C#•3mo ago

✅ How to correctly write conditions with expressions?

I have this code:

public static class BookPolicy
    {
        public static void AllowRead(Book book)
        {
            var isHorrorBook = Specifications.IsHorrorBook().Compile();
            var isPonyBook = Specifications.IsPonyBook().Compile();

            if (isHorrorBook(book) || isPonyBook(book))
                throw new Exception("Horror or pony books don't allow here");
        }
    }

    public static class Specifications
    {
        public static Expression<Func<Book, bool>> IsHorrorBook() =>
            book => book.Title.Contains("Frankenstein");
        
        public static Expression<Func<Book, bool>> IsPonyBook() =>
            book => book.Title.Contains("Pony");
    }

public static class BookPolicy
    {
        public static void AllowRead(Book book)
        {
            var isHorrorBook = Specifications.IsHorrorBook().Compile();
            var isPonyBook = Specifications.IsPonyBook().Compile();

            if (isHorrorBook(book) || isPonyBook(book))
                throw new Exception("Horror or pony books don't allow here");
        }
    }

    public static class Specifications
    {
        public static Expression<Func<Book, bool>> IsHorrorBook() =>
            book => book.Title.Contains("Frankenstein");
        
        public static Expression<Func<Book, bool>> IsPonyBook() =>
            book => book.Title.Contains("Pony");
    }

everything is ok but i would like to know is it possible to write smth like this? if(book is Horror or Pony) {...} I don't understand how to do this with expressions

2 Replies

Anton•3mo ago

yes Well, not exactly But similar I think you can do

(book.GetType() == typeof(Horror) || book.GetType() == typeof(Pony)) ? DoStuff1() : DoStuff2()

(book.GetType() == typeof(Horror) || book.GetType() == typeof(Pony)) ? DoStuff1() : DoStuff2()

chatgpt at work (it's just tedious to write out manually)

using System;
using System.Linq.Expressions;

public class Book { }
public class Horror : Book { }
public class Pony : Book { }

public class Program
{
    public static void Main()
    {
        // Assuming these are the provided expressions
        Expression<Func<Book, bool>> isHorror = book => book.GetType() == typeof(Horror);
        Expression<Func<Book, bool>> isPony = book => book.GetType() == typeof(Pony);

        ParameterExpression bookParam = isHorror.Parameters[0];

        var parameterReplacer = new ParameterReplacer(bookParam);
        Expression isPonyReplaced = parameterReplacer.Visit(isPony.Body);

        Expression combinedCondition = Expression.OrElse(isHorror.Body, isPonyReplaced);

        MethodCallExpression doStuff1Call = Expression.Call(typeof(Program).GetMethod(nameof(DoStuff1)));
        MethodCallExpression doStuff2Call = Expression.Call(typeof(Program).GetMethod(nameof(DoStuff2)));

        Expression ternaryExpression = Expression.Condition(combinedCondition, doStuff1Call, doStuff2Call);

        var lambda = Expression.Lambda<Action<Book>>(ternaryExpression, bookParam);
    }

    public static void DoStuff1()
    {
        Console.WriteLine("DoStuff1 called");
    }

    public static void DoStuff2()
    {
        Console.WriteLine("DoStuff2 called");
    }

    // Helper class to replace parameters in an expression
    private class ParameterReplacer : ExpressionVisitor
    {
        private readonly ParameterExpression _parameter;

        public ParameterReplacer(ParameterExpression parameter)
        {
            _parameter = parameter;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return _parameter; // Replace with the correct parameter
        }
    }
}

using System;
using System.Linq.Expressions;

public class Book { }
public class Horror : Book { }
public class Pony : Book { }

public class Program
{
    public static void Main()
    {
        // Assuming these are the provided expressions
        Expression<Func<Book, bool>> isHorror = book => book.GetType() == typeof(Horror);
        Expression<Func<Book, bool>> isPony = book => book.GetType() == typeof(Pony);

        ParameterExpression bookParam = isHorror.Parameters[0];

        var parameterReplacer = new ParameterReplacer(bookParam);
        Expression isPonyReplaced = parameterReplacer.Visit(isPony.Body);

        Expression combinedCondition = Expression.OrElse(isHorror.Body, isPonyReplaced);

        MethodCallExpression doStuff1Call = Expression.Call(typeof(Program).GetMethod(nameof(DoStuff1)));
        MethodCallExpression doStuff2Call = Expression.Call(typeof(Program).GetMethod(nameof(DoStuff2)));

        Expression ternaryExpression = Expression.Condition(combinedCondition, doStuff1Call, doStuff2Call);

        var lambda = Expression.Lambda<Action<Book>>(ternaryExpression, bookParam);
    }

    public static void DoStuff1()
    {
        Console.WriteLine("DoStuff1 called");
    }

    public static void DoStuff2()
    {
        Console.WriteLine("DoStuff2 called");
    }

    // Helper class to replace parameters in an expression
    private class ParameterReplacer : ExpressionVisitor
    {
        private readonly ParameterExpression _parameter;

        public ParameterReplacer(ParameterExpression parameter)
        {
            _parameter = parameter;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return _parameter; // Replace with the correct parameter
        }
    }
}

also fyi they aren't cached the whole tree gets recreated each time you call that method make it a readonly field for it to get cached

SWEETPONY•3mo ago

ah thanks for helping me

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✅ How to correctly write conditions with expressions?