• C#知识点-表达式目录树


    原文链接:https://www.cnblogs.com/loverwangshan/p/10254730.html

    阅读目录

    1:什么是表达式树

    2:表达式目录树与委托

    3:使用Expression来进行不同对象的相同名字的属性映射

    4:ORM与表达式树目录的关系

    一:什么是表达式树

    Expression我们称为是表达式树,是一种数据结构体,用于存储需要计算,运算的一种结构,这种结构可以只是存储,而不进行运算。通常表达式目录树是配合Lambda一起来使用的,lambda可以是匿名方法,当然也可以使用Expression来动态的创建!下面我们举例来说明什么是表达式目录树。

    先创建一个People的实体,下面会用到

    /// <summary>
    /// 实体类
    /// </summary>
    public class People
    {
        public int Age { get; set; }
        public string Name { get; set; }
        public int Id;
    }
    View Code

    我们可以通过下面创建表达式目录树,我们称之为A种方式:

    Expression<Func<People, bool>> lambda = x => x.Id.ToString().IndexOf("5") >= 0;
    View Code

    我们还可以使用Expression来动态创建,我们称之为B种方式:

    var peopleParam = Expression.Parameter(typeof(People), "x");//创建一个x,类型为people
    //得到x.Id
    MemberExpression idParam = Expression.Field(peopleParam, "Id");
    
    //得到ToString方法
    MethodInfo toStringWay = typeof(int).GetMethod("ToString", new Type[] { });
    
    //得到IndexOf的方法,然后new Type[]这个代表是得到参数为string的一个方法
    MethodInfo indexOfWay = typeof(string).GetMethod("IndexOf", new Type[] { typeof(string) });
    
    //通过下面方法得到x.Id.ToString()
    MethodCallExpression tostringResult = Expression.Call(idParam, toStringWay, new Expression[] { });
    
    //通过下面方法得到x.Id.ToString().IndexOf("5") ,MethodCallExpression继承于Expression
    MethodCallExpression indexOfResult = Expression.Call(tostringResult, indexOfWay, new Expression[] { Expression.Constant("5") });
    
    //x.Id.ToString().IndexOf("5")>=0
    var lambdaBody = Expression.GreaterThanOrEqual(indexOfResult, Expression.Constant(0));
    
    //得到x => x.Id.ToString().IndexOf("5") >= 0,后面的一个参数指的是x,如果有多个则指定多个
    Expression<Func<People,bool>> lambdaResult = Expression.Lambda<Func<People, bool>>(lambdaBody, new ParameterExpression[]
                                                                                                    { peopleParam });
    
    //通过lambdaResult.Compile()得到Func<People,bool>这样的委托,然后Invoke是调用委托
    bool result = lambdaResult.Compile().Invoke(new People() { Id = 155 });
    View Code

    A种和B种得到的结果是一致的,只不过第一种是通过lambda匿名方法来构建,第二种是通过动态的Expression来构建。另外下面的原理也是一样的

    //普通的Lambda表达式
     Func<int,int,int> func = (x,y)=>  x + y - 2;
    //表达式目录树的Lambda表达式声明方式
    Expression<Func<int, int, int>> expression = (x, y) => x + y - 2;   
    
    //表达式目录树的拼接方式实现
    ParameterExpression parameterx =  Expression.Parameter(typeof(int), "x");
    ParameterExpression parametery =  Expression.Parameter(typeof(int), "y");
    ConstantExpression constantExpression = Expression.Constant(2, typeof(int));
    BinaryExpression binaryAdd = Expression.Add(parameterx, parametery);
    BinaryExpression binarySubtract = Expression.Subtract(binaryAdd, constantExpression);
    Expression<Func<int, int, int>> expressionMosaic = Expression.Lambda<Func<int, int, int>>(binarySubtract, new ParameterExpression[]
    {
           parameterx,
           parametery
    });
    
    int ResultLambda = func(5, 2);
    int ResultExpression = expression.Compile()(5, 2);
    int ResultMosaic = expressionMosaic.Compile()(5, 2);
    Console.WriteLine($"func:{ResultLambda}");
    Console.WriteLine($"expression:{ResultExpression}");
    Console.WriteLine($"expressionMosaic:{ResultMosaic}");
    View Code

