表达式树

记录表达式树的学习过程

表达式树将代码表示为可以检测、修改、或执行的一种结构，一种定义代码的结构。

表达式树是代码的完整表示形式：可以看到任何子表达式的值。 可以看到方法和属性名称。 可以看到任何常数表达式的值。 还可以将自己转换为可执行的委托，并执行代码。

通过表达式树 API，可创建几乎任何有效代码构造的树。 但不能在表达式树中创建某些 C# 习惯用语，第一 异步的async、await，第二是循环（循环有什么习惯用语？我凌乱了）。

 表达式树是不可变的数据结构， 只能换新树。

访问表达式树变量

 public static void Main()
 {            
     Expression<Func<int, int>> addFive = num => num + 5;
     // ExpressionType有Add、AddChecked、And、AndAlso、Divide、Equal、Lambda、Multiply等。
     if (addFive.NodeType== ExpressionType.Lambda)
     {
         var lambdaExp = (LambdaExpression)addFive;
         var parameter = lambdaExp.Parameters.First();
         Console.WriteLine(parameter.Name);
         Console.WriteLine(parameter.Type);

         Console.WriteLine(lambdaExp.Parameters.Last().Name); //跟上面同一个
         Console.WriteLine(lambdaExp.Parameters.Last().Type);                
     }
 }

创建表达式树

public static void Main()
{           
    // 常量表达式树
    ConstantExpression one = Expression.Constant(1, typeof(int));
    ConstantExpression two = Expression.Constant(2, typeof(int));
    // 二进制表达式树
    BinaryExpression addition = Expression.Add(one, two);
}

表达式树API

ExpressionType:（ConvertChecked 和Convert明明c#写法一样，怎么表达不同节点类型，迷茫... Multiply和MultiplyChecked，迷茫...，NewArrayBounds 多维数组）

//
// 摘要:
//     An addition operation, such as a + b, without overflow checking, for numeric
//     operands.
Add = 0,
//
// 摘要:
//     An addition operation, such as (a + b), with overflow checking, for numeric operands.
AddChecked = 1,
//
// 摘要:
//     A bitwise or logical AND operation, such as (a & b) in C# and (a And b) in Visual
//     Basic.
And = 2,
//
// 摘要:
//     A conditional AND operation that evaluates the second operand only if the first
//     operand evaluates to true. It corresponds to (a && b) in C# and (a AndAlso b)
//     in Visual Basic.
AndAlso = 3,
//
// 摘要:
//     An operation that obtains the length of a one-dimensional array, such as array.Length.
ArrayLength = 4,
//
// 摘要:
//     An indexing operation in a one-dimensional array, such as array[index] in C#
//     or array(index) in Visual Basic.
ArrayIndex = 5,
//
// 摘要:
//     A method call, such as in the obj.sampleMethod() expression.
Call = 6,
//
// 摘要:
//     A node that represents a null coalescing operation, such as (a ?? b) in C# or
//     If(a, b) in Visual Basic.
Coalesce = 7,
//
// 摘要:
//     A conditional operation, such as a > b ? a : b in C# or If(a > b, a, b) in Visual
//     Basic.
Conditional = 8,
//
// 摘要:
//     A constant value.
Constant = 9,
//
// 摘要:
//     A cast or conversion operation, such as (SampleType)obj in C#or CType(obj, SampleType)
//     in Visual Basic. For a numeric conversion, if the converted value is too large
//     for the destination type, no exception is thrown.
Convert = 10,
//
// 摘要:
//     A cast or conversion operation, such as (SampleType)obj in C#or CType(obj, SampleType)
//     in Visual Basic. For a numeric conversion, if the converted value does not fit
//     the destination type, an exception is thrown.
ConvertChecked = 11,

