.NET

I think all Resharper users are familiar with snippets. Resharper’s snippet basically generates a piece of code, which can be easily customised on the fly. All you have to do is to press TAB, and you can change names of auto-generated variables etc. Today, I decided to write my own snippet, which would make writing unit tests easier. In general, all my unit test methods looks like this

[Test]
public void MethodName_Scenario_ExpectedResult_Test()
{
    //test logic goes in here            
}

[Test]

public void MethodName_Scenario_ExpectedResult_Test()

{

//test logic goes in here

}

So let’s write a snippet which will generate a method with this signature. In order to do this, let’s choose a Templates Explorer… from the Resharper’s menu.

In the opened window, click on the plus sign and a new tab should appear.

Let’s name our snippet test and paste the following code into the main editor.

[Test]
public void MethodName_Scenario_ExpectedResult_Test()
{
           
}

[Test]

public void MethodName_Scenario_ExpectedResult_Test()

{

}

Now, we’ve created a simplified snippet for generating our test function signature. However, this snippet is not really useful as its user cannot navigate through the underscored chunks. In order to employ the Resharper’s navigation features, we have to slightly modify our snippet. Let’s replace the previous code with this one:

[Test]
public void $MethodName$_$Scenario$_$ExpectedResult$_Test()
{
    $END$
}

[Test]

public void $MethodName$_$Scenario$_$ExpectedResult$_Test()

{

$END$

}

Please notice that now some parts of the method’s name are written between dollar signs. Thanks to this syntax, the Resharper is able to iterate over these parts and lets us change their names on the fly. $END$ is a special control sequence which tells the Resharper where to stop iteration over variables. At this moment, our snippet is fully functional and we can use it by typing the word test and hitting the TAB button.

1. Introduction

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

public  void Update(Car entity)
{
    if(entity ==null)
        throw new ArgumentNullException("entity");
}

public void Update(Car entity)

{

if(entity ==null)

throw new ArgumentNullException("entity");

}

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

public static class ValidationHelper
{
    public static void CheckNotNull(object entity, string argumentName)
    {
        if (entity == null)
            throw new ArgumentNullException(argumentName);
    }
}

public  void Update(Car entity)
{
    ValidationHelper.CheckNotNull(entity, "entity");
    //rest of the code
}

public static class ValidationHelper

{

public static void CheckNotNull(object entity, string argumentName)

{

if (entity == null)

throw new ArgumentNullException(argumentName);

}

public void Update(Car entity)

{

ValidationHelper.CheckNotNull(entity, "entity");

//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

[AttributeUsage(AttributeTargets.Parameter)]
public abstract class ArgumentValidationAttribute : Attribute
{
    public abstract void ValidateArgument(object value, string argumentName);
}

[AttributeUsage(AttributeTargets.Parameter)]

public abstract class ArgumentValidationAttribute : Attribute

{

public abstract void ValidateArgument(object value, string argumentName);

}

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

public class NotNullAttribute : ArgumentValidationAttribute
{
    public override void ValidateArgument(object value,string argumentName)
    {
        if (value == null)
            throw new ArgumentNullException(argumentName, string.Format("Value {0} can not be null", argumentName));
    }
}

public class NotNullAttribute : ArgumentValidationAttribute

{

public override void ValidateArgument(object value,string argumentName)

{

if (value == null)

throw new ArgumentNullException(argumentName, string.Format("Value {0} can not be null", argumentName));

}

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.

public class ValidationInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        var parameters = invocation.Request.Method.GetParameters();
        for (int index = 0; index < parameters.Length; index++)
        {
            foreach (ArgumentValidationAttribute attr in parameters[index].GetCustomAttributes(typeof(ArgumentValidationAttribute), true))
                attr.ValidateArgument(invocation.Request.Arguments[index], parameters[index].Name);
        }
        invocation.Proceed();       
    }
}

public class ValidationInterceptor : IInterceptor

{

public void Intercept(IInvocation invocation)

{

var parameters = invocation.Request.Method.GetParameters();

for (int index = 0; index < parameters.Length; index++)

