C# Expression详解（高级）

LINQ在本地查询IEnumerbale主要是用委托来作为传参，而解析型查询IQueryable则用Expression来作为传参：

public static IEnumerable<T> Where<T>(this IEnumerable<T> enumable, Func<T, bool> func)
public static IQueryable<T> Where<T>(this IQueryable<T> queryable, Expression<Func<T, bool>> func)

一、Expression是什么

1、如何定义

• Expression<Func<TSource, bool>>就是表达式目录树
• Expression不能带有大括号，只能有一行代码

2、和委托的区别

• 在委托外面包裹一层Expression<>就是表达式目录树

• 表达式目录树可以通过Compile()转换成一个委托

3、Expression本质

• 表达式目录树是一个类的封装，描述了一个结构，有身体部分和参数部分
• 身体部分分为左边和右边，内部描述了左边和右边之间的关系，可以不断的往下拆分，类似于二叉树
• 表达式目录树展开后的每一个节点也是一个表达式目录树

Expression<Func<People, bool>> expression = p => p.Id == 10;
Func<People, bool> func = expression.Compile();
bool bResult = func.Invoke(new People()
{
    Id = 10,
    Name = "张三"
});

二、Expression与Expression Tree

首先我们来写下一些代码：

Expression<Func<int, int>> expression = (num) => num + 5;
Console.WriteLine($"NodeType:{expression.NodeType}");
Console.WriteLine($"Body:{expression.Body}");
Console.WriteLine($"Body Type: {expression.Body.GetType()}");
Console.WriteLine($"Body NodeType: {expression.Body.NodeType}");

输出如下：

NodeType:Lambda
Body:(num + 5)
Body Type: System.Linq.Expressions.SimpleBinaryExpression
Body NodeType: Add

我们将expression转为LambdaExpression看看都有啥：

if (expression.NodeType == ExpressionType.Lambda)
{
    var lambda = (LambdaExpression)expression;
    var parameter = lambda.Parameters.Single();
    Console.WriteLine($"parameter.Name:{parameter.Name}");
    Console.WriteLine($"parameter.Type:{parameter.Type}");
    Console.WriteLine($"parameter.ReturnType:{lambda.ReturnType}");
}

输出如下：

parameter.Name:num
parameter.Type:System.Int32
parameter.ReturnType:System.Int32

由于我们知道expression.Body是BinaryExpression，那么我们就将其转为它，然后我们继续看下去：

if (expression.Body.NodeType == ExpressionType.Add)
{
    var binaryExpreesion = (BinaryExpression)expression.Body;


    Console.WriteLine($"Left Type:{binaryExpreesion.Left.GetType()}");
    Console.WriteLine($"Left NodeType:{binaryExpreesion.Left.NodeType}");
    
    Console.WriteLine($"Right Type:{binaryExpreesion.Right.GetType()}");
    Console.WriteLine($"Right NodeType:{binaryExpreesion.Right.NodeType}");
    
    if (binaryExpreesion.Left is ParameterExpression parameterExpreesion)
    {
        Console.WriteLine($"parameterExpreesion.Name:{parameterExpreesion.Name}");
        Console.WriteLine($"parameterExpreesion.Type:{parameterExpreesion.Type}");
    }
    
    if (binaryExpreesion.Right is ConstantExpression constantExpreesion)
    {
        Console.WriteLine($"constantExpreesion.Value:{constantExpreesion.Value}" );
    }

}

输出如下：

Left Type:System.Linq.Expressions.PrimitiveParameterExpression`1[System.Int32]
Left NodeType:Parameter
Right Type:System.Linq.Expressions.ConstantExpression
Right NodeType:Constant
parameterExpreesion.Name:num
parameterExpreesion.Type:System.Int32
constantExpreesion.Value:5

最后我们将表达式树转为委托：

var @delegate = expression.Compile();
Console.WriteLine(@delegate?.Invoke(2));

