C# Expression Visitor

Posted 2021-11-28 zhaxichun

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Runtime.InteropServices;

namespace app
{
    public abstract class Visitor
    {
        private readonly Expression node;

        protected Visitor(Expression node)
        {
            this.node = node;
        }

        public abstract void Visit(string prefix);
        public ExpressionType NodeType => this.node.NodeType;

        public static Visitor CreateFormatExpression(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:
                case ExpressionType.Equal:
                case ExpressionType.Multiply:
                    return new BinaryVisitor((BinaryExpression)node);
                case ExpressionType.Conditional:
                    return new ConditionalVisitor((ConditionalExpression)node);
                case ExpressionType.Call:
                    return new MethodCallVisitor((MethodCallExpression)node);
                default:
                    Console.Error.WriteLine($"Node not processed yet: {node.NodeType}");
                    return default(Visitor);
            }
        }
    }

    public class LambdaVisitor : Visitor
    {
        private readonly LambdaExpression node;

        public LambdaVisitor(LambdaExpression node) : base(node)
        {
            this.node = node;
        }
        
        
        public override void Visit(string prefix)
        {
            Console.WriteLine($"{prefix} this expession is a {NodeType} expression type");
            Console.WriteLine($"{prefix} this name of the lambda is {((node.Name == null) ? "<null>": node.Name)}");
            Console.WriteLine($"{prefix} the return is {node.ReturnType.ToString()}");
            Console.WriteLine($"{prefix} the expression has {node.Parameters.Count} arguments.They are");
            foreach (var argumentExpression in node.Parameters)
            {
                var argumentVisitor = Visitor.CreateFormatExpression(argumentExpression);
                argumentVisitor.Visit(prefix + ‘\t‘);
            }

            Console.WriteLine($"{prefix} The expression body is:");
            var bodyVisitor = Visitor.CreateFormatExpression(node.Body);
            bodyVisitor.Visit(prefix + ‘\t‘);
        }
    }
    
    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($"{prefix}This binary expression is a {NodeType} expression");
            var left = Visitor.CreateFormatExpression(node.Left);
            Console.WriteLine($"{prefix}The Left argument is:");
            left.Visit(prefix + "\t");
            var right = Visitor.CreateFormatExpression(node.Right);
            Console.WriteLine($"{prefix}The Right argument is:");
            right.Visit(prefix + "\t");
        }
    }
    
    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}");
        }
    }
    
    public class ConditionalVisitor : Visitor
{
    private readonly ConditionalExpression node;
    public ConditionalVisitor(ConditionalExpression node) : base(node)
    {
        this.node = node;
    }

    public override void Visit(string prefix)
    {
        Console.WriteLine($"{prefix}This expression is a {NodeType} expression");
        var testVisitor = Visitor.CreateFormatExpression(node.Test);
        Console.WriteLine($"{prefix}The Test for this expression is:");
        testVisitor.Visit(prefix + "\t");
        var trueVisitor = Visitor.CreateFormatExpression(node.IfTrue);
        Console.WriteLine($"{prefix}The True clause for this expression is:");
        trueVisitor.Visit(prefix + "\t");
        var falseVisitor = Visitor.CreateFormatExpression(node.IfFalse);
        Console.WriteLine($"{prefix}The False clause for this expression is:");
        falseVisitor.Visit(prefix + "\t");
    }
}

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

    public override void Visit(string prefix)
    {
        Console.WriteLine($"{prefix}This expression is a {NodeType} expression");
        if (node.Object == null)
            Console.WriteLine($"{prefix}This is a static method call");
        else
        {
            Console.WriteLine($"{prefix}The receiver (this) is:");
            var receiverVisitor = Visitor.CreateFormatExpression(node.Object);
            receiverVisitor.Visit(prefix + "\t");
        }

        var methodInfo = node.Method;
        Console.WriteLine($"{prefix}The method name is {methodInfo.DeclaringType}.{methodInfo.Name}");
        // There is more here, like generic arguments, and so on.
        Console.WriteLine($"{prefix}The Arguments are:");
        foreach(var arg in node.Arguments)
        {
            var argVisitor = Visitor.CreateFormatExpression(arg);
            argVisitor.Visit(prefix + "\t");
        }
    }
}
    
}