表达式树（Expression）的执行、解释与创建

前言：在这里不进行概念性描述，能看到这篇文章说明你已经知道如何使用表达式树

执行

这里需要说明下表达式树是表示代码的数据结构，并不是经过编译且可执行的代码，如果想要执行由表达式树表示的 .NET 代码，必须将其转换为可执行的 IL 指令。这时候我们需要调用编译函数创建委托（Delegate），再执行。

执行编译后的委托通常看到三种方式：func(入参)、func.Invoke(入参)、func.DynamicInvoke(入参)

前两种用于在确认委托类型是可直接调用，DynamicInvoke一般在不确定委托的具体类型时调用

 {
     //  
     //  定义Expression
     //  @return  int
     //  @params  int(a),int(b)
     //
     Expression<Func<int, int, int>> expression = (a, b) => a + b;
     //编译创建委托
     Func<int, int, int> func = expression.Compile();
     //入参执行
     Console.WriteLine(func(1, 2));
     Console.WriteLine(func.Invoke(1, 2));
     Console.WriteLine(func.DynamicInvoke(1, 2));
 }


 {
     //  
     //  创建Expression
     //  @return  int
     //  @params  UserInfo(user)
     //
     //入参表达式
     var param = Expression.Parameter(typeof(UserInfo), "user");
     //参数属性访问器表达式
     var member = Expression.MakeMemberAccess(param, typeof(UserInfo).GetMember("Age").FirstOrDefault()!);
     //常量表达式
     var constant = Expression.Constant(999);
     //二进制计算表达式
     var body = Expression.Add(member, constant);
     //Lambda表达式
     var lambda = Expression.Lambda(body, param);
     Console.WriteLine(lambda.Compile().DynamicInvoke(new UserInfo() { Age = 1 }));
     //Console.WriteLine(((Func<UserInfo, int>)lambda.Compile()).DynamicInvoke(new UserInfo() { Age = 1 }));
 }

注意Lambda 表达式将对表达式中引用的任何局部变量创建闭包，必须保证作为委托的一部分的任何变量在调用 Compile 的位置处和执行结果委托时可用，例如以下这种情况将会抛出异常

 public class Resource : IDisposable
 {
     private bool _isDisposed = false;
     public int Argument
     {
         get
         {
             if (!_isDisposed)
                 return 5;
             else throw new ObjectDisposedException("资源已被释放");
         }
     }

     public void Dispose()
     {
         _isDisposed = true;
     }
 }

 {
     var constant = new Resource();
     Expression<Func<int, int>> expression = (b) => constant.Argument + b;
     var func = expression.Compile();
     try
     {
         //  释放后入参执行
         constant.Dispose();
         Console.WriteLine(func(1));
     }
     catch (Exception ex)
     {
         Console.WriteLine(ex);
     // 异常
     //System.ObjectDisposedException: Cannot access a disposed object.Object     name: '资源已被释放'
     }
 }

 

解释

解析表达式树能够让我们更好的理解和创建复杂的代码结构树，下面是一个简单的demo我直接从官网复制的。他解释了这个表达式的类型、参数、返回类型、以及表达式体的左右结构

  Expression<Func<int, int, int>> addition = (a, b) => a + b;
  Console.WriteLine($"This expression is a {addition.NodeType} expression type");
  Console.WriteLine($"The name of the lambda is {((addition.Name == null) ? "<null>" : addition.Name)}");
  Console.WriteLine($"The return type is {addition.ReturnType.ToString()}");
  Console.WriteLine($"The expression has {addition.Parameters.Count} arguments. They are:");
  foreach (var argumentExpression in addition.Parameters)
  {
      Console.WriteLine($"\tParameter Type: {argumentExpression.Type.ToString()}, Name: {argumentExpression.Name}");
  }
  var additionBody = (BinaryExpression)addition.Body;
  Console.WriteLine($"The body is a {additionBody.NodeType} expression");
  Console.WriteLine($"The left side is a {additionBody.Left.NodeType} expression");
  var left = (ParameterExpression)additionBody.Left;
  Console.WriteLine($"\tParameter Type: {left.Type.ToString()}, Name: {left.Name}");
  Console.WriteLine($"The right side is a {additionBody.Right.NodeType} expression");
  var right = (ParameterExpression)additionBody.Right;
  Console.WriteLine($"\tParameter Type: {right.Type.ToString()}, Name: {right.Name}");