输出：

7 //2+5

实际上，通过Expression<Func<int, int>> expression = (num) => num + 5;，赋值后的expression变成了一个表达式树，它的结构是这样的：

而有意思的是二元表达式树BinaryExpression是一个二叉树，而LambdaExpression则是一个支持参数的表达式，能够通过其Parameters属性知道传入的参数的类型和数量，通过ReturnType知道返回值是什么类型

而我们再看看整个关于Expression的继承关系链：

因此，我们也可以显式的通过各自Expreesion的实现子类来创建跟lambda表达式一样的结果：

var parameterExpreesion1 = Expression.Parameter(typeof(int), "num");
BinaryExpression binaryExpression1 = Expression.MakeBinary(ExpressionType.Add, parameterExpreesion1, Expression.Constant(5));
Expression<Func<int, int>> expression1 = Expression.Lambda<Func<int, int>>(binaryExpression1, parameterExpreesion1);

if (expression1.Body.NodeType == ExpressionType.Add)
{
    var binaryExpreesion1 = (BinaryExpression)expression1.Body;


    Console.WriteLine($"Left Type:{binaryExpreesion1.Left.GetType()}");
    Console.WriteLine($"Left NodeType:{binaryExpreesion1.Left.NodeType}");
    
    Console.WriteLine($"Right Type:{binaryExpreesion1.Right.GetType()}");
    Console.WriteLine($"Right NodeType:{binaryExpreesion1.Right.NodeType}");
    
    if (binaryExpreesion1.Left is ParameterExpression parameterExpreesion2)
    {
        Console.WriteLine($"parameterExpreesion.Name:{parameterExpreesion2.Name}");
        Console.WriteLine($"parameterExpreesion.Type:{parameterExpreesion2.Type}");
    }
    
    if (binaryExpreesion1.Right is ConstantExpression constantExpreesion1)
    {
        Console.WriteLine($"constantExpreesion.Value:{constantExpreesion1.Value}");
    }
    
    var @delegate1 = expression1.Compile();
    Console.WriteLine(@delegate1(2));

输出结果：

Left Type:System.Linq.Expressions.PrimitiveParameterExpression`1[System.Int32]
Left NodeType:Parameter
Right Type:System.Linq.Expressions.ConstantExpression
Right NodeType:Constant
parameterExpreesion.Name:num
parameterExpreesion.Type:System.Int32
constantExpreesion.Value:5
result:7

我们则发现，结果是一模一样的，但是费劲了很多，因此用lamda构建表达式树是一个非常愉快的语法糖，让你能够愉快的在使用表达式和表达式树

三、Expression动态拼装

1、最基础版本

Expression<Func<int>> expression = () => 123 + 234;
//常量表达式
ConstantExpression expression1 = Expression.Constant(123);
ConstantExpression expression2 = Expression.Constant(234);
//二元表达式
BinaryExpression binaryExpression = Expression.Add(expression1, expression2);
Expression<Func<int>> expressionReslut = Expression.Lambda<Func<int>>(binaryExpression);
Func<int> func = expressionReslut.Compile();
int iResult = func.Invoke();

2、带参数版本

Expression<Func<int, int>> expression1 = m => m + 1;
Func<int, int> func = expression1.Compile();
int iResult = func.Invoke(5);
//参数表达式
ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "m");
//常量表达式
ConstantExpression constant = Expression.Constant(1, typeof(int));
//二元表达式
BinaryExpression addExpression = Expression.Add(parameterExpression, constant);
Expression<Func<int, int>> expression = Expression.Lambda<Func<int, int>>(addExpression, new ParameterExpression[1]
{
    parameterExpression
});
Func<int, int> func1 = expression.Compile();
int iResult1 = func1.Invoke(5);

