Whatever I tried I cannot set an extension property on a User object, here is a reproducible piece of code:

```
public async Task CleanTest(string extName)
      {
         ExtensionProperty ep = new ExtensionProperty
         {
            Name = extName,
            DataType = "String",
            TargetObjects = { "User" }
         };

         App app = (App)(await _client.Applications.Where(a => a.AppId == _managementAppClientId).ExecuteSingleAsync());
         app.ExtensionProperties.Add(ep);
         await app.UpdateAsync();

         GraphUser user = (GraphUser)(await _client.Users.Where(u => u.UserPrincipalName.Equals("email")).ExecuteSingleAsync());
         string propName = FormatExtensionPropertyName(extName); //formats properly as extesion_xxx_name
         user.SetExtendedProperty(propName, "testvalue");
         //user.SetExtendedProperty(extName, "testvalue");
         await user.UpdateAsync(); // fails here
      }

```

user.UpdateAsync() according to Fiddler doesn't even go out and application fails with an exception:

"The property 'extension\_e206e28ff36244b19bc56c01160b9cf0\_UserEEEqdbtgd3ixx2' does not exist on type 'Microsoft.Azure.ActiveDirectory.GraphClient.Internal.User'. Make sure to only use property names that are defined by the type."