Fluent validations. A hands-on introduction.

Tell your clients what they are doing wrong :)

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TL;DR

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Introduction

Straight to the point! 😄

Let’s imaging we need to develop and REST-API. For the sake of the example, we will build the API using Asp.Net core with C#.

This API will have one endpoint that will:

• Receive a parameter from a header named header-name.
• Receive a parameter from the query named query-property-name.
• Receive two dates from the request body named pastDate and futureDate.

Basically, a curl request to this endpoint would look something like this:

curl --location --request GET 'https://localhost:5001/fv?query-prop-name=10' \
--header 'header-prop-name: header-value' \
--header 'Content-Type: application/json' \
--data-raw '{
    "Date1": 1,
    "Date2": 2
}'

Validations

Because we want to see how well FluentValidations work, let’s say we have the following rules our request must follow:

• The body must be informed.
• The futureDate must be larger than the pastDate.
• The pastDate must be larger than or equal to 1.
• The header must be informed and can not be empty.
• The query parameter must be informed, can not be empty, and must be greater than or equal to 0.

To do so, create the endpoint in C# as follows:

[ApiController]
[Route("[controller]")]
public class FVController : ControllerBase
{
    public FVController()
    {
    }
    
    [HttpGet]
    public IActionResult Get([FromQuery] RequestToValidate request)
    {
        return Ok();
    }
}

where the RequestToValidate.cs class is:

public class RequestToValidate
{
    [FromHeader(Name = ApiConstants.HeaderName)]
    public string PropertyFromHeader { get; set; }
    [FromQuery(Name = ApiConstants.QueryPropertyName)]
    public int? PropertyFromQuery { get; set; }
    [FromBody]
    public RequestBody Body { get; set; }
    public class RequestBody
    {
        public int PastDate { get; set; }
        public int FutureDate { get; set; }
    }
}

Then, inject in the DI container the following class by adding this to your startup.cs class:

public void ConfigureServices(IServiceCollection services)
{    
    services.AddControllers();
    services.AddMvc().AddFluentValidation();
    services.AddTransient<IValidator<RequestToValidate>, RequestToValidateValidator>();
}

Run the API by pressing F5 and call the GET endpoint. You will receive a response like this one:

{
    "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
    "title": "One or more validation errors occurred.",
    "status": 400,
    "traceId": "|cd6c6f05-4f8c3911f4c0410d.",
    "errors": {
        "header-name": [
            "header-name must be informed as a header"
        ],
        "query-property-name": [
            "query-property-name must be informed as a query parameter"
        ],
        "body.PastDate": [
            "PastDate must be larger or equal to 1.",
            "FutureDate must be after PastDate"
        ]
    }
}

And