3、带有多个参数

Expression<Func<int, int, int>> expression = (m, n) => m * n + 2;
Func<int, int, int> func = expression.Compile();
int iResult = func.Invoke(10, 20);
//参数表达式
ParameterExpression parameterExpressionM = Expression.Parameter(typeof(int), "m");
ParameterExpression parameterExpressionN = Expression.Parameter(typeof(int), "n");
//二元表达式
BinaryExpression multiply = Expression.Multiply(parameterExpressionM, parameterExpressionN);
//常量表达式
ConstantExpression constantExpression = Expression.Constant(2);
//二元表达式
BinaryExpression plus = Expression.Add(multiply, constantExpression);
Expression<Func<int, int, int>> expression1 = Expression.Lambda<Func<int, int, int>>(plus, new ParameterExpression[2]
{
    parameterExpressionM,
    parameterExpressionN
});
Func<int, int, int> func1 = expression1.Compile();
int iResult1 = func1.Invoke(10, 20);

4、对象字段值比较

类似于这种比较复杂的，建议大家可以反编译看看

Expression<Func<People, bool>> predicate = c => c.Id == 10;
Func<People, bool> func = predicate.Compile();
bool bResult = func.Invoke(new People()
{
    Id = 10
});

//参数表达式
ParameterExpression parameterExpression = Expression.Parameter(typeof(People), "c");
//反射获取属性
FieldInfo fieldId = typeof(People).GetField("Id");
//通过parameterExpression来获取调用Id
MemberExpression idExp = Expression.Field(parameterExpression, fieldId);
//常量表达式
ConstantExpression constant10 = Expression.Constant(10, typeof(int));
//二元表达式
BinaryExpression expressionExp = Expression.Equal(idExp, constant10);
Expression<Func<People, bool>> predicate1 = Expression.Lambda<Func<People, bool>>(expressionExp, new ParameterExpression[1]
{
    parameterExpression
});

Func<People, bool> func1 = predicate1.Compile();
bool bResult1 = func1.Invoke(new People()
{
    Id = 10
});

5、多条件

如果遇到很长的表达式目录树，拼装建议从右往左拼装

Expression<Func<People, bool>> predicate = c => c.Id.ToString() == "10" && c.Name.Equals("张三") && c.Age > 35;
Func<People, bool> func = predicate.Compile();
bool bResult = func.Invoke(new People()
{
    Id = 10,
    Name = "张三",
    Age = 36
});

ParameterExpression parameterExpression = Expression.Parameter(typeof(People), "c");
//c.Age > 35
ConstantExpression constant35 = Expression.Constant(35);
PropertyInfo propAge = typeof(People).GetProperty("Age");
MemberExpression ageExp = Expression.Property(parameterExpression, propAge);
BinaryExpression cagExp = Expression.GreaterThan(ageExp, constant35);
//c.Name.Equals("张三")
ConstantExpression constantrichard = Expression.Constant("张三");
PropertyInfo propName = typeof(People).GetProperty("Name");
MemberExpression nameExp = Expression.Property(parameterExpression, propName);
MethodInfo equals = typeof(string).GetMethod("Equals", new Type[] { typeof(string) });
MethodCallExpression NameExp = Expression.Call(nameExp, equals, constantrichard);
//c.Id.ToString() == "10"
ConstantExpression constantExpression10 = Expression.Constant("10", typeof(string));
FieldInfo fieldId = typeof(People).GetField("Id");
var idExp = Expression.Field(parameterExpression, fieldId);
MethodInfo toString = typeof(int).GetMethod("ToString", new Type[0]);
var toStringExp = Expression.Call(idExp, toString, Array.Empty<Expression>());
var EqualExp = Expression.Equal(toStringExp, constantExpression10);
//c.Id.ToString() == "10"&& c.Name.Equals("张三")&& c.Age > 35
var plus = Expression.AndAlso(EqualExp, NameExp);
var exp = Expression.AndAlso(plus, cagExp);
Expression<Func<People, bool>> predicate1 = Expression.Lambda<Func<People, bool>>(exp, new ParameterExpression[1]
{
    parameterExpression
});
Func<People, bool> func1 = predicate1.Compile();
bool bResult1 = func1.Invoke(new People()
{
    Id = 10,
    Name = "张三",
    Age = 36
});

