C#中的表达式树

    本人之前从未接触过表达式树的概念，所以特意从网上找到两篇这方面的资料学习了下。本文为阅读笔记性质博客！

    表达式树是.NET 3.5之后引入的，它是一个强大灵活的工具（比如用在LINQ中构造动态查询）。

    先来看看Expression类的API接口：

using System.Collections.ObjectModel;
 
namespace System.Linq.Expressions
{
    // Summary:
    //     Represents a strongly typed lambda expression as a data structure in the
    //     form of an expression tree. This class cannot be inherited.
    //
    // Type parameters:
    //   TDelegate:
    //     The type of the delegate that the System.Linq.Expressions.Expression<tdelegate>
    //     represents.
    public sealed class Expression<tdelegate> : LambdaExpression
    {
        // Summary:
        //     Compiles the lambda expression described by the expression tree into executable
        //     code.
        //
        // Returns:
        //     A delegate of type TDelegate that represents the lambda expression described
        //     by the System.Linq.Expressions.Expression<tdelegate>.
        public TDelegate Compile();
    }
}

    表达式树的语法如下：

Expression<Func<type,returnType>> = (param) => lamdaexpresion;

    我们先来看一个简单例子：

Expression<Func<int, int, int>> expr = (x, y) => x+y;

    这就是一个表达式树了。使用Expression Tree Visualizer工具（直接调试模式下看也可以，只不过没这个直观）在调试模式下查看这个表达式树（就是一个对象），如下：

    可以看到表达式树主要由下面四部分组成：

1、Body 主体部分
2、Parameters 参数部分
3、NodeType 节点类型
4、Lambda表达式类型

    对于前面举的例子，主体部分即x+y，参数部分即(x,y)。Lambda表达式类型是Func<Int32, Int32, Int32>。注意主体部分可以是表达式，但是不能包含语句，如下这样：

Expression<Func<int, int, int>> expr = (x, y) => { return x+y; };

     会报编译错误“Lambada expression with state body cannot be converted to expression tree”：即带有语句的Lambda表达式不能转换成表达式树。

    用前面的方法虽然可以创建表达式树，但是不够灵活，如果要灵活构建表达式树，可以像下面这样：

ParameterExpression exp1 = Expression.Parameter(typeof(int), "a");
ParameterExpression exp2 = Expression.Parameter(typeof(int), "b");

BinaryExpression exp = Expression.Multiply(exp1,exp2);
var lamExp = Expression.Lambda<Func<int, int, int>>(exp, new ParameterExpression[] { exp1, exp2 });

    exp1、exp2即表达式树的参数，exp是表达式树的主体。如果我利用Reflector反编译Expression<Func<int, int, int>> expr = (x, y) => { return x+y; };得到下面的C#代码：

ParameterExpression CS$0$0000;
ParameterExpression CS$0$0001;
Expression<Func<int, int, int>> expr = Expression.Lambda<Func<int, int, int>>(Expression.Multiply(CS$0$0000 = Expression.Parameter(typeof(int), "x"), CS$0$0001 = Expression.Parameter(typeof(int), "y")), new ParameterExpression[] { CS$0$0000, CS$0$0001 });

    可以看到它基本和上面的手动构建代码一致。再来看一个简单的例子：

Expression<Func<Customer, bool>> filter =
    cust => Equal(Property(cust,"Region"),"North");

    可以用下面的代码手动构建效果等同于上面的表达式树：

// declare a parameter of type Customer named cust

ParameterExpression custParam = Expression.Parameter(

    typeof(Customer), "custParam");

// compare (equality) the Region property of the

// parameter against the string constant "North"

BinaryExpression body = Expression.Equal(

    Expression.Property(custParam, "Region"),

    Expression.Constant("North", typeof(string)));

// formalise this as a lambda

Expression<Func<Customer, bool>> filter =

    Expression.Lambda<Func<Customer, bool>>(body, cust);

    然后我们可以通过表达式树的Compile方法将表达式树编译成Lambda表达式，如下：

Func<Customer, bool> filterFunc = filter.Compile();

    

