Class user usable. Implementation of excpetions with summary

MCTG/MCTGLib/User.cs

@@ -33,18 +33,17 @@ namespace MCTGLib
             get => picture;
             set
             {
-                picture = value;
-                if (picture == null)
+                if (string.IsNullOrWhiteSpace(value))
                 {
-                    picture = "PhotoParDefaut";
-                }
+                    throw new ArgumentNullException("Erreur pour la photo d'utilisateur!");
                 }
             
+          
         }
         /// <summary>
-        /// Property to get Name of users, without setter because we don't want users 
-        /// to change their Name.
+        /// Property to get Name of users and a setter 
         /// </summary>
+        /// <exception cref="ArgumentNullException" >Setter have Exception which is trigger when Name is null</exception>
         public string Name
         {
             get { return name; }
@@ -52,15 +51,26 @@ namespace MCTGLib
             {
                 if (string.IsNullOrWhiteSpace(value)) 
                 {
-                    throw new ArgumentNullException();
+                    throw new ArgumentNullException("Il y a une erreur de Nom d'utilisateur!");
                 }
                 name = value;
             }
         }
+        /// <summary>
+        /// Property to get Surname of users and a setter 
+        /// </summary>
+        /// <exception cref="ArgumentNullException" >Setter have Exception which is trigger when Surname is null</exception>
         public string Surname
         {
             get { return surname; }
-            private set { surname = value; }
+            private set
+            {
+                if (string.IsNullOrWhiteSpace(value))
+                {
+                    throw new ArgumentNullException("Il y a une erreur de Prénom d'utilisateur");
+                }
+                surname = value;
+            }
         }
         public string Mail
         {