#nullable enable

using System;

using System.Linq;

using System.Linq.Expressions;

using System.Collections.Generic;

using AgileObjects.ReadableExpressions;

​

public class LambdaConverter<TArg, TReturnType> : ExpressionVisitor

{

    private Type? NewArgType { get; set; } = null;

​

    public Expression<Func<TNewArg, TReturnType>> CastArgTo<TNewArg>(Expression<Func<TArg, TReturnType>> lambda)

    {

        NewArgType = typeof(TNewArg);

        return (Expression<Func<TNewArg, TReturnType>>)base.Visit(lambda);

    }

​

    protected override Expression VisitParameter(ParameterExpression node)

    {

        Console.WriteLine("Visiting param");

        if (NewArgType is null)

        {

            return node;

        }

        Console.WriteLine($"Returning different param of new arg type {NewArgType.Name}");

        return Expression.Parameter(NewArgType);

    }

}

​

public class BaseEntity

{

    public int Value { get; init; } = 123;

    public override string ToString() => "BaseEntity";

}

​

public class Foo : BaseEntity

{

    public override string ToString() => "Foo";

}

​

public class Program

{

    public static void Main()

    {

        Expression<Func<BaseEntity,int>> original = foo => foo.Value;

        BaseEntity b = new Foo();

        var converter = new LambdaConverter<BaseEntity,int>();

        var converted = converter.CastArgTo<Foo>(original);

        //Console.WriteLine($"converted: {converted.Compile()(b)}");

    }

}