Creating a new C# API: Validate incoming requests

Tim Deschryver

The key to successful long-term software is a good architectural design. A good design doesn't only allow developers to come in and write new features with ease, but it's also is adaptive to changes without resistance.

This is what Eric Evans calls a supple design in his book Domain Driven Design:


To have a project accelerate as development proceeds—rather than get weighed down by its own legacy—demands a design that is a pleasure to work with, inviting to change. A supple design.

A good design focuses on the core of the application, the domain.

Sadly, it's easy to clutter the domain with responsibilities that don't belong here. With each addition, it becomes harder to read and understand the core domain.

Therefore, it's important to guard that application logic drops into the domain layer. One of the culprits of this, is the validation of incoming requests. In this blog post, we'll learn how to validate the incoming request before it reaches the domain level. With the result that the domain layer remains a layer that's focused on the core domain.

This blog post assumes that the API uses the command pattern to translate incoming requests to commands or queries. All the snippets throughout this blog post are using the MediatR package.

The command pattern has the benefit that it can decouple the core logic from the API layer (we also don't want thick and messy controllers). Most of the packages that implement the command pattern, expose a middleware pipeline that can be hooked into. This pipeline is useful because it offers a solution and a centralized location to add common application logic that needs to be executed for each command.

A MediatR Request link

Before we start validating the incoming request, let's first cover the basics of a MediatR request.

A request is created by creating a POCO that implements the MediatR.IRequest interface.


With the new record type, introduced in C# 9, defining a request becomes a one-liner. The additional benefit is that the record instance is immutable and side-effect free, making the command predictable and reliable.

In the example below a AddProductToCartCommand is defined. I use the Command suffix to indicate that the request is a command, meaning it will try to execute an operation. Another suffix you'll see in our application is Query, which indicates that data will be queried.

Such a request just holds data, and is bound to a specific handler. The handler contains the logic to execute the request (command or query).

To dispatch the above command, the incoming HTTP request is mapped to a command in the controller. Then, the command is dispatched to the MediatR pipeline using the IMediator instance.

MediatR Validator Behavior link

We don't want to validate a command (AddProductToCartCommand in the example) in the controller, nor in the domain. Instead, we want to validate it in the pipeline with a pipeline behavior. By using a pipeline behavior it's possible to execute logic before and/or after a command is handled by its handler. With behaviors, we can centralize common-validation logic and enforce a standard.

In our case, we want to validate the command before it reaches its handler. When the command is found valid, then the handler is executed. Otherwise, when it's invalid, it will short-circuit and we'll resolve the invalid request later in this blog post.

This way we don't have to worry anymore if the command is valid when it reaches the handler. While this seems like a trivial change, it declutters every handler in the domain layer.

In the snippet below, we create a new pipeline behavior ValidatorPipelineBehavior to validate the commands. When a command is sent, the ValidatorPipelineBehavior handler receives the command before it reaches the command handler. The ValidatorPipelineBehavior validates if that command is valid by invoking the validators corresponding to that type. The request is only allowed to pass to the next handler when the request is valid. If not, an InputValidationException exception is thrown.

We'll look at how we create our validators in Validation with FluentValidation. For now, it's important to know that when a request is invalid, the validation messages are returned (with a reference to the invalid property). The validation details are added to the exception and will be used later to create the response.

Validation with FluentValidation link

The behavior we created in the previous section is using the FluentValidation package. With FluentValidation, we create "rules" for a specific class. This is done in a separate class that implements the AbstractValidator<T> abstract class.

The reasons why I like to use FluentValidation are:

The validator should contain rules, defined by the RuleFor method, that simply validate the shape of the request. I find that adding complex business logic here is an antipattern, and that those cases should remain in the domain.

In our case, we create a validator for each command. Just like the one below, where we create a validator, AddProductToCartCommandValidator, for the AddProductToCartCommand command. The validator just verifies that all fields have a value.