四、Expression应用之Mapper映射

需求：需要把People字段值映射到PeopleCopy字段

1、硬编码

性能好，不灵活；不能共用

PeopleCopy peopleCopy0 = new PeopleCopy()
{
    Id = people.Id,
    Name = people.Name,
    Age = people.Age
};

2、反射

灵活，但是性能不好

using System;

namespace MyExpression.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;
        }
    }
}

调用

PeopleCopy peopleCopy1 = ReflectionMapper.Trans<People, PeopleCopy>(people);

3、序列化

灵活，但是性能不好

using Newtonsoft.Json;

namespace MyExpression.MappingExtend
{
    public class SerializeMapper
    {
        /// <summary>
        /// 序列化
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            string strJson = JsonConvert.SerializeObject(tIn);
            return JsonConvert.DeserializeObject<TOut>(strJson);
        }
    }
}

调用

PeopleCopy peopleCopy2 = SerializeMapper.Trans<People, PeopleCopy>(people);

4、Expression动态拼接+普通缓存

• 把People变成PeopleCopy的过程封装在一个委托中，这个委托通过表达式目录树Compile出来，过程动态拼装适应不同的类型
• 第一次生成的时候，保存一个委托在缓存中，如果第二次来，委托就可以直接从缓存中获取到，直接运行委托，效率高

using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace MyExpression.MappingExtend
{
    public class ExpressionMapper
    {
        /// <summary>
        /// 字典缓存，保存的是委托，委托内部是转换的动作
        /// </summary>
        private static Dictionary<string, object> _Dic = new Dictionary<string, object>();

        /// <summary>
        /// Expression动态拼接+普通缓存
        /// </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 = $"funckey_{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);
        }
    }
}

调用

PeopleCopy peopleCopy3 = ExpressionMapper.Trans<People, PeopleCopy>(people);

5、Expression动态拼接+泛型缓存

泛型缓存，就是为每一组类型的组合，生成一个副本，性能最高

using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace MyExpression.MappingExtend
{
    /// <summary>
    /// Expression动态拼接+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));
                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 = lambda.Compile();//拼装是一次性的
        }
        public static TOut Trans(TIn t)
        {
            return _FUNC(t);
        }
    }
}

调用

PeopleCopy peopleCopy4 = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);

6、性能比较

Expression动态拼接+泛型缓存性能高，而且灵活

long common = 0;
long generic = 0;
long cache = 0;
long reflection = 0;
long serialize = 0;
{
    Stopwatch watch = new Stopwatch();
    watch.Start();
    for (int i = 0; i < 1_000_000; 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 < 1_000_000; i++)
    {
        PeopleCopy peopleCopy = ReflectionMapper.Trans<People, PeopleCopy>(people);
    }
    watch.Stop();
    reflection = watch.ElapsedMilliseconds;
}
{
    Stopwatch watch = new Stopwatch();
    watch.Start();
    for (int i = 0; i < 1_000_000; i++)
    {
        PeopleCopy peopleCopy = SerializeMapper.Trans<People, PeopleCopy>(people);
    }
    watch.Stop();
    serialize = watch.ElapsedMilliseconds;
}
{

    Stopwatch watch = new Stopwatch();
    watch.Start();
    for (int i = 0; i < 1_000_000; i++)
    {
        PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people);
    }
    watch.Stop();
    cache = watch.ElapsedMilliseconds;
}
{
    Stopwatch watch = new Stopwatch();
    watch.Start();
    for (int i = 0; i < 1_000_000; 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");

运行结果

common = 32 ms
reflection = 1026 ms
serialize = 2510 ms
cache = 236 ms
generic = 31 ms