    下面举例说明以下Expression.Block

    ParameterExpression varExpr = Expression.Variable(typeof(int), "x"); //add(int x);
    var ex1 = Expression.Assign(varExpr, Expression.Constant(1)); //x = 1; var ex1 = x;
    var ex2 = Expression.Add(ex1, Expression.Constant(5)); //var ex2 = ex1 + 5;//6
    var ex4 = Expression.Add(ex2, Expression.Constant(9)); //var ex4 = ex2 + 9; //15
    var ex5 = Expression.Add(ex4, Expression.Constant(8)); // var ex5 = ex4 + 8; //23
    BlockExpression blockExpr = Expression.Block(
        new ParameterExpression[] { varExpr },
        ex1,
        ex2,
        ex4,
        ex5
    );
    View Code

    该代码等效于,返回的结果都以最后一个Expression为主,则为ex5这个表达式

    public int add(int x)
    {
        x = 1;
        var ex1 = x;
        var ex2 = ex1 + 5;//6
        var ex4 = ex2 + 9; //15
        var ex5 = ex4 + 8; //23
        return ex5; //23
    }
    View Code

    Expression.Block没有返回值

    {   
        Expression A = Expression.Constant("第一大");
        Expression B = Expression.Constant("第二大");
        Expression ex = Expression.GreaterThan(Expression.Constant(1), Expression.Constant(2));
    
        var method = typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) });
        var AM = Expression.Call(method, A);
        var BM = Expression.Call(method, B);
    
        var condition = Expression.IfThenElse(ex, AM, BM);
        var blockExpr = Expression.Block(condition); //IfThenElse是没有返回值的
    
        foreach (var expr in blockExpr.Expressions)
            Console.WriteLine(expr.ToString());
    
        var lambdaExpression = Expression.Lambda<Action>(blockExpr).Compile();
        lambdaExpression();
    }
    View Code

    下图是Expression的一些变量

     二:表达式目录树与委托

    Expression一般都是都是配合委托一起来使用的,比如和委托Action(没有返回值),Func(至少有一个返回参数,且最后一个值为返回参数),Action,Func既可以直接传入一个与之匹配的实体方法,又可以传入lambda表达式这种匿名类(这种是声明lambda表达式的一种快捷方式)。Expression,Action,Func关键词是在.net 3.5之后出现的。Expression<Func<>>是可以转成Func的(通过compile()这个方法转换)。反过来则不行。我们可以理解为Func<>经过定义后,就无法改变它了。而表达式树(Expression<Func<>>则是可以进行变更的。Lambda使用lambda表达声明表达式目录树的时候注意不能有{},即:

    Func<int, int, int> func = (m, n) => m * n + 2;
    View Code

    上面这样是可以的。但是下面这样是不被允许的:

     Expression<Func<int, int, int>> exp1 = (m, n) =>
      {
              return m * n + 2;
      };//不能有语句体   只能是一行,不能有大括号
    View Code

    下面的例子来解析一下委托和表达式目录树

    #region PrivateMethod
     private static void Do1(Func<People, bool> func)
     {
         List<People> people = new List<People>();
         people.Where(func);
     }
     private static void Do1(Expression<Func<People, bool>> func)
     {
         List<People> people = new List<People>()
         {
             new People(){Id=4,Name="123",Age=4},
             new People(){Id=5,Name="234",Age=5},
             new People(){Id=6,Name="345",Age=6},
         };
    
         List<People> peopleList = people.Where(func.Compile()).ToList();
     }
    
     private static IQueryable<People> GetQueryable(Expression<Func<People, bool>> func)
     {
         List<People> people = new List<People>()
         {
             new People(){Id=4,Name="123",Age=4},
             new People(){Id=5,Name="234",Age=5},
             new People(){Id=6,Name="345",Age=6},
         };
    
         return people.AsQueryable<People>().Where(func);
     }
     #endregion
    View Code

    然后调用的时候为如下:

    Expression<Func<People, bool>> lambda1 = x => x.Age > 5;
    Expression<Func<People, bool>> lambda2 = x => x.Id > 5;
    Expression<Func<People, bool>> lambda3 = lambda1.And(lambda2);
    Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);
    Expression<Func<People, bool>> lambda5 = lambda1.Not();
    Do1(lambda3);
    Do1(lambda4);
    Do1(lambda5);
    View Code

