 46         {
 47             //大于或等于
 48             return Expression.GreaterThanOrEqual(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(T)));
 49         }
 50         /// <summary>
 51         /// 相当于小于等于操作
 52         /// ——just0ne
 53         /// </summary>
 54         /// <param name="thisParameterExpression">查询对象</param>
 55         /// <param name="propertieName">属性名称</param>
 56         /// <param name="propertieValue">属性值</param>
 57         /// <returns>新的过滤</returns>
 58         public static Expression GotoLessThanOrEqual<T>(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 59         {
 60             //小于或等于
 61             return Expression.LessThanOrEqual(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(T)));
 62         }
 63         /// <summary>
 64         /// 相当于>操作
 65         /// ——just0ne
 66         /// </summary>
 67         /// <param name="thisParameterExpression">查询对象</param>
 68         /// <param name="propertieName">属性名称</param>
 69         /// <param name="propertieValue">属性值</param>
 70         /// <returns>新的过滤</returns>
 71         public static Expression GotoGreaterThan<T>(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 72         {
 73             //大于
 74             return Expression.GreaterThan(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(T)));
 75         }
 76         /// <summary>
 77         /// 相当于小于操作
 78         /// ——just0ne
 79         /// </summary>
 80         /// <param name="thisParameterExpression">查询对象</param>
 81         /// <param name="propertieName">属性名称</param>
 82         /// <param name="propertieValue">属性值</param>
 83         /// <returns>新的过滤</returns>
 84         public static Expression GotoLessThan<T>(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 85         {
 86             //小于
 87             return Expression.LessThan(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(T)));
 88         }
 89         /// <summary>
 90         /// 相当于>=操作
 91         /// ——just0ne
 92         /// </summary>
 93         /// <param name="thisParameterExpression">查询对象</param>
 94         /// <param name="propertieName">属性名称</param>
 95         /// <param name="propertieValue">属性值</param>
 96         /// <returns>新的过滤</returns>
 97         public static Expression GotoGreaterThanOrEqualByDateTime(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
 98         {
 99             //大于或等于
100             return Expression.GreaterThanOrEqual(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(DateTime?)));
101         }
102         /// <summary>
103         /// 相当于小于或等于操作
104         /// ——just0ne
105         /// </summary>
106         /// <param name="thisParameterExpression">查询对象</param>
107         /// <param name="propertieName">属性名称</param>
108         /// <param name="propertieValue">属性值</param>
109         /// <returns>新的过滤</returns>
110         public static Expression GotoLessThanOrEqualByDateTime(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
111         {
112             //小于或等于
113             return Expression.LessThanOrEqual(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(DateTime?)));
114         }
115         /// <summary>
116         /// 相当于>操作
117         /// ——just0ne
118         /// </summary>
119         /// <param name="thisParameterExpression">查询对象</param>
120         /// <param name="propertieName">属性名称</param>
121         /// <param name="propertieValue">属性值</param>
122         /// <returns>新的过滤</returns>
123         public static Expression GotoGreaterThanByDateTime(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
124         {
125             //大于
126             return Expression.GreaterThan(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(DateTime?)));
127         }
128         /// <summary>
129         /// 相当于小于操作
130         /// —一just0ne
131         /// </summary>
132         /// <param name="thisParameterExpression">查询对象</param>
133         /// <param name="propertieName">属性名称</param>
134         /// <param name="propertieValue">属性值</param>
135         /// <returns>新的过滤</returns>
136         public static Expression GotoLessThanByDateTime(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
137         {
138             //小于
139             return Expression.LessThan(Expression.Property(thisParameterExpression, propertieName), Expression.Constant(propertieValue, typeof(DateTime?)));
140         }
141 
142         /// <summary>
143         /// 一一just0ne
144         /// 包含操作 相当余 a=> arr.Contains(a.ID)
145         /// 如果arr中数据量过大则不适用linq
146         /// </summary>
147         /// <param name="thisParameterExpression">查询对象</param>
148         /// <param name="propertieName">属性名称</param>
149         /// <param name="propertieValue">属性值</param>
150         /// <returns>新的过滤</returns>
151         public static Expression ContainsOperations(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
152         {
153             MethodInfo method = null;
154             MemberExpression member = Expression.Property(thisParameterExpression, propertieName);
155             var containsMethods = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public).Where(m => m.Name == "Contains");
156             foreach (var m in containsMethods)
157             {
158                 if (m.GetParameters().Count() == 2)
159                 {
160                     method = m;
161                     break;
162                 }
163             }
164             method = method.MakeGenericMethod(member.Type);
165             var exprContains = Expression.Call(method, new Expression[] { Expression.Constant(propertieValue), member });
166             return exprContains;
167         }
168 
169         /// <summary>
170         /// 一一just0ne
171         /// 包含操作 相当于  a=>a.ID.Contains(key)
172         /// </summary>
173         /// <param name="thisParameterExpression">查询对象</param>
174         /// <param name="propertieName">属性名称</param>
175         /// <param name="propertieValue">属性值</param>
176         /// <returns>新的过滤</returns>
177         public static Expression Contains(this ParameterExpression thisParameterExpression, string propertieName, object propertieValue)
178         {
179             var propertyExp = Expression.Property(thisParameterExpression, propertieName);
180             MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
181             var someValue = Expression.Constant(propertieValue, typeof(string));
182             var containsMethodExp = Expression.Call(propertyExp, method, someValue);
183             return containsMethodExp;
184         }
185 
186     }