五、ExpressionVisitor解析Expression

1、Expression解析

• Expression是通过访问者模式进行解析的，官方提供了ExpressionVisitor抽象类
• ExpressionVisitor的Visit方法是解析表达式目录树的一个入口，Visit方法判断Expression是一个什么表达式目录树，走不同的细分方法进行进一步解析
• ExpressionVisitor的VisitBinary方法是对二元表达式的解析，所有复杂的表达式都会拆解成二元表达式进行解析

2、Expression修改

自定义一个OperationsVisitor，继承自ExpressionVisitor，复写父类的VisitBinary方法，修改Expression的解析

OperationsVisitor定义

using System.Linq.Expressions;

namespace MyExpression
{
    /// <summary>
    /// 自定义Visitor
    /// </summary>
    public class OperationsVisitor : ExpressionVisitor
    {
        /// <summary>
        /// 覆写父类方法；//二元表达式的访问
        /// 把表达式目录树中相加改成相减，相乘改成相除
        /// </summary>
        /// <param name="b"></param>
        /// <returns></returns>
        protected override Expression VisitBinary(BinaryExpression b)
        {
            if (b.NodeType == ExpressionType.Add)//相加
            {
                Expression left = this.Visit(b.Left);
                Expression right = this.Visit(b.Right);
                return Expression.Subtract(left, right);//相减
            }
            else if (b.NodeType==ExpressionType.Multiply) //相乘
            {
                Expression left = this.Visit(b.Left);
                Expression right = this.Visit(b.Right);
                return Expression.Divide(left, right); //相除
            }
            return base.VisitBinary(b);
        }
    }
}

Expression解析转换

Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
Console.WriteLine(exp.ToString());
OperationsVisitor visitor = new OperationsVisitor();
Expression expNew = visitor.Visit(exp);
Console.WriteLine(expNew.ToString());

运行结果

(m, n) => ((m * n) + 2)
(m, n) => ((m / n) - 2)

3、封装多条件连接扩展方法

扩展方法实现

/// <summary>
/// 合并表达式 And Or Not扩展方法
/// </summary>
public static class ExpressionExtend
{
    /// <summary>
    /// 合并表达式 expr1 AND expr2
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="expr1"></param>
    /// <param name="expr2"></param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        if (expr1 == null || expr2 == null)
        {
            throw new Exception("null不能处理");
        }
        ParameterExpression newParameter = Expression.Parameter(typeof(T), "x");
        NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);
        Expression left = visitor.Visit(expr1.Body);
        Expression right = visitor.Visit(expr2.Body);
        BinaryExpression body = Expression.And(left, right);
        return Expression.Lambda<Func<T, bool>>(body, newParameter);
    }

    /// <summary>
    /// 合并表达式 expr1 or expr2
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="expr1"></param>
    /// <param name="expr2"></param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        if (expr1 == null || expr2 == null)
        {
            throw new Exception("null不能处理");
        }
        ParameterExpression newParameter = Expression.Parameter(typeof(T), "x");
        NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);
        Expression left = visitor.Visit(expr1.Body);
        Expression right = visitor.Visit(expr2.Body);
        BinaryExpression body = Expression.Or(left, right);
        return Expression.Lambda<Func<T, bool>>(body, newParameter);
    }

    /// <summary>
    /// 表达式取非
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="expr"></param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expr)
    {
        if (expr == null)
        {
            throw new Exception("null不能处理");
        }
        ParameterExpression newParameter = expr.Parameters[0];
        UnaryExpression body = Expression.Not(expr.Body);
        return Expression.Lambda<Func<T, bool>>(body, newParameter);
    }
}

自定义Visitor

internal class NewExpressionVisitor : ExpressionVisitor
{
    public ParameterExpression _NewParameter { get; private set; }
    public NewExpressionVisitor(ParameterExpression param)
    {
        this._NewParameter = param;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return this._NewParameter;
    }
}

数据过滤方法定义

/// <summary>
/// 筛选数据执行
/// </summary>
/// <param name="func"></param>
private static void Do(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();
}