输出：

 

如上所示是一个很简单的表达式树，依次解析表达式树的本身信息->表达式结构体->结构体的左右表达式，在表达式中每一个参数变量都是一个表达式结构。如果想要解析更复杂的表达式，我们需要递归从外层依次从左往右剥离这个表达式树，在ExpressionType这个枚举中我们能看到所有的表达式类型（80余种），笔者自己在代码种匹配了27种表达式类型的解析。下面给出一些案例和完成的解析代码：

完整解析类：

// Base Visitor class:
public abstract class Visitor
{
    private readonly Expression node;
    protected Visitor(Expression node) => this.node = node;
    public abstract void Visit(string prefix);
    public ExpressionType NodeType => node.NodeType;

    protected void ForegroundColor(ConsoleColor consoleColor, string key, bool isLine)
    {
        Console.ForegroundColor = consoleColor;
        if (isLine)
        {
            Console.WriteLine(key);
        }
        else
        {
            Console.Write(key);
        }
        Console.ResetColor();
    }

    //27
    public static Visitor CreateFromExpression(Expression node) =>
     node.NodeType switch
     {
         ExpressionType.Lambda => new LambdaVisitor((LambdaExpression)node),

         ExpressionType.Add => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.AddChecked => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.AddAssign => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.AddAssignChecked => new BinaryVisitor((BinaryExpression)node),
      
         ExpressionType.Subtract => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.SubtractChecked => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.SubtractAssign => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.SubtractAssignChecked => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.Multiply => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.MultiplyChecked => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.MultiplyAssign => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.MultiplyAssignChecked => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.Divide => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.DivideAssign => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.Modulo => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.ModuloAssign => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.GreaterThan => new BinaryVisitor((BinaryExpression)node),
         ExpressionType.OrElse => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.Constant => new ConstantVisitor((ConstantExpression)node),
  

         ExpressionType.Parameter => new ParameterVisitor((ParameterExpression)node),
         ExpressionType.Equal => new BinaryVisitor((BinaryExpression)node),

         ExpressionType.Conditional => new ConditionalVisitor((ConditionalExpression)node),
         ExpressionType.Call => new MethodCallVisitor((MethodCallExpression)node),

         ExpressionType.MemberAccess => new MemberAccessVisitor((MemberExpression)node),
         ExpressionType.Convert => new ConvertVisitor((UnaryExpression)node),

         ExpressionType.MemberInit => new NewVisitor((MemberInitExpression)node),
       
         _ => new BinaryVisitor((BinaryExpression)node),
     };
}


//  New Visitor: 类型初始化
public class NewVisitor : Visitor {

    private readonly MemberInitExpression node;
    public NewVisitor(MemberInitExpression node) : base(node) => this.node = node;

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        Console.Write($"{prefix}New Type：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.NewExpression}", true);

        Console.WriteLine($"{prefix}Members：");
        foreach (MemberAssignment item in node.Bindings)
        {
            Console.Write($"{prefix}\t");
            ForegroundColor(ConsoleColor.DarkGray, $"{item.Member}", true);
            var member = CreateFromExpression(item.Expression);
            member.Visit(prefix + "\t\t");
        }
    }
}


//  Convert Visitor: 一元运算符
public class ConvertVisitor : Visitor
{
    private readonly UnaryExpression node;
    public ConvertVisitor(UnaryExpression node) : base(node) => this.node = node;

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        Console.Write($"{prefix}To Type：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Type}", true);

        Console.Write($"{prefix}To Type Body：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Operand}", true);

        var left = CreateFromExpression(node.Operand);
        Console.WriteLine($"{prefix}Analysis Body：");
        left.Visit(prefix + "\t");
    }
}


//  MemberAccess Visitor: 属性访问器
public class MemberAccessVisitor : Visitor
{
    private readonly MemberExpression node;
    public MemberAccessVisitor(MemberExpression node) : base(node) => this.node = node;

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);