以上就是一些基本的拼接都有了接下来是如何进行使用还是贴上代码进行说明

  class Program
    {
        static void Main(string[] args)
        {
            string[] nameArr = new string[] { "just0ne", "kphui", "point" };
            var userDataList = GetUserDataList().AsQueryable();
            //初始化
            var parameterExpression = Expression.Parameter(typeof(UserData));
            var filter = (Expression)Expression.Constant(true);
            //寻找年岁大于10
            filter = filter.And(parameterExpression.GotoGreaterThan<int>("Age", 10));
            string key = Console.ReadLine();
            if (!String.IsNullOrEmpty(key))
            {
                var keyFilter = (Expression)Expression.Constant(false);
                keyFilter = keyFilter.Or(parameterExpression.Contains("Name", key));
                keyFilter = keyFilter.Or(parameterExpression.Contains("Phone", key));
                filter = filter.And(keyFilter);
            }
            filter = filter.And(parameterExpression.ContainsOperations("Name", nameArr));
            var lamadaFilter = Expression.Lambda<Func<UserData, bool>>(filter, parameterExpression);
            var userDatas = userDataList.Where(lamadaFilter).ToList();
            Console.WriteLine(Newtonsoft.Json.JsonConvert.SerializeObject(userDatas));
        }

        public static List<UserData> GetUserDataList()
        {
            return new List<UserData>()
            {
                new UserData(){ Age=18, Id=1, Name="just0ne", Phone="13856****26" },
                new UserData(){ Age=22, Id=2, Name="point", Phone="17521****52" },
                new UserData(){ Age=21, Id=3, Name="geekdog", Phone="15562****36" },
                new UserData(){ Age=14, Id=4, Name="kphui" , Phone="13577****26"},
                new UserData(){ Age=13, Id=5, Name="lg" , Phone="13456****26"},
                new UserData(){ Age=16, Id=6, Name="ming", Phone="13356****26" },
                new UserData(){ Age=18, Id=7, Name="tencent", Phone="13256****26" },
                new UserData(){ Age=10, Id=8, Name="justin" , Phone="13156****26"},
                new UserData(){ Age=31, Id=9, Name="hujw", Phone="13823****26" },
                new UserData(){ Age=27, Id=10, Name="lqm" , Phone="13876****26"},
                new UserData(){ Age=26, Id=11, Name="jiujiu" , Phone="13846****26"},
            };
        }
    }

    public class UserData
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public string Phone { get; set; }

        public string Email { get; set; }

        public int Age { get; set; }
    }

运行程序会输出如下的内容:

以上只是针对如何使用表达式树做个例子如有不同的可以留言哦！

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原文地址：https://www.cnblogs.com/just0ne/p/11553064.html