Expression拼接

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);//且
Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);//或
Expression<Func<People, bool>> lambda5 = lambda1.Not();//非
Do(lambda3);
Do(lambda4);
Do(lambda5);

六、ExpressionVisitor应用之ToSql

需求：实现ORM框架Expression映射成SQL

自定义一个ConditionBuilderVisitor

继承自ExpressionVisitor，复写父类的方法，Expression解析过程中实现SQL的拼接

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Reflection;

namespace MyExpression
{
    public class ConditionBuilderVisitor : ExpressionVisitor
    {
        private Stack<string> _StringStack = new Stack<string>();

        /// <summary>
        /// 返回拼装好的sql条件表达式
        /// </summary>
        /// <returns></returns>
        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(" " + ToSqlOperator(node.NodeType) + " ");
            base.Visit(node.Left);//解析左边
            this._StringStack.Push("(");

            return node;
        }

        /// <summary>
        /// 解析属性
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitMember(MemberExpression node)
        {
            if (node == null) throw new ArgumentNullException("MemberExpression");
            if (node.Expression is ConstantExpression)
            {
                var value1 = this.InvokeValue(node);
                var value2 = this.ReflectionValue(node);
                this._StringStack.Push("'" + value2 + "'");
            }
            else
            {
                this._StringStack.Push(" [" + node.Member.Name + "] ");
            }
            return node;
        }

        private string ToSqlOperator(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");
            }
        }

        private object InvokeValue(MemberExpression member)
        {
            var objExp = Expression.Convert(member, typeof(object));//struct需要
            return Expression.Lambda<Func<object>>(objExp).Compile().Invoke();
        }

        private object ReflectionValue(MemberExpression member)
        {
            var obj = (member.Expression as ConstantExpression).Value;
            return (member.Member as FieldInfo).GetValue(obj);
        }

        /// <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;
        }
    }
}

ConstantSqlString泛型缓存缓存生成的sql

using System;
using System.Linq;

namespace MyExpression
{
    public class ConstantSqlString<T>
    {
        /// <summary>
        /// 泛型缓存，一个类型一个缓存
        /// </summary>
        private static string FindSql = null;

        /// <summary>
        /// 获取查询sql
        /// </summary>
        static ConstantSqlString()
        {
            Type type = typeof(T);
            FindSql = $"Select {string.Join(',', type.GetProperties().Select(c => $"[{c.Name}]").ToList())} from {type.Name}";
        }

        /// <summary>
        /// 获取查询sql+条件筛选
        /// </summary>
        /// <param name="exp"></param>
        /// <returns></returns>
        public static string GetQuerySql(string exp)
        {
            return $"{FindSql} where {exp}";
        }
    }
}

普通多条件

Expression<Func<People, bool>> lambda = x => x.Age > 5
                                             && x.Id > 5
                                             && x.Name.StartsWith("1") //  like '1%'
                                             && x.Name.EndsWith("1") //  like '%1'
                                             && x.Name.Contains("1");//  like '%1%'
ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
vistor.Visit(lambda);
string sql = ConstantSqlString<People>.GetQuerySql(vistor.Condition());
Console.WriteLine(sql);

外部参数变量

string name = "AAA";
Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Name == name || x.Id > 5;
ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
vistor.Visit(lambda);
string sql = ConstantSqlString<People>.GetQuerySql(vistor.Condition());
Console.WriteLine(sql);

内部常量多条件

Expression<Func<People, bool>> lambda = x => x.Age > 5 || (x.Name == "A" && x.Id > 5);
ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
vistor.Visit(lambda);
string sql = ConstantSqlString<People>.GetQuerySql(vistor.Condition());
Console.WriteLine(sql);

运行结果

Select [Age],[Name] from People where ((((( [Age]  > 5) AND ( [Id]  > 5)) AND ( [Name]  LIKE '1%')) AND ( [Name]  LIKE '%1')) AND ( [Name]  LIKE '%1%'))
Select [Age],[Name] from People where ((( [Age]  > 5) AND ( [Name]  = 'AAA')) OR ( [Id]  > 5))
Select [Age],[Name] from People where (( [Age]  > 5) OR (( [Name]  = A) AND ( [Id]  > 5)))