    三:使用Expression来进行不同对象的相同名字的属性映射

    如果我们有一个新的对象和People属性基本上一致,如下:

    /// <summary>
    /// 实体类Target
    /// PeopleDTO
    /// </summary>
    public class PeopleCopy
    {
    
        public int Age { get; set; }
        public string Name { get; set; }
        public int Id;
    }
    View Code

    现在我们想要把People的中Age,Name,Id等赋值给PeopleCopy,第一种我们直接想到的是硬编码,然后如下:

    People people = new People()
    {
        Id = 11,
        Name = "加菲猫",
        Age = 31
    };
    //PeopleCopy copy = (PeopleCopy)people; //这种强制转换肯定是不行的
    
    PeopleCopy peopleCopy = new PeopleCopy()
    {
        Id = people.Id,
        Name = people.Name,
        Age = people.Age
    };
    View Code

    但是如果有多个类型转换,要写N次,然后不同用且费力,所以我们会想到通用的方法,比如使用:【反射】,【序列化反序列化】,【缓存+表达式目录】,【泛型+表达式目录】,【AutoMapper】,我们可以用这五种方法都小试一下!

    1:反射完成对象属性映射

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace ExpressionDemo.MappingExtend
    {
        public class ReflectionMapper
        {
            /// <summary>
            /// 反射
            /// </summary>
            /// <typeparam name="TIn"></typeparam>
            /// <typeparam name="TOut"></typeparam>
            /// <param name="tIn"></param>
            /// <returns></returns>
            public static TOut Trans<TIn, TOut>(TIn tIn)
            {
                TOut tOut = Activator.CreateInstance<TOut>();
                foreach (var itemOut in tOut.GetType().GetProperties())
                {
                    var propIn = tIn.GetType().GetProperty(itemOut.Name);
                    itemOut.SetValue(tOut, propIn.GetValue(tIn));
                }
                foreach (var itemOut in tOut.GetType().GetFields())
                {
                    var fieldIn = tIn.GetType().GetField(itemOut.Name);
                    itemOut.SetValue(tOut, fieldIn.GetValue(tIn));
                }
                return tOut;
            }
        }
    }
    View Code

    2:使用序列化和反序列化来完成对象属性映射:

    using Newtonsoft.Json;
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace ExpressionDemo.MappingExtend
    {
        /// <summary>
        /// 使用第三方序列化反序列化工具
        /// 
        /// 还有automapper
        /// </summary>
        public class SerializeMapper
        {
            /// <summary>
            /// 序列化反序列化方式
            /// </summary>
            /// <typeparam name="TIn"></typeparam>
            /// <typeparam name="TOut"></typeparam>
            public static TOut Trans<TIn, TOut>(TIn tIn)
            {
                return JsonConvert.DeserializeObject<TOut>(JsonConvert.SerializeObject(tIn));
            }
        }
    }
    View Code

    3:缓存+表达式目录树

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace ExpressionDemo.MappingExtend
    {
        /// <summary>
        /// 生成表达式目录树 缓存
        /// </summary>
        public class ExpressionMapper
        {
            /// <summary>
            /// 字典缓存--hash分布
            /// </summary>
            private static Dictionary<string, object> _Dic = new Dictionary<string, object>();
    
            /// <summary>
            /// 字典缓存表达式树
            /// </summary>
            /// <typeparam name="TIn"></typeparam>
            /// <typeparam name="TOut"></typeparam>
            /// <param name="tIn"></param>
            /// <returns></returns>
            public static TOut Trans<TIn, TOut>(TIn tIn)
            {
                string key = string.Format("funckey_{0}_{1}", typeof(TIn).FullName, typeof(TOut).FullName);
                if (!_Dic.ContainsKey(key))
                {
                    ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
                    List<MemberBinding> memberBindingList = new List<MemberBinding>();
                    foreach (var item in typeof(TOut).GetProperties())
                    {
                        MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                        MemberBinding memberBinding = Expression.Bind(item, property);
                        memberBindingList.Add(memberBinding);
                    }
                    foreach (var item in typeof(TOut).GetFields())
                    {
                        MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                        MemberBinding memberBinding = Expression.Bind(item, property);
                        memberBindingList.Add(memberBinding);
                    }
                    MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
                    Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
                    {
                        parameterExpression
                    });
                    Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的
                    _Dic[key] = func;
                }
                return ((Func<TIn, TOut>)_Dic[key]).Invoke(tIn);
            }
        }
    }
    View Code