//
// 摘要:
//     A division operation, such as (a / b), for numeric operands.
Divide = 12,
//
// 摘要:
//     A node that represents an equality comparison, such as (a == b) in C# or (a =
//     b) in Visual Basic.
Equal = 13,
//
// 摘要:
//     A bitwise or logical XOR operation, such as (a ^ b) in C# or (a Xor b) in Visual
//     Basic.
ExclusiveOr = 14,
//
// 摘要:
//     A "greater than" comparison, such as (a > b).
GreaterThan = 15,
//
// 摘要:
//     A "greater than or equal to" comparison, such as (a >= b).
GreaterThanOrEqual = 16,
//
// 摘要:
//     An operation that invokes a delegate or lambda expression, such as sampleDelegate.Invoke().
Invoke = 17,
//
// 摘要:
//     A lambda expression, such as a => a + a in C# or Function(a) a + a in Visual
//     Basic.
Lambda = 18,
//
// 摘要:
//     A bitwise left-shift operation, such as (a << b).
LeftShift = 19,
//
// 摘要:
//     A "less than" comparison, such as (a < b).
LessThan = 20,
//
// 摘要:
//     A "less than or equal to" comparison, such as (a <= b).
LessThanOrEqual = 21,
//
// 摘要:
//     An operation that creates a new System.Collections.IEnumerable object and initializes
//     it from a list of elements, such as new List<SampleType>(){ a, b, c } in C# or
//     Dim sampleList = { a, b, c } in Visual Basic.
ListInit = 22,
//
// 摘要:
//     An operation that reads from a field or property, such as obj.SampleProperty.
MemberAccess = 23,
//
// 摘要:
//     An operation that creates a new object and initializes one or more of its members,
//     such as new Point { X = 1, Y = 2 } in C# or New Point With {.X = 1, .Y = 2} in
//     Visual Basic.
MemberInit = 24,

 //
        // 摘要:
        //     An arithmetic remainder operation, such as (a % b) in C# or (a Mod b) in Visual
        //     Basic.
        Modulo = 25,
        //
        // 摘要:
        //     A multiplication operation, such as (a * b), without overflow checking, for numeric
        //     operands.
        Multiply = 26,
        //
        // 摘要:
        //     An multiplication operation, such as (a * b), that has overflow checking, for
        //     numeric operands.
        MultiplyChecked = 27,
        //
        // 摘要:
        //     An arithmetic negation operation, such as (-a). The object a should not be modified
        //     in place.
        Negate = 28,
        //
        // 摘要:
        //     A unary plus operation, such as (+a). The result of a predefined unary plus operation
        //     is the value of the operand, but user-defined implementations might have unusual
        //     results.
        UnaryPlus = 29,
        //
        // 摘要:
        //     An arithmetic negation operation, such as (-a), that has overflow checking. The
        //     object a should not be modified in place.
        NegateChecked = 30,
        //
        // 摘要:
        //     An operation that calls a constructor to create a new object, such as new SampleType().
        New = 31,
        //
        // 摘要:
        //     An operation that creates a new one-dimensional array and initializes it from
        //     a list of elements, such as new SampleType[]{a, b, c} in C# or New SampleType(){a,
        //     b, c} in Visual Basic.
        NewArrayInit = 32,
        //
        // 摘要:
        //     An operation that creates a new array, in which the bounds for each dimension
        //     are specified, such as new SampleType[dim1, dim2] in C# or New SampleType(dim1,
        //     dim2) in Visual Basic.
        NewArrayBounds = 33,
        //
        // 摘要:
        //     A bitwise complement or logical negation operation. In C#, it is equivalent to
        //     (~a) for integral types and to (!a) for Boolean values. In Visual Basic, it is
        //     equivalent to (Not a). The object a should not be modified in place.
        Not = 34,

