Stop ASP.NET MVC from calling all getters in a model class - asp.net

Question

If you define the following two properties on your model class this will crash with a NullReferenceException during model binding:
public Customer Customer { get; private set; } //set in the action method
public bool Name => Customer.Name;
This is because Customer is still null during model binding and ASP.NET MVC calls the getter for Name.
The stack is:
System.ComponentModel.ReflectPropertyDescriptor.GetValue(Object component) +525
System.Web.Mvc.ModelMetadata.get_Model() +34
System.Web.Mvc.DataAnnotationsModelValidator.Validate(Object container) +151
System.Web.Mvc.<Validate>d__1.MoveNext() +387
System.Web.Mvc.DefaultModelBinder.OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext) +163
System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +83
System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1754
System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) +460
System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +137
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +982
System.Web.Mvc.<>c__DisplayClass22.<BeginExecuteCore>b__1e() +39
System.Web.Mvc.Async.AsyncResultWrapper.<.cctor>b__0(IAsyncResult asyncResult, Action action) +21
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +53
System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +36
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +38
System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +44
System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +65
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +399
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +137
From the stack it looks like model validation is querying all getters. I'm not using model validation.
How can I deal with this situation? Can I make ASP.NET MVC not call all getters without any (apparent) reason?

Answer 1

So the Model Binder new's up an instance of your Model, then is probably doing reflection over the model's properties to look for named matches with the values in the FormCollection. What's happening is that the Customer prop is null when that dangerous Name prop is called, thus the NullRef.
The order by which .NET is checking those properties might not be actually random, but your code will be much improved by just treating it as such. Calling a method/prop directly on a Class that's nullable by default is a terrible idea, unless you check it for null. You have two options here, either (1) redesign your Model class so that the Constructor initializes the "Customer" property, or (b) add a null-check in that "Name" method.
Here's the easiest approach to just null-checking it when you grab it:
public bool Name => Customer?.Name ?? false;
This does not solve the underlying issue, which is that you have a Model that has nullable props chained together. Don't worry about your Model's constructor messing up your model binding. The Model Binder will (1) initialize your model class, then (2) try to hydrate it. So initializing the Customer class/prop in your Model's constructor won't impact any mapping of UI fields to say that Customer's fields.

Answer 2

The DefaultModelBinder in MVC version 5.2.3 does validation in addition to binding, and there is no way to shut it off completely. Other SO posts mention turning off the implicit required attribute for value types with the following line of code in your Global.asax.cs, Application_Start() method...
DataAnnotationsModelValidatorProvider.AddImplicitRequiredAttributeForValueTypes = false;
See: https://stackoverflow.com/a/2224651 (which references a forum with an answer directly from the asp.net team).
Given your stack trace, that might fix your immediate problem. However, that probably won't be enough due to the DefaultModelBinder calling the getters even outside of its validation code for no stated reason (the source code makes no comment as to why it does that).
To solve the problem on my projects, where I use calculated properties all the time similar to your example, I implemented a custom model binder based on the original DefaultModelBinder source code that does not call any getters.
See a more detailed explanation and my full solution here: https://stackoverflow.com/a/54431404/10987278.