    4:泛型+表达式目录树

    using System;
    using System.Collections.Generic;
    using System.Linq.Expressions;
    
    namespace ExpressionDemo.MappingExtend
    {
        /// <summary>
        /// 生成表达式目录树  泛型缓存
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        public class ExpressionGenericMapper<TIn, TOut>//Mapper`2
        {
            private static Func<TIn, TOut> _FUNC = null;
            static ExpressionGenericMapper()
            {
                ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
                List<MemberBinding> memberBindingList = new List<MemberBinding>();
                foreach (var item in typeof(TOut).GetProperties())
                {
                    MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name)); //p.Age
                    MemberBinding memberBinding = Expression.Bind(item, property); //Age=p.Age
                    memberBindingList.Add(memberBinding);
                }
                foreach (var item in typeof(TOut).GetFields())
                {
                    MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                    MemberBinding memberBinding = Expression.Bind(item, property);
                    memberBindingList.Add(memberBinding);
                }
                //new PeopleCopy() {Age = p.Age, Name = p.Name, Id = p.Id}
                MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
                //p => new PeopleCopy() {Age = p.Age, Name = p.Name, Id = p.Id}
                Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
                {
                        parameterExpression
                });
                _FUNC = lambda.Compile();//拼装是一次性的
            }
            public static TOut Trans(TIn t)
            {
                return _FUNC(t);
            }
        }
    }
    View Code

    5:使用.netFramwork框架自带的AutoMapper,首先我们要nuget添加引用AutoMapper即可直接使用,具体代码为:

    using AutoMapper;
    
    namespace ExpressionDemo.MappingExtend
    {
        public class AutoMapperTest
        {
            public static TOut Trans<TIn, TOut>(TIn tIn)
            {
                return Mapper.Instance.Map<TOut>(tIn);
            }
        }
    }
    View Code

    五种方法我们分别调用一下,然后测试一下性能,代码如下:

    {
                    People people = new People()
                    {
                        Id = 11,
                        Name = "加菲猫",
                        Age = 31
                    };
                    //使用AutoMapper之前必须要初始化对应的关系
                    Mapper.Initialize(x => x.CreateMap<People, PeopleCopy>()); 
                   
                    long common = 0;
                    long generic = 0;
                    long cache = 0;
                    long reflection = 0;
                    long serialize = 0;
                    long autoMapper = 0;
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = new PeopleCopy()
                            {
                                Id = people.Id,
                                Name = people.Name,
                                Age = people.Age
                            };
                        }
                        watch.Stop();
                        common = watch.ElapsedMilliseconds;
                    }
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = AutoMapperTest.Trans<People, PeopleCopy>(people);
                        }
                        watch.Stop();
                        autoMapper = watch.ElapsedMilliseconds;
                    }
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = ReflectionMapper.Trans<People, PeopleCopy>(people);
                        }
                        watch.Stop();
                        reflection = watch.ElapsedMilliseconds;
                    }
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = SerializeMapper.Trans<People, PeopleCopy>(people);
                        }
                        watch.Stop();
                        serialize = watch.ElapsedMilliseconds;
                    }
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people);
                        }
                        watch.Stop();
                        cache = watch.ElapsedMilliseconds;
                    }
                    {
                        Stopwatch watch = new Stopwatch();
                        watch.Start();
                        for (int i = 0; i < 1000000; i++)
                        {
                            PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);
                        }
                        watch.Stop();
                        generic = watch.ElapsedMilliseconds;
                    }
    
                    Console.WriteLine($"common = { common} ms");
                    Console.WriteLine($"reflection = { reflection} ms");
                    Console.WriteLine($"serialize = { serialize} ms");
                    Console.WriteLine($"cache = { cache} ms");
                    Console.WriteLine($"generic = { generic} ms");
                    Console.WriteLine($"automapper = { autoMapper} ms");
                    //性能比automapper还要高
                }
    View Code

    运行结果如下:

    通过结果发现:反射和序列化运用的时间最多,而我们惊奇的发现表达式目录树+泛型缓存比框架自带的AutoMapper时间还短!有木有感觉超级腻害~!

