Set property value before using any Interfaces

samples/ControlCatalog/Models/Person.cs

@@ -21,12 +21,12 @@ namespace ControlCatalog.Models
             get => _firstName;
             set
             {
+                _firstName = value;
                 if (string.IsNullOrWhiteSpace(value))
                     SetError(nameof(FirstName), "First Name Required");
                 else
                     SetError(nameof(FirstName), null);
 
-                _firstName = value;
                 OnPropertyChanged(nameof(FirstName));
             }
 
@@ -37,12 +37,12 @@ namespace ControlCatalog.Models
             get => _lastName;
             set
             {
+                _lastName = value;
                 if (string.IsNullOrWhiteSpace(value))
                     SetError(nameof(LastName), "Last Name Required");
                 else
                     SetError(nameof(LastName), null);
 
-                _lastName = value;
                 OnPropertyChanged(nameof(LastName));
             }
         }
@@ -95,4 +95,4 @@ namespace ControlCatalog.Models
                 return null;
         }
     }
-}
+}