    但是Compile调用过程涉及动态代码生成，所以出于性能考虑最好只调用一次，然后缓存起来。或者像下面这样在静态构造块中使用（也只会调用一次）：

public static class Operator<T>
{
    private static readonly Func<T, T, T> add;
    public static T Add(T x, T y)
    {
        return add(x, y);
    }
    static Operator()
    {
        var x = Expression.Parameter(typeof(T), "x");
        var y = Expression.Parameter(typeof(T), "y");
        var body = Expression.Add(x, y);
        add = Expression.Lambda<Func<T, T, T>>(
            body, x, y).Compile();
    }
}

    

    Expression类包含下面几类静态方法(.NET 3.5中）：

Arithmetic: Add, AddChecked, Divide, Modulo, Multiply, MultiplyChecked, Negate, NegateChecked, Power, 
Subtract, SubtractChecked, UnaryPlus

Creation: Bind, ElementInit, ListBind, ListInit, MemberBind, MemberInit, New, NewArrayBounds, NewArrayInit

Bitwise: And, ExclusiveOr, LeftShift (<<), Not, Or, RightShift (>>)

Logical: AndAlso (&&), Condition (? :), Equal, GreaterThan, GreaterThanOrEqual, LessThan, 
LessThanOrEqual, NotEqual, OrElse (||), TypeIs

Member Access: ArrayIndex, ArrayLength, Call, Field, Property, PropertyOrField

Other: Convert, ConvertChecked, Coalesce (??), Constant, Invoke, Lambda, Parameter, TypeAs, Quote

    下面我们类似前面重载一个浅拷贝的例子（比使用反射开销小）：

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace ExpressionTreeLab
{
    class Program
    {
        static void Main(string[] args)
        {
            var p = new Person()
                {
                    Name = "jxq",
                    Age = 23
                };
            var shallowCopy = Operator<Person>.ShallowCopy(p);
            shallowCopy.Name = "feichexia";
            Console.WriteLine(shallowCopy.Name);
            Console.WriteLine(p.Name);

            Console.ReadKey();
        }

        public class Person
        {
            public string Name { get; set; }
            public int Age { get; set; }
        }

        public static class Operator<T>
        {
            private static readonly Func<T, T> ShallowClone; 

            public static T ShallowCopy(T sourcObj)
            {
                return ShallowClone.Invoke(sourcObj);
            }

            static Operator()
            {
                var origParam = Expression.Parameter(typeof(T), "orig");

                // for each read/write property on T, create a  new binding 
                // (for the object initializer) that copies the original's value into the new object 
                var setProps = from prop in typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance)
                                        where prop.CanRead && prop.CanWrite
                                        select (MemberBinding)Expression.Bind(prop, Expression.Property(origParam, prop));

                var body = Expression.MemberInit( // object initializer 
                    Expression.New(typeof(T)), // ctor 
                    setProps // property assignments 
                );

                ShallowClone = Expression.Lambda<Func<T, T>>(body, origParam).Compile();
            }
        }
    }
}

    继续看Expression.AndAlso的使用，它可以用来替代类似下面这种多条件与的情况：

Func<Person, Person, bool> personEqual = (person1, person2) => person1.Name == person2.Name && person1.Age == person2.Age;
 if(personEqual(p1, p2))
{
    Console.WriteLine("两个对象所有属性值都相等！");
}

    代码如下：

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace ExpressionTreeLab
{
    class Program
    {
        static void Main(string[] args)
        {
            var p1 = new Person()
                {
                    Name = "jxq",
                    Age = 23
                };
            var p2 = new Person()
                {
                    Name = "jxq",
                    Age = 23
                };

            if (Operator<Person>.ObjectPropertyEqual(p1, p2))
            {
                Console.WriteLine("两个对象所有属性值都相等！");
            }
            
            Console.ReadKey();
        }

        public class Person
        {
            public string Name { get; set; }
            public int Age { get; set; }
        }

        public static class Operator<T>
        {
            private static readonly Func<T, T, bool> PropsEqual; 

            public static bool ObjectPropertyEqual(T obj1, T obj2)
            {
                return PropsEqual.Invoke(obj1, obj2);
            }

            static Operator()
            {
                var x = Expression.Parameter(typeof(T), "x");
                var y = Expression.Parameter(typeof(T), "y");

                // 获取类型T上的可读Property
                var readableProps = from prop in typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance)
                                        where prop.CanRead
                                        select prop;