    四:ORM与表达式树目录的关系

    我们平常项目中经常用到EF,其实都是继承Queryable,然后我们使用的EF通常都会使用 var items = anserDo.GetAll().Where(x => x.OrganizationId == input.oid || input.oid == 0) ,where其实传的就是表达式目录树。那我们来一步一步解析EF底层实现的具体逻辑。

    lambada表达式上面说了能使用Expression来动态拼接,当然它还有一个神奇的功能,能动态的解耦。Expression有个类ExpressionVisitor

     

     这个类中的Visit(Expression node)是解读表达式的入口,然后能够神奇的区分参数和方法体,然后将表达式调度到此类中更专用的访问方法中,然后一层一层的解析下去,一直到最终的叶节点!

    将表达式调度到此类中更专用的访问方法中:我们来举例说明:

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace ExpressionDemo.Visitor
    {
        public class OperationsVisitor : ExpressionVisitor
        {
            public Expression Modify(Expression expression)
            {
                return this.Visit(expression);
            }
    
            protected override Expression VisitBinary(BinaryExpression b)
            {
                if (b.NodeType == ExpressionType.Add)
                {
                    Expression left = this.Visit(b.Left);
                    Expression right = this.Visit(b.Right);
                    return Expression.Subtract(left, right);
                }
    
                return base.VisitBinary(b);
            }
    
            protected override Expression VisitConstant(ConstantExpression node)
            {
                return base.VisitConstant(node);
            }
        }
    }
    View Code

    下面调用:

    1 {
    2     //修改表达式目录树
    3     Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
    4     OperationsVisitor visitor = new OperationsVisitor();      
    5     Expression expNew = visitor.Modify(exp);
    6 }
    View Code

    visit这个这个方法能够识别出来 m*n+2 是个二叉树,会通过下面的图然后一步一步的进行解析,如果遇到m*n 这会直接调用VisitBinary(BinaryExpression b)这个方法,如果遇到m或者n会调用VisitParameter(ParameterExpression node)这个方法,

    如果遇到2常量则会调用VisitConstant(ConstantExpression node),这就是visit神奇的调度功能!

    我们EF写的where等lambda表达式,就是通过ExpressionVisitor这个类来反解析的!之前没有学习过表达式目录树,以为ef本来就应该这样写,有没有和我一样认为的?

    我们现在模拟写一个lambda转换sql的方法

    using ExpressionDemo.DBExtend;
    using System;
    using System.Collections.Generic;
    using System.Linq.Expressions;
    
    namespace ExpressionDemo.Visitor
    {
        public class ConditionBuilderVisitor : ExpressionVisitor
        {
            private Stack<string> _StringStack = new Stack<string>();
    
            public string Condition()
            {
                string condition = string.Concat(this._StringStack.ToArray());
                this._StringStack.Clear();
                return condition;
            }
    
            /// <summary>
            /// 如果是二元表达式
            /// </summary>
            /// <param name="node"></param>
            /// <returns></returns>
            protected override Expression VisitBinary(BinaryExpression node)
            {
                if (node == null) throw new ArgumentNullException("BinaryExpression");
    
                this._StringStack.Push(")");
                base.Visit(node.Right);//解析右边
                this._StringStack.Push(" " + node.NodeType.ToSqlOperator() + " ");
                base.Visit(node.Left);//解析左边
                this._StringStack.Push("(");
    
                return node;
            }
            /// <summary>
            /// 
            /// </summary>
            /// <param name="node"></param>
            /// <returns></returns>
            protected override Expression VisitMember(MemberExpression node)
            {
                if (node == null) throw new ArgumentNullException("MemberExpression");
                this._StringStack.Push(" [" + node.Member.Name + "] ");
                return node;
            }
            /// <summary>
            /// 常量表达式
            /// </summary>
            /// <param name="node"></param>
            /// <returns></returns>
            protected override Expression VisitConstant(ConstantExpression node)
            {
                if (node == null) throw new ArgumentNullException("ConstantExpression");
                this._StringStack.Push(" '" + node.Value + "' ");
                return node;
            }
            /// <summary>
            /// 方法表达式
            /// </summary>
            /// <param name="m"></param>
            /// <returns></returns>
            protected override Expression VisitMethodCall(MethodCallExpression m)
            {
                if (m == null) throw new ArgumentNullException("MethodCallExpression");
    
