Argument validation with attributes and Ninject interceptor

1. Introduction

It is a common practice to validate function arguments before running actual code. Usually validation looks like that:

There is nothing wrong with this code, however we have to repeat this piece of code in every function. Of course we could create some helper class for argument validation and call approperiate validation function before launching rest of the code

However I was searching for more universal way for argument validation. That is why, I decided to try function interceptors and attributes.

2. Creating validation attributes

Let’s start for creating base attribute

In the next step let’s create specialized attribute for “not null” validation. This argument may look this way:

3. Using IInterceptor interface

That was an easy part, now it’s necessary to somehow intercept function execution and run attribute validation.In .NET there is no build-in mechanism to do that,that is why I’ll use extension for Ninject – Ninject Interception. First of all we have to add to our project three dll’s

  • Ninject
  • Ninject.Extensions.Interception
  • Ninject.Extensions.Interception.LinFu

You can easily add them using NuGet. Having all necessary files added to solution,let’s create function interceptor – ValidaitionInterceptor.

As You can see ValidationInterceptor implements interface IInterceptorwhich belongs to Ninject.Extensions.Interception. This interface has one method Intercept which is called before function from registered component is called. In this function I iterate over all function attributes of type ArgumentValidationAttribute and call ValidateArgument function. If validation passess function invocation.Proceed() is called, which continues execution of original function.

4. Example of usage

In order to take advantage of benefits of function interceptors, we have to resolve objects from Ninject container. First of all I’ll create some examplorary classes

In the next step I’ll create simple wrapper for Ninject container and register CarRepository in Kernel

Please, notice that registration of CarRepository was used with option Intercept().With(). Thanks to this configuration,everytime we call method from CarRepository, Intercept method from ValidaitionInterceptor will be called first. Now, it is time to use our repository,we could do that this way

Argument entity in method Update from interface IRepository was decorated with NotNull, that is why our validation mechanism should throw exception.
Argument validation

5. Validation of return value

Using IInterceptor we could go one step further and also validate return value of method. In order to do that, we have to slightly modify Intercept method in ValidaitionInterceptor class.

We also have to change attribute usage of ArgumentValidationAttribute to

Now, we can decorate function GetAll with NotNullattribute

Since this moment, everytime function GetAll returns null, the ArgumentNullException will be thrown.

6. Drawbacks

Unfortunately using attributes for argument validation have some drawbacks. First of all, argument validation is based on function interceptors, and as I mentioned before only function, which are in objects, which are resolved from IoC container can be intercepted. What is more, interceptors allow us to intercept only functions marked as virtual or inhirited from interface. Source code for this post can be found here

Argument validation with attributes and Ninject interceptor