using System;

using System.Linq;  

​

public class Program

{

    public static void Main()

    {

        var ecritures = new []{

            new Ecriture{ Debit = 200 },

            new Ecriture{ Credit = 550 }

        };

        var solde = ecritures.Sum(e=> e.Credit - e.Debit);

        Console.WriteLine("Solde:" + solde);

    }

    public class Ecriture{

        public decimal? Debit{get;set;}

        public decimal? Credit{get;set;}

    }

}