（Quote节点类型是什么？）

 //
        // 摘要:
        //     A bitwise complement or logical negation operation. In C#, it is equivalent to
        //     (~a) for integral types and to (!a) for Boolean values. In Visual Basic, it is
        //     equivalent to (Not a). The object a should not be modified in place.
        Not = 34,
        //
        // 摘要:
        //     An inequality comparison, such as (a != b) in C# or (a <> b) in Visual Basic.
        NotEqual = 35,
        //
        // 摘要:
        //     A bitwise or logical OR operation, such as (a | b) in C# or (a Or b) in Visual
        //     Basic.
        Or = 36,
        //
        // 摘要:
        //     A short-circuiting conditional OR operation, such as (a || b) in C# or (a OrElse
        //     b) in Visual Basic.
        OrElse = 37,
        //
        // 摘要:
        //     A reference to a parameter or variable that is defined in the context of the
        //     expression. For more information, see System.Linq.Expressions.ParameterExpression.
        Parameter = 38,
        //
        // 摘要:
        //     A mathematical operation that raises a number to a power, such as (a ^ b) in
        //     Visual Basic.
        Power = 39,
        //
        // 摘要:
        //     An expression that has a constant value of type System.Linq.Expressions.Expression.
        //     A System.Linq.Expressions.ExpressionType.Quote node can contain references to
        //     parameters that are defined in the context of the expression it represents.
        Quote = 40,
        //
        // 摘要:
        //     A bitwise right-shift operation, such as (a >> b).
        RightShift = 41,

 Expression：BinaryExpression、IndexExpression、MethodCallExpression、UnaryExpression、BlockExpression、GotoExpression、DynamicExpression、LambdaExpression、MemberExpression等。

ExpressionVisitor。（什么东东？）

执行表达式：

任何 LambdaExpression 或派生自 LambdaExpression 的类型都可以转换为IL。大多数情况下，LambdaExpression会与其对应的委托之间创建映射，通过Expression<Func<int>>将Lambda表达式树转换对应可执行的目标类型Func<int>。

var constant = 5; // constant is captured by the expression tree
Expression<Func<int, int>> expression = (b) => constant + b;
var rVal = expression.Compile(); //表达式编译成委托对象，该委托表示表达式树中的代码。
return rVal;

表达式树是不可变的，如果多次使用委托，可以将编译之后的委托缓存起来。

Lambda表达式引用的任何局部变量，有可能被释放，生成的委托调用时，发现引用的局部变量如果被释放就会报错ObjectDisposedException。引用其他程序集的方法或属性不存在也会报错ReferencedAssemblyNotFoundException。

 

 解释表达式：一个表达式树是什么节点类型（借助ExpressionType判断），然后转换成对应表达式树。有些表达式树可以由其他表达式树组成的（递归）。

using System;
using System.Linq.Expressions;

namespace ConsoleApp4
{
    class Program
    {
        public static void Main()
        {
            Expression<Func<int, int, int>> addition = (a, b) => a + b;
            var visitor = Visitor.CreateFromExpression(addition);
            visitor.Visit("current addition");
        }
    }
   
    public abstract class Visitor
    {
        // readonly是指不能第二次修改此引用。
        private readonly Expression node;
        public Visitor(Expression node)
        {
            this.node = node;
        }
        public abstract void Visit(string prefix);

        // 不能使用等于号
        public ExpressionType NodeType => this.node.NodeType;

        // 当前表达式树是什么类型，转成对应的表达式树。
        public static Visitor CreateFromExpression(Expression node)
        {
            switch (node.NodeType)
            {
                case ExpressionType.Constant:
                    return new ConstantVisitor((ConstantExpression)node);
                case ExpressionType.Lambda:
                    return new LambdaVisitor((LambdaExpression)node);
                case ExpressionType.Parameter:
                    return new ParameterVisitor((ParameterExpression)node);
                case ExpressionType.Add:
                    return new BinaryVisitor((BinaryExpression)node);
                default:
                    Console.Error.WriteLine($"Node not processed yet:{node.NodeType}");                    
                    return default(Visitor);                
            }
        }
    }

    // Lambda Visitor
    public class LambdaVisitor : Visitor
    {
        private readonly LambdaExpression node;