        Console.Write($"{prefix}In：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Member.DeclaringType}", true);

        string type = node.Member.MemberType switch
        {

            MemberTypes.Event => ((EventInfo)node.Member).EventHandlerType.Name,
            MemberTypes.Field => ((FieldInfo)node.Member).FieldType.Name,
            MemberTypes.Method => ((MethodInfo)node.Member).ReturnType.Name,
            MemberTypes.Property => ((PropertyInfo)node.Member).PropertyType.Name,
            _ => throw new ArgumentException
                 (
                  "Input MemberInfo must be if type EventInfo, FieldInfo, MethodInfo, or PropertyInfo"
                 )
        };
        Console.Write($"{prefix}Type：");
        ForegroundColor(ConsoleColor.DarkGray, $"{type}", true);

        Console.Write($"{prefix}Name：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Member.Name}", true);
    }
}



// Lambda Visitor: Lambda表达式
public class LambdaVisitor : Visitor
{
    private readonly LambdaExpression node;
    public LambdaVisitor(LambdaExpression node) : base(node) => this.node = node;
    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        Console.Write($"{prefix}Name：");
        ForegroundColor(ConsoleColor.DarkGray, $"{((node.Name == null) ? "<null>" : node.Name)}", true);

        Console.Write($"{prefix}ReturnType：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.ReturnType}", true);

        Console.Write($"{prefix}Parameters：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Parameters.Count}", true);

        for (int i = 0; i < node.Parameters.Count; i++)
        {
            var argumentVisitor = CreateFromExpression(node.Parameters[i]);
            argumentVisitor.Visit(prefix + "\t");
        }

        Console.WriteLine($"{prefix}Analysis Body：");
        var bodyVisitor = CreateFromExpression(node.Body);
        bodyVisitor.Visit(prefix + "\t");
    }
}

// Binary Expression Visitor:二进制运算符
public class BinaryVisitor : Visitor
{
    private readonly BinaryExpression node;
    public BinaryVisitor(BinaryExpression node) : base(node) => this.node = node;

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);


        var left = CreateFromExpression(node.Left);
        Console.WriteLine($"{prefix}Left Analysis：");
        left.Visit(prefix + "\t");
        var right = CreateFromExpression(node.Right);
        Console.WriteLine($"{prefix}Right Analysis：");
        right.Visit(prefix + "\t");
    }
}

 
// Parameter visitor: 参数类型
public class ParameterVisitor : Visitor
{
    private readonly ParameterExpression node;
    public ParameterVisitor(ParameterExpression node) : base(node)
    {
        this.node = node;
    }

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Name：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Name}", false);

        Console.Write($"    Type：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Type}", false);

        Console.Write($"    ByRef：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.IsByRef}", true);
    }
}


// Constant visitor: 常数类型
public class ConstantVisitor : Visitor
{
    private readonly ConstantExpression node;
    public ConstantVisitor(ConstantExpression node) : base(node) => this.node = node;
    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        Console.Write($"{prefix}Type：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Type}", true);

        Console.Write($"{prefix}Value：");
        ForegroundColor(ConsoleColor.DarkGray, $"{node.Value}", true);
    }
}

// Conditional visitor: 条件处理
public class ConditionalVisitor : Visitor
{
    private readonly ConditionalExpression node;
    public ConditionalVisitor(ConditionalExpression node) : base(node)
    {
        this.node = node;
    }
    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        var testVisitor = Visitor.CreateFromExpression(node.Test);
        Console.WriteLine($"{prefix}Analysis Test:");
        testVisitor.Visit(prefix + "\t");
        var trueVisitor = Visitor.CreateFromExpression(node.IfTrue);
        Console.WriteLine($"{prefix}Analysis IfTrue:");
        trueVisitor.Visit(prefix + "\t");
        var falseVisitor = Visitor.CreateFromExpression(node.IfFalse);
        Console.WriteLine($"{prefix}Analysis IfFalse:");
        falseVisitor.Visit(prefix + "\t");
    }
}

// MethodCall visitor: 方法调用
public class MethodCallVisitor : Visitor
{
    private readonly MethodCallExpression node;
    public MethodCallVisitor(MethodCallExpression node) : base(node)
    {
        this.node = node;
    }

    public override void Visit(string prefix)
    {
        Console.Write($"{prefix}Expression Node Type：");
        ForegroundColor(ConsoleColor.DarkBlue, $"{NodeType}", false);

