c#表达式树入门,看这个就够了

原创

洪移潮

修改于 2024-10-31 21:00:23

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文章被收录于专栏：C#从中级到高级C#从中级到高级

题记:

由于反射需要大量的性能开销,所以推荐用表达式树或者emit,但是emit 如果不熟悉指令编程的话,使用成本很大,所以优先推荐表达式树,但是网上给出来的文档都非常的复杂,只是带你使用,刚好我团队的小伙伴也不太理解,所以我来整理一篇简单入门版本的

ps:问: 反射有3种方式,一个是获取值,一个是赋值,一个是调用方法（如构造器静态方法普通方法等），哪个才是性能元凶

先总结: 表达式树就是代码的拼接, 所以有以下三个区域

入参
代码块
返回值

入参参数是 Expression.Parameter()
代码块常见的代码块加/减/乘/除/等于/获取字段值/获取属性值/调用方法/new对象 Expression.Property()获取某对象字段的值 test.M Expression.Assign(), 给对象属性或字段赋值 test.M = 1 Expression.Multiply()，加减乘除的方法 Expression.Call()，调用某对象的方法，可以是静态 Expression.Constant() 常值比如 1 其实应该是 Constant("This is t2 name", typeof(String)); Expression.New()创建对象 Expression.NotEqual ELSE For循环等等,这个入门后自己查资料即可 Expression.Convert(),强制性转换如 Expression.Convert()配合Call
返回值返回值其实就是2个：无返回值即可执行的代码块{}，返回一个值如 new对象，入参的对象，常量值，或者 List数组 Expression.Block() Expression.MemberInit() Expression.ListInit() 其实理论上来说，表达式树没有固定的返回的值，因为表达式树中不允许出现return，只是调用Compile()的推测， 所以其实返回值只有一个Block，其他全部是代码块，只不过我觉得这时候应该区分出来，这个也是所有的表达式树在调用Lamda 的时候标准写法都是 Block包装，可以省略。

以下是例子的由简单到复杂的汇总，代码在后面的框里面

Expression<Func<int, int, int>> func = (m, n) => m * n;
Expression<Func<T01, String>> func = (T01 m) => m.Name;
Expression<Action<T01，String>> func = (T01 m, String str) => m.Name = str;
Expression<Func<String, T01>> expression1 = (String str)=>new T01() { Name = str };
Expression<Action<T01,T02>> action = (T01 t1, T02 t2)=> t2.Name = t1.Name;
Fun<T01, T02> func = T01 t1 => new T02(){ Name = t1.Name}

简单例子：Expression<Func<int, int, int>> func = (m, n) => m * n;

以下所有的表达式树都带你从入参到代码块到返回值到Compile()的步骤来拼接

//入参
ParameterExpression t1 = Expression.Parameter(typeof(Int32), "t1");
ParameterExpression t2 = Expression.Parameter(typeof(Int32), "t2");
//代码块
BinaryExpression multiply = Expression.Multiply(t1, t2);
//返回值Block
BlockExpression block = Expression.Block(multiply);
//编译
Expression<Func<int, int, int>> expression =  Expression.Lambda<Func<int, int, int>>(block, t1, t2);
Func<int, int, int> func = expression.Compile();

上面说了表达式树不允许出现显示的代码块不允许出现return，所以注意以下错误的写法

Expression<Func<int, int,int>> expression = (m,n)=> m * n;
//以下是错误形式
Expression<Action<int, int>> expression = (m,n)=> { };
Expression<Action<int, int>> expression = (m,n)=> {m + n };
Expression<Func<int, int,int>> expression = (m,n)=> { return m + n; };

注意: 没有复杂逻辑块,如 {},不允许显示写 return,大家可以试试上面注释code

进阶之操作类型属性

public class T01
{
    public String Name { get; set; }
}

public class T02
{
    public String Name { get; set; }
}

1 操作 Expression<Func<T01, String>> func = (T01 m) => m.Name;

//如果想 得到  Fuc<Test, int> t = Test test => test.M
//入参
ParameterExpression m = Expression.Parameter(typeof(T01), "m");
//代码块
MemberExpression left = Expression.Property(m, typeof(T01).GetProperty(nameof(T01.Name)));
//错误的写法是 
//Expression.Property(null, typeof(T01).GetProperty(nameof(T01.Name))),
//因为有些以为 我都 传了typeof(Test) 肯定获取到对应属性的值,
//其实表达式树是 编译代码的，所以每一步都得实现，少一个不可。
//这里的 是 m.Name 其实分为3步  m  .   Name，都必须有

//返回值
BlockExpression block1 = Expression.Block(left);
Expression<Func<T01, String>> expression = Expression.Lambda<Func<T01, String>>(block1, m);
Func<T01, String> func = expression.Compile();

2 操作属性赋值： Expression<Action<T01>> func = (T01 m) => m.Name = "new name";