七、多条件拼接示例

1、动态拼接lambda表达式

/// <summary>
/// Lambda表达式拼接扩展类
/// </summary>
public static class Utility
{
    /// <summary>
    /// Lambda表达式拼接
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="first"></param>
    /// <param name="second"></param>
    /// <param name="merge"></param>
    /// <returns></returns>
    public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        // 构建参数映射(从第二个参数到第一个参数)
        var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
        // 用第一个lambda表达式中的参数替换第二个lambda表达式中的参数
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
        // 将lambda表达式体的组合应用于第一个表达式中的参数 
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }

    /// <summary>
    /// and扩展
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="first"></param>
    /// <param name="second"></param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.And);
    }

    /// <summary>
    /// or扩展
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="first"></param>
    /// <param name="second"></param>
    /// <returns></returns>
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.Or);
    }
}

/// <summary>
/// 
/// </summary>
public class ParameterRebinder : ExpressionVisitor
{
    private readonly Dictionary<ParameterExpression, ParameterExpression> map;

    public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
    {
        this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="map"></param>
    /// <param name="exp"></param>
    /// <returns></returns>
    public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
    {
        return new ParameterRebinder(map).Visit(exp);
    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="p"></param>
    /// <returns></returns>
    protected override Expression VisitParameter(ParameterExpression p)
    {
        ParameterExpression replacement;
        if (map.TryGetValue(p, out replacement))
        {
            p = replacement;
        }
        return base.VisitParameter(p);
    }
}

//常见用途，比如我们数据层封装了如下方法：

/// <summary>
/// 获取一个实体
/// </summary>
/// <param name="expWhere">查询条件</param>
/// <returns></returns>
public virtual T Find(Expression<Func<T, bool>> expWhere) {
    T entity = null;
    using (IDbContext MsSqlDB = CreateSqlContext()) {
        IQuery<T> q = MsSqlDB.Query<T>();
        entity = q.Where(expWhere).FirstOrDefault();
    }
    return entity;
}

//上层调用时候可以动态拼接lambda表达式：
Expression<Func<Books, bool>> where = c => true;
if (bookID == "-1")
{
    where = where.And(a => a.Id == "001");
}
if (bookName == "-1")
{
    where = where.Or(a => a.Name == "Test");
}
new BaseLocalBll<Books>().Find(where);

2、使用Expression进行查询拼接

public static class DynamicLinqExpressions
{

    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>(Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>(Expression.And(expr1.Body, invokedExpr), expr1.Parameters);
    }
}

//使用方法
protected void btnSearch_Click(object sender, EventArgs e)
{
    using(LinqDBDataContext db = new LinqDBDataContext())
    {
        var list = db.StuInfo;
        var where = DynamicLinqExpressions.True<StuInfo>();
        if(txtName.Text.Trim().Length!=0)
        {
            where = where.And(p => p.StuName.Contains(txtName.Text.Trim()));
        }
        if(txtAge.Text.Trim().Length!=0)
        {
            where = where.And(p => p.StuAge == Convert.ToInt32(txtAge.Text.Trim()));
        }
        var result = list.Where(where.Compile()).ToList();
        this.repStuInfo.DataSource = result;
        this.repStuInfo.DataBind();
    }
}

3、简单的多条件拼接方法

var list = new List<Expression<Func<Student, bool>>>();
list.Add(o => true);
if (stuID != 0) list.Add(o => o.ID == stuID);
if (sex != "") list.Add(o => o.Sex == sex);
if (age != null) list.Add(o => o.Age > age);
Expression<Func<Student, bool>> expWhere = null;
foreach (var expression in list)
{
    expWhere = expWhere.And(expression);
}

Students.Where(expWhere.Compile()).ToList();