Test.cs

List<Quantite> list = new List<Quantite>
{
    new Quantite
    {
        Poids = 0,
        Palette = 33,
        UniteSpecifique = "test"
    }
};

foreach (var quant in list)
{
    var quantite = quant.GetType()
                        .GetProperties()
                        .Select(x => new { nom = x.Name, value = x.GetValue(quant), type = x.PropertyType })
                        .ToList();

    foreach (var property in quantite)
    {
        if (property.type == typeof(int?))
        {
            Console.WriteLine($"{property.nom} has type int?");
        }
        else if (property.type == typeof(string))
        {
            Console.WriteLine($"{property.nom} has type string");
        }
        else if (property.type == typeof(decimal?))
        {
            Console.WriteLine($"{property.nom} has type decimal?");
        }
        // etc
    }
}