{

foreach (ArgumentValidationAttribute attr in parameters[index].GetCustomAttributes(typeof(ArgumentValidationAttribute), true))

attr.ValidateArgument(invocation.Request.Arguments[index], parameters[index].Name);

}

invocation.Proceed();

}

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

public interface IRepository<T> where T:class
{
    List<T> GetAll();
    void Update([NotNull] T entity);
}

public class Car
{
    public int Id { get; set; }
    public string Model { get; set; }
}

public class CarRepository : IRepository<Car>
{
    public  List<Car> GetAll()
    {
        return null;
    }

    public  void Update(Car entity)
    {
            
    }
}

public interface IRepository<T> where T:class

{

List<T> GetAll();

void Update([NotNull] T entity);

}

public class Car

{

public int Id { get; set; }

public string Model { get; set; }

}

public class CarRepository : IRepository<Car>

{

public List<Car> GetAll()

{

return null;

}

public void Update(Car entity)

{

}

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

public static class IOCContainer
{
    private static readonly IKernel Kernel;
    static IOCContainer()
    {
        Kernel = new StandardKernel();
    }

    public static void Configure()
    {
        Kernel.Bind(typeof(IRepository<Car>)).To(typeof(CarRepository)).Intercept().With<ValidaitionInterceptor>();
    }

    public static T Get<T>()
    {
        return Kernel.Get<T>();
    }
}

public static class IOCContainer

{

private static readonly IKernel Kernel;

static IOCContainer()

{

Kernel = new StandardKernel();

}

public static void Configure()

{

Kernel.Bind(typeof(IRepository<Car>)).To(typeof(CarRepository)).Intercept().With<ValidaitionInterceptor>();

}

public static T Get<T>()

{

return Kernel.Get<T>();

}

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

class Program
{
    static void Main(string[] args)
    {
        IOCContainer.Configure();
        var carRepository = IOCContainer.Get<IRepository<Car>>();
        carRepository.Update(null);
    }
}

class Program

{

static void Main(string[] args)

{

IOCContainer.Configure();

var carRepository = IOCContainer.Get<IRepository<Car>>();

carRepository.Update(null);

}

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

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.

public class ValidaitionInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        var parameters = invocation.Request.Method.GetParameters();
        for (int index = 0; index < parameters.Length; index++)
        {
            foreach (ArgumentValidationAttribute attr in parameters[index].GetCustomAttributes(typeof(ArgumentValidationAttribute), true))
                attr.ValidateArgument(invocation.Request.Arguments[index], parameters[index].Name);
        }

        invocation.Proceed();

        foreach (ArgumentValidationAttribute attr in invocation.Request.Method.ReturnTypeCustomAttributes.GetCustomAttributes(typeof(ArgumentValidationAttribute), true))
        {
            attr.ValidateArgument(invocation.ReturnValue, "return value");
        }
    }
}

public class ValidaitionInterceptor : IInterceptor

{

public void Intercept(IInvocation invocation)

{

var parameters = invocation.Request.Method.GetParameters();

for (int index = 0; index < parameters.Length; index++)

{

foreach (ArgumentValidationAttribute attr in parameters[index].GetCustomAttributes(typeof(ArgumentValidationAttribute), true))

attr.ValidateArgument(invocation.Request.Arguments[index], parameters[index].Name);

}

invocation.Proceed();

foreach (ArgumentValidationAttribute attr in invocation.Request.Method.ReturnTypeCustomAttributes.GetCustomAttributes(typeof(ArgumentValidationAttribute), true))

{

attr.ValidateArgument(invocation.ReturnValue, "return value");

}

We also have to change attribute usage of ArgumentValidationAttribute to

[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.ReturnValue)]

1	[AttributeUsage(AttributeTargets.Parameter \| AttributeTargets.ReturnValue)]

Now, we can decorate function GetAll with NotNullattribute

[return: NotNull]
List<T> GetAll();

1 2	[return: NotNull] List<T> GetAll();

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

.NET BLOG

.NET, C# blog

Resharper – custom snippet

Argument validation with attributes and Ninject interceptor

1. Introduction

2. Creating validation attributes

3. Using IInterceptor interface

4. Example of usage

5. Validation of return value

6. Drawbacks