with the modifications it's better

MCTG/MCTGApp/Program.cs

@@ -13,7 +13,7 @@ foreach (User user in users)
     try
     {
         Console.WriteLine(user.Name);
-        Console.WriteLine(user.Priorities.ToString());
+        Console.WriteLine(user.Priorities[0].ToString());
     } catch (ArgumentNullException erreurNomVide)
     {
         Console.WriteLine(erreurNomVide.Message);

MCTG/MCTGLib/Priority.cs

@@ -12,6 +12,8 @@ namespace MCTGLib
     /// </summary>
     public enum Priority
     { Economic, Fast, Easy, Light, Gourmet };
+
+   
 }
 
 

MCTG/MCTGLib/User.cs

@@ -15,9 +15,9 @@ namespace MCTGLib
     {
         #region Private Attributes
 
-        private string Name;
+        private string name;
-        private string Surname;
+        private string surname;
-        private string Mail;
+        private string mail;
 
         #endregion
 
@@ -45,16 +45,35 @@ namespace MCTGLib
         /// Property to get Name of users, without setter because we don't want users 
         /// to change their Name.
         /// </summary>
-        public string UserName
+        public string Name
         {
-            get { return Name; }
+            get { return name; }
+            private set 
+            {
+                if (string.IsNullOrWhiteSpace(value)) 
+                {
+                    throw new ArgumentNullException();
+                }
+                name = value;
+            }
         }
+        public string Surname
+        {
+            get { return surname; }
+            private set { surname = value; }
+        }
+        public string Mail
+        {
+            get { return mail; }
+            private set { mail = value; }
+        }
+
 
         private List<Priority> priorities;
         public List<Priority> Priorities
         {
-            get;
+            get => priorities;
-            set;
+            set=> priorities = value;
         }
 
 
@@ -71,12 +90,12 @@ namespace MCTGLib
             Name = name;
             Surname = surname;
             Mail = mail;
-            Priorities = new List<Priority> {
+            priorities = new List<Priority> {
-            Priority.Gourmet,
+                Priority.Gourmet,
-            Priority.Economic,
+                Priority.Economic,
-            Priority.Fast,
+                Priority.Fast,
-            Priority.Light,
+                Priority.Light,
-            Priority.Easy};
+                Priority.Easy};
             ProfilPict = picture;
 
         }