        Console.Write($"    =>  ");
        ForegroundColor(ConsoleColor.Green, $"{node}", true);

        Console.Write($"{prefix}Static or Receiver：");
        if (node.Object == null)
        {
            ForegroundColor(ConsoleColor.DarkGray, $"static", true);
        }
        else
        {
            ForegroundColor(ConsoleColor.DarkGray, $"receiver (this)", true);
            Console.WriteLine($"{prefix}Method Analysis：");
            var receiverVisitor = Visitor.CreateFromExpression(node.Object);
            receiverVisitor.Visit(prefix + "\t");
        }

        var methodInfo = node.Method;
        Console.Write($"{prefix}Method Name：");
        ForegroundColor(ConsoleColor.DarkGray, $"{methodInfo.DeclaringType}.{methodInfo.Name}", true);

        Console.WriteLine($"{prefix}Arguments：");
        foreach (var arg in node.Arguments)
        {
            var argVisitor = Visitor.CreateFromExpression(arg);
            argVisitor.Visit(prefix + "\t");
        }
    }

}

demo1：（demo1的出输出太长无法贴图，建议读者自己尝试）

  public class TestMonths
  {
      public  int Conver32(int a)
      {
          return Convert.ToInt32(a);
      }
  }

   TestMonths testMonths = new TestMonths();
   var gg = 99;
   Expression<Func<UserInfo, int, int, double>> exception = (user, a, b) => user.Age + (a / b) * a - b + a - (b + b) + (a % b) + (a > b ? a : b) / user.Level * gg + testMonths.Conver32(user.Level);
   LambdaVisitor visitor = new LambdaVisitor(exception);
   visitor.Visit("解析二进制运算:");
   Console.WriteLine("\t");

demo2：

 Expression<Func<UserInfo, UserInfoModel>> lambda = (user) => new UserInfoModel
 {
     Age = user.Age,
     Level = EF.Functions.DataLength(user.Name) ?? 0,
     Name = user.Name
 };
 LambdaVisitor visitor3 = new LambdaVisitor(lambda);
 visitor3.Visit("解析查询表达式:");

 

创建

只有了解了他的构造，我们才懂得如何创建，下面给出两个例子分别是最常用的属性访问器和条件判断表达式

  //属性访问器
  {
      //需要构建的表达式
      Expression<Func<UserInfo, int>> left = (user) => user.Age;

      //1.创建访问对象表达式  =>(user)
      var param = Expression.Parameter(typeof(UserInfo), "user");

      //
      //2.创建访问属性表达式 =>(user.Age)
      //      @expression 我们需要访问成员的归属对象
      //      @member     成员访问器
      var member = Expression.MakeMemberAccess(param, typeof(UserInfo).GetMember("Age").FirstOrDefault()!);

      //3.创建Lambda表达式
      var lambda = Expression.Lambda(member, param);

      //测试编译输出 18
      Console.WriteLine(lambda.Compile().DynamicInvoke(new UserInfo { Age = 18 }));

  }

  //条件判断表达式
  {
      //需要构建的表达式
      Expression<Func<UserInfo, bool>> left = (user) => user.Age > 9;

      //1.创建访问对象表达式   =>(user)
      var param = Expression.Parameter(typeof(UserInfo), "user");

      //
      //2.创建访问属性表达式   =>(user.Age)
      //      @expression 我们需要访问成员的归属对象
      //      @member     成员访问器
      var member = Expression.MakeMemberAccess(param, typeof(UserInfo).GetMember("Age").FirstOrDefault()!);

      //
      //3.创建常量表达式    =>(9)
      var constant = Expression.Constant(9);

      //
      //4.创建条件判断表达式    =>(>)
      var greaterThan = Expression.GreaterThan(member, constant);

      //5.创建Lambda表达式
      var lambda = Expression.Lambda(greaterThan, param);

      //测试编译输出 True
      Console.WriteLine(lambda.Compile().DynamicInvoke(new UserInfo { Age = 18 }));
  }

如上所述，通过解析表达式反推创建表达式树是一种不错的方法，还有更加复杂的创建方式不再介绍，笔者只提供思路。

在这里推荐一个不错的库可以直接将表达式树解释成C#或VB代码ExpressionTreeToString

只要你掌握了规则，我相信这对你来说不是难事。