        // :base(node)代表先执行父类的构造函数
        public LambdaVisitor(LambdaExpression node) : base(node)
        {
            this.node = node;
        }

        public override void Visit(string prefix)
        {
            // 每个Expression都有NodeType，因此可以放到父类。
            Console.WriteLine($"{prefix}This expression is a {NodeType} expression type");
            Console.WriteLine($"{prefix}The name of the lambda is{((node.Name == null) ? "<null>" : node.Name)}");
            Console.WriteLine($"{prefix}The return type is{node.ReturnType.ToString()}");            
            Console.WriteLine($"{prefix}The expression has {node.Parameters.Count} argument(s). They are:");
            // Visit each parameter:           
            foreach(var argumentExpression in node.Parameters)
            {
                // 在父类弄个简单工厂
                var argumentVisitor = Visitor.CreateFromExpression(argumentExpression);
                argumentVisitor.Visit(prefix + "	");
            }
            Console.WriteLine($"{prefix}The expression body is:");
            // Visit the body:
            var bodyVisitor = Visitor.CreateFromExpression(node.Body);
            bodyVisitor.Visit(prefix + "	");
        }
    }
    
    public class BinaryVisitor : Visitor
    {
        private readonly BinaryExpression node;
        public BinaryVisitor(BinaryExpression node) : base(node)
        {
            this.node = node;
        }
        public override void Visit(string prefix)
        {
            Console.WriteLine($"{prefix}This binary expression is a{NodeType} expression");
            var left = Visitor.CreateFromExpression(node.Left);
            Console.WriteLine($"{prefix}The left argument is:");
            left.Visit(prefix + "	");
            var right = Visitor.CreateFromExpression(node.Right);
            Console.WriteLine($"{prefix}The Right argument is:");
            right.Visit(prefix + "	");
        }
    }

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

        public override void Visit(string prefix)
        {
            Console.WriteLine($"{prefix}This is an {NodeType} expression type");            
            Console.WriteLine($"{prefix}Type: {node.Type.ToString()}, Name: {node.Name}, ByRef: {node.IsByRef}");
        }
    }

    public class ConstantVisitor : Visitor
    {
        private readonly ConstantExpression node;
        public ConstantVisitor(ConstantExpression node) : base(node)
        {
            this.node = node;
        }

        public override void Visit(string prefix)
        {
            Console.WriteLine($"{prefix}This is an {NodeType} expression type");
            Console.WriteLine($"{prefix}The type of the constant value is {node.Type}");
            Console.WriteLine($"{prefix}The value of the constant value is {node.Value}");
        }
    }
}

 括号不是表达式树节点的一部分，括号只是传递一种优先级。

生成表达式树：借助Expression静态生成表达式树方法。

//       Expression<Func<double, double, double>> distanceCalc =
//(x, y) => Math.Sqrt(x * x + y * y);            

var xParameter = Expression.Parameter(typeof(double), "x");
var yParameter = Expression.Parameter(typeof(double), "y");
var xSquared = Expression.Multiply(xParameter, xParameter);
var ySquared = Expression.Multiply(yParameter, yParameter);
var sum = Expression.Add(xSquared, ySquared);

var sqrtMethod = typeof(Math).GetMethod("Sqrt", new[] { typeof(double) });
var distance = Expression.Call(sqrtMethod, sum);

var distanceLambda = Expression.Lambda(distance, xParameter, yParameter);

转换表达式树：访问原表达式树，并在访问的同时生成新树。 新树可包含对原始节点的引用和插入新的树节点。

private static Expression ReplaceNodes(Expression original)
{
    if (original.NodeType == ExpressionType.Constant)
    {
        // Expression.Constant(10)新节点
        return Expression.Multiply(original, Expression.Constant(10));
    }
    else if (original.NodeType == ExpressionType.Add)
    {
        var binaryExpression = (BinaryExpression)original;
        return Expression.Add(
            ReplaceNodes(binaryExpression.Left),
            ReplaceNodes(binaryExpression.Right));
    }
    return original;
}