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
//@returnint
//@paramsint(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
//@returnint
//@paramsUserInfo(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, $"{node.Parameters.Count}", true);
for (int i = 0; i < node.Parameters.Count; i++)
{
var argumentVisitor = CreateFromExpression(node.Parameters);
argumentVisitor.Visit(prefix + "\t");
}
Console.WriteLine($"{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");
}
}
}View Codedemo1：（demo1的出输出太长无法贴图，建议读者自己尝试）
public class TestMonths
{
publicint 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。
来源:https://www.cnblogs.com/largeprob/p/18026782
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