Registering MediatR and FluentValidation into the application link

Now that we have our validation pipeline behavior, and we've also created a validator, we can register them to the DI container.

Problem Details for HTTP APIs link

Everything is now ready to make the first request. When we try it out and send an invalid request, we receive an Internal Server Error (500) response. This is good, but this doesn't reflect a good experience for the consumers, and I'm sure we can do better. No, we must do better.

To create a better experience for the consumers, users, developers, or even a 3rd party, an enhanced result will make it clear why a request fails. This practice makes the integration with your API easier, better, and also faster.

I'm putting an emphasis on this because I recently had to integrate with a service that didn't keep this in mind. This lead to many frustrations on my end, and I was happy when the integration was finally over, even when I knew that it wasn't perfect. I'm sure that the implementation would have been faster, and that the end result would have been better if there was given more thought to the response of a failed request.

Because of this experience, I try my best to help my future-self and other developers by providing a better response. Luckily, I don't have to think of one because there's a standardized response for bad requests. This is also known as Problem Details for HTTP APIs.


The next part of this blog post could be outdated if you're using .NET 7. For more info see Problem Details responses everywhere with ASP.NET Core and .NET 7 by Filip W.

The .NET framework provides a class that implements the specifications of a problem detail, ProblemDetails. In fact, a .NET API already returns a problem detail response for some invalid requests. For example, the following response is returned when an invalid parameter is used in a route parameter (e.g a string instead of an int).

Mapping responses to Problem Details link

To implement our problem details, in our case the InputValidationException exception, we can overwrite the response with either the exception middleware, or with an exception filter.

In the snippet below, we're using the middleware to retrieve the details of an exception when one is raised in the application. Based on these exception details, the problem detail object is build up.

All thrown exceptions are caught by the middleware, so you can create specific problem details for each exception. In the following example, only the InputValidationException exception is mapped, the rest of the exceptions are all treated the same.

With the exception handler in place, the following response is returned when the pipeline behavior detects an invalid command. For example, when the AddProductToCartCommand command is sent with a negative amount.


Instead of creating a custom exception handler and mapping the exception to problem details, it's also possible to use the Hellang.Middleware.ProblemDetails package. The Hellang.Middleware.ProblemDetails package makes it easy to map exceptions to problem details, with almost no code.

Consistent Problem Details link

There's one last problem. The above snippets expect that a MediatR request is created within the controller. But an API endpoint that contains a body is automatically be validated by the .NET Model Validator. When the endpoint receives an invalid body a default response is returned before it's caught by our pipeline. This means that our problem details implementation won't be used to return a consistent response.

I didn't expect this at first, and it took me a while to figure out why this happens and how to make sure the response objects stay consistent. As a possible fix, we can suppress this default behavior so the invalid request will be handled by our pipeline.

But this has a drawback. By suppressing the invalid model filter, invalid primitive types aren't caught anymore. For an endpoint that expects a number but receives a string, the expected primitive type (in this case the number) is assigned to the default value (0 in this case). Turning off the invalid model filter may lead to unexpected bugs due to this. Previously, this action would lead to a bad request (400).

That's why I prefer to reuse the InputValidationException exception when the endpoint receives a bad input implementing a InvalidModelStateResponseFactory.

Conclusion link

In this post, we've seen how to centralize the validation logic before a command reaches the domain layer by using a MediatR pipeline behavior. This has the benefit that all the commands are validated, and handlers only receive valid commands. Resulting that the domain remains clean and simple.

Because there's a clear separation, we only has to focus on the task that's in plain sight. Another advantage that you'll notice during the development process, is that unit tests are more focused and easier to write. In the future, it's also easier to replace the validation layer, if needed.

We've also learned that there's a standardized response to specify errors with Problem Details. By following the Problem Details specification we don't have to reinvent the wheel, and we create a better experience for the consumers of our API.

Incoming links

Feel free to update this blog post on GitHub, thanks in advance!

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