                Expression combination = null;
                foreach (var readableProp in readableProps)
                {
                    var thisPropEqual = Expression.Equal(Expression.Property(x, readableProp),
                                                         Expression.Property(y, readableProp));

                    if(combination == null)
                    {
                        combination = thisPropEqual;
                    }
                    else
                    {
                        combination = Expression.AndAlso(combination, thisPropEqual);
                    }
                }

                if(combination == null)   // 如果没有需要比较的东西，直接返回false
                {
                    PropsEqual = (p1, p2) => false;
                }
                else
                {
                    PropsEqual = Expression.Lambda<Func<T, T, bool>>(combination, x, y).Compile();
                }
            }
        }
    }
}

    在.NET 4.0中扩展了一些Expression的静态方法，使得编写动态代码更容易：

Mutation: AddAssign, AddAssignChecked, AndAssign, Assign, DivideAssign, ExclusiveOrAssign, LeftShiftAssign, ModuloAssign, MultiplyAssign, MultiplyAssignChecked, OrAssign, PostDecrementAssign, PostIncrementAssign, PowerAssign, PreDecrementAssign, PreIncrementAssign, RightShiftAssign, SubtractAssign, SubtractAssignChecked

Arithmetic: Decrement, Default, Increment, OnesComplement

Member Access: ArrayAccess, Dynamic

Logical: ReferenceEqual, ReferenceNotEqual, TypeEqual

Flow: Block, Break, Continue, Empty, Goto, IfThen, IfThenElse, IfFalse, IfTrue, Label, Loop, Return, Switch, SwitchCase, Unbox, Variable

Exceptions: Catch, Rethrow, Throw

Debug: ClearDebugInfo, DebugInfo

    下面是一个利用表达式树编写动态代码的例子（循环打印0到9）：

using System;
using System.Linq.Expressions;

namespace ExpressionTreeLab
{
    class Program
    {
        static void Main(string[] args)
        {
            var exitFor = Expression.Label("exitFor"); // jump label
            var x = Expression.Variable(typeof(int), "x");
            var body = 
                Expression.Block(
                    new[] { x }, // declare scope variables
                    Expression.Assign(x, Expression.Constant(0, typeof(int))), // init
                    Expression.Loop(
                        Expression.IfThenElse(
                            Expression.GreaterThanOrEqual( // test for exit
                                x,
                                Expression.Constant(10, typeof(int))
                            ),
                            Expression.Break(exitFor), // perform exit
                            Expression.Block( // perform code
                                Expression.Call(
                                    typeof(Console), "WriteLine", null, x),
                                Expression.PostIncrementAssign(x)
                            )
                        ), exitFor
                     )  // Loop ends
                 );

            var runtimeLoop = Expression.Lambda<Action>(body).Compile();
            runtimeLoop();

            Console.Read();
        }

    }
}

    另外WhereIn扩展实现如下，如果前面的例子都熟悉了的话，这个自然也很容易看懂了：

    /// <summary>
    ///   使之支持Sql in语法
    /// </summary>
    /// <typeparam name = "T"></typeparam>
    /// <typeparam name = "TValue"></typeparam>
    /// <param name = "query"></param>
    /// <param name = "obj"></param>
    /// <param name = "values"></param>
    /// <returns></returns>
    public static IQueryable<T> WhereIn<T, TValue>(this IQueryable<T> query, Expression<Func<T, TValue>> obj, IEnumerable<TValue> values)
    {
        return query.Where(BuildContainsExpression(obj, values));
    }

    private static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(
        Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
    {
        if (null == valueSelector)
        {
            throw new ArgumentNullException("valueSelector");
        }
        if (null == values)
        {
            throw new ArgumentNullException("values");
        }
        var p = valueSelector.Parameters.Single();
        if (!values.Any()) return e => false;

        var equals = values.Select(value => (Expression) Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof (TValue))));
        var body = equals.Aggregate(Expression.Or);
        return Expression.Lambda<Func<TElement, bool>>(body, p);
    }

    调用方式如下：

db.Users.WhereIIn(u => u.Id, new int[] { 1, 2, 3 });

    关于使用表达式树构建LINQ动态查询，请参考Dynamic Linq Queries with Expression Trees

 

参考资料：

http://www.codeproject.com/Tips/438804/Expression-Tree

http://www.infoq.com/articles/expression-compiler