                string format;
                switch (m.Method.Name)
                {
                    case "StartsWith":
                        format = "({0} LIKE {1}+'%')";
                        break;
    
                    case "Contains":
                        format = "({0} LIKE '%'+{1}+'%')";
                        break;
    
                    case "EndsWith":
                        format = "({0} LIKE '%'+{1})";
                        break;
    
                    default:
                        throw new NotSupportedException(m.NodeType + " is not supported!");
                }
                this.Visit(m.Object);
                this.Visit(m.Arguments[0]);
                string right = this._StringStack.Pop();
                string left = this._StringStack.Pop();
                this._StringStack.Push(String.Format(format, left, right));
    
                return m;
            }
        }
    }
    View Code
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
    using System.Threading.Tasks;
    
    namespace ExpressionDemo.DBExtend
    {
        internal static class SqlOperator
        {
            internal static string ToSqlOperator(this ExpressionType type)
            {
                switch (type)
                {
                    case (ExpressionType.AndAlso):
                    case (ExpressionType.And):
                        return "AND";
                    case (ExpressionType.OrElse):
                    case (ExpressionType.Or):
                        return "OR";
                    case (ExpressionType.Not):
                        return "NOT";
                    case (ExpressionType.NotEqual):
                        return "<>";
                    case ExpressionType.GreaterThan:
                        return ">";
                    case ExpressionType.GreaterThanOrEqual:
                        return ">=";
                    case ExpressionType.LessThan:
                        return "<";
                    case ExpressionType.LessThanOrEqual:
                        return "<=";
                    case (ExpressionType.Equal):
                        return "=";
                    default:
                        throw new Exception("不支持该方法");
                }
    
            }
        }
    }
    View Code

    然后调用的时候如下:

    {
        //修改表达式目录树
        Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
        OperationsVisitor visitor = new OperationsVisitor();               
        Expression expNew = visitor.Modify(exp);
    }
    
    {
        Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Id > 5
                                                 && x.Name.StartsWith("1")
                                                 && x.Name.EndsWith("1")
                                                 && x.Name.Contains("1");
    
        string sql = string.Format("Delete From [{0}] WHERE {1}"
            , typeof(People).Name
            , " [Age]>5 AND [ID] >5"
            );
        ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
        vistor.Visit(lambda);
        Console.WriteLine(vistor.Condition());
    }
    {
        Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Name == "A" || x.Id > 5;
        ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
        vistor.Visit(lambda);
        Console.WriteLine(vistor.Condition());
    }
    {
        Expression<Func<People, bool>> lambda = x => x.Age > 5 || (x.Name == "A" && x.Id > 5);
        ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
        vistor.Visit(lambda);
        Console.WriteLine(vistor.Condition());
    }
    {
        Expression<Func<People, bool>> lambda = x => (x.Age > 5 || x.Name == "A") && x.Id > 5;
        ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
        vistor.Visit(lambda);
        Console.WriteLine(vistor.Condition());
    }
    View Code

    目前Expression只支持ExpressionType的84种操作符Add, AndAlso等等,然后VisitMethodCall这个方法中表示lambda能解析出来的方法名字,如果需要可以自行修改会得到对应的sql语句的where条件!

  • 相关阅读:
    Vuex2.0+Vue2.0构建备忘录应用实践
    一步步构造自己的vue2.0+webpack环境
    .NET入行之工作前
    webpack入门之简单例子跑起来
    vue中,class、内联style绑定、computed属性
    wap问答系统工作总结
    ASP.NET Core Api网关Ocelot的中文文档
    在pom.xml中添加Spring依赖
    【java基础】从反射开始(Reflection)
    【java基础】 == 和 equals() 的区别
  • 原文地址:https://www.cnblogs.com/bigbox777/p/12463156.html
Copyright © 2020-2023  润新知