//入参
ParameterExpression m = Expression.Parameter(typeof(T01), "m");
//代码块
//拼接 m.Name
MemberExpression left = Expression.Property(m, typeof(T01).GetProperty(nameof(T01.Name)));
// m.Name = "new name"
BinaryExpression assignExpression1 = Expression.Assign(left, Expression.Constant("new name", typeof(String)));
//返回值 此处无
BlockExpression block = Expression.Block(assignExpression1);
Expression<Action<T01>> expression = Expression.Lambda<Action<T01>>(block, m);
Action<T01> func = expression.Compile();
T01 t01 = new T01() { Name = "old name" };
func(t01);
Console.WriteLine(t01.Name);//new name

3 操作属性赋值： Expression<Action<T01，String>> func = (T01 m, String str) => m.Name = str;

//入参
ParameterExpression m = Expression.Parameter(typeof(T01), "m");
ParameterExpression str = Expression.Parameter(typeof(String), "str");
//代码块
//拼接 m.Name
MemberExpression left = Expression.Property(m, typeof(T01).GetProperty(nameof(T01.Name)));
// m.Name = str
//Assign 赋值的
BinaryExpression assignExpression1 = Expression.Assign(left, left);
//返回值
BlockExpression block = Expression.Block(assignExpression1);
Expression<Action<T01, String>> expression = Expression.Lambda<Action<T01, String>>(block, m, str);
Action<T01, String> func = expression.Compile();
T01 t01 = new T01() { Name = "old name" };
func(t01,"new name");
Console.WriteLine(t01.Name);//new name

4 操作属性赋值：Expression<Func<String, T01>> expression1 = (String str)=>new T01() { Name = str };

这里有个问题是 new T01(){};

4.1. 这里是对象初始化模块，而不是 var t01 = new T02();t02.Name="";

所以一定得区分，而对象初始化模块用得 Bind() 关系处理的，需要和Assign()区分开来

4.2. 注意Expression.MemberInit(Expression.New(typeof(T01))) == Expression.New(typeof(T01))，如果没有绑定关系其实可以去掉 MemberInit()的写法，就和可以去掉所有的Block一样，以及 Expression.Property()只传递一个 name 值一样，其实里面做了简化

//入参
ParameterExpression str = Expression.Parameter(typeof(String), "str");
//代码块
MemberAssignment bind = Expression.Bind(typeof(T01).GetProperty(nameof(T01.Name)), str);
//返回值
MemberInitExpression newT01 = Expression.MemberInit(Expression.New(typeof(T01)), bind);
BlockExpression block = Expression.Block(newT01);
Expression<Func<String, T01>> expression = Expression.Lambda<Func<String, T01>>(block,str);
Func<String, T01> func = expression.Compile();
T01 t01 = new T01() { Name = "old name" };
t01 = func("new name");
Console.WriteLine(t01.Name);//new name

5 操作 Expression<Action<T01,T02>> action = (T01 t1, T02 t2)=> t2.Name = t1.Name;

//入参
ParameterExpression t1 = Expression.Parameter(typeof(T01), "t1");
ParameterExpression t2 = Expression.Parameter(typeof(T02), "t2");
//代码块
//第二步,t2.Name 拆解为 参数t2, 属性符号 . 属性名称 Name
var member1 = Expression.Property(t2, nameof(T02.Name));
//拆解 t1.Name
var member2 = Expression.Property(t1, nameof(T01.Name));
//将member2赋值给member1
var member = Expression.Assign(member1, member2);

Expression block = Expression.Block(member);
Expression<Action<T01, T02>> expression = Expression.Lambda<Action<T01, T02>>(block, t1, t2);
var func = expression.Compile();
T01 t01 = new T01() { Name = "This is t1 name" };
T02 t02 = new T02() { Name = "This is t2 name" };

func(t01, t02);
Console.WriteLine(t02.Name); //输出的是 This is t1 name;

6 写一个Fun<T01, T02> func = T01 t1 => new T02(){ Name = t1.Name}的转换器

//定义入参
ParameterExpression t1 = Expression.Parameter(typeof(T01), "t1");
//代码块
//拆解 t1.Name
var member2 = Expression.Property(t1, nameof(T01.Name));
//因为这里是new,所以 这里实际是 绑定 T02.Name与 T01.Name的关系
var member = Expression.Bind(typeof(T02).GetProperty(nameof(T02.Name)), member2);
//当创建对象的时候,依据绑定关系 new T02()
//当然也可以 var member = Expression.Bind(typeof(T02).GetProperty(nameof(T02.Name)), Expression.Constant("xxxxx"));
//如果没有 member 那么可以直接 New() 不需要MemberInit()
Expression block = Expression.Block(Expression.MemberInit(Expression.New(typeof(T02)), member));
Expression<Func<T01, T02>> expression = Expression.Lambda<Func<T01, T02>>(block, t1);
var func = expression.Compile();
T01 t01 = new T01() { Name = "This is t1 name" };
T02 t02 = new T02() { Name = "This is t2 name" };

t02 = func(t01);
Console.WriteLine(t02.Name);//这里输出的  "This is t1 name"

注意这里是 Bind() 而不是 Assign(), 那这里可以用 Assign()吗?

答案是不行.因为 Assign 的写法就是 T02 t02 = new T02(); t02.Name = t01.Name;return t02; 这个原因上面解释过, 这里在强调下, 表达式树不允许代码块,不允许return,只允许单操作.

最后在强调下,表达式树其实就是入参出参返回的代码拼接,然后执行成我们想要的代码,虽然看起来复杂,但是实际就是一步步来的.

留个作业,

将上面的 Fun<T01, T02> func 改成通用的写法. 下面直接贴了,我在本地没调试,大家可以自己调试下
如何实现 Expression.Call 的执行方式
将注释上面的功能都实现一遍

public class MapHelper
{
    private static Dictionary<String, Object> dicFunc = new Dictionary<string, Object>();
            /// 没有判断泛型 IsGenericType
        /// 没操作 属性 不同类型 转不同类型的操作(重新调用Map类即可), ps => IEnumerable 转换需要调用 Expression.Call( IEnumerable.Select(Map) )来处理转换条件
        /// 没操作 IsNullable 转为值类型数据转换  需要 Expression.PropertyType("value")来强制性赋值到 基础类型
        /// 没操作 特性转换
        /// 可以理解成只做 类型相同字段 的  值赋值转换
    public static void Map<T1, T2>(T1 source, T2 destination) where T1 : class, new() where T2 : class, new()
    {
        Type tIn = typeof(T1);
        Type tResult = typeof(T2);
        String key = $"{tIn}_Convert_{tResult}";
        if (!dicFunc.TryGetValue(key, out var func))
        {
            lock (dicFunc)
            {
                if (!dicFunc.TryGetValue(key, out func))
                {
                    ParameterExpression t1 = Expression.Parameter(tIn);
                    ParameterExpression t2 = Expression.Parameter(tResult);
                    List<Expression> list = new List<Expression>();
                    Dictionary<PropertyInfo, PropertyInfo> map = new Dictionary<PropertyInfo, PropertyInfo>();
                    foreach (var item in tResult.GetProperties().Where(x => x.PropertyType.IsPublic && x.CanWrite && x.CanRead))
                    {
                        String name = item.Name;
                        var p1 = tIn.GetProperty(name);
                        if (p1 == null) continue;
                        if (!p1.PropertyType.IsPublic || !p1.CanWrite || !p1.CanRead || p1.PropertyType != item.PropertyType) continue;
                        MemberExpression mIn = Expression.Property(t1, p1);
                        MemberExpression mResult = Expression.Property(t2, item);
                        var memberBinding = Expression.Assign(mResult, mIn);
                        list.Add(memberBinding);
                    }
                    Expression<Action<T1, T2>> lambda = Expression.Lambda<Action<T1, T2>>(Expression.Block(list), t1, t2);
                    var action = lambda.Compile();
                    dicFunc[key] = action;
                    func = action;
                }
            }
        }
        ((Action<T1, T2>)func)(source, destination);
    }
    public static TResult Map<TIn, TResult>(TIn source) where TIn : class, new() where TResult : class, new()
    {
        Type tIn = typeof(TIn);
        Type tResult = typeof(TResult);
        String key = $"{tIn}_New_{tResult}";
        if (!dicFunc.TryGetValue(key, out var func))
        {
            lock (dicFunc)
            {
                if (!dicFunc.TryGetValue(key, out func))
                {
                    ParameterExpression t1 = Expression.Parameter(tIn);

                    List<MemberBinding> list = new List<MemberBinding>();
                    foreach (var item in tResult.GetProperties().Where(x => x.PropertyType.IsPublic && x.CanWrite && x.CanRead))
                    {
                        String name = item.Name;
                        var p1 = tIn.GetProperty(name);
                        if (p1 == null) continue;
                        if (!p1.PropertyType.IsPublic || !p1.CanWrite || !p1.CanRead || p1.PropertyType != item.PropertyType) continue;
                        MemberExpression property1 = Expression.Property(t1, p1);
                        var memberBinding = Expression.Bind(item, property1);
                        list.Add(memberBinding);
                    }
                    Expression<Func<TIn, TResult>> lambda;
                    if (list.Count == 0)
                    {
                        lambda = Expression.Lambda<Func<TIn, TResult>>(Expression.Block(Expression.Constant(null, tResult)), t1);
                    }
                    else
                    {
                        lambda = Expression.Lambda<Func<TIn, TResult>>(Expression.MemberInit(Expression.New(tResult), list), t1);
                    }
                    var action = lambda.Compile();
                    dicFunc[key] = action;
                    func = action;
                }
            }
        }
        return ((Func<TIn, TResult>)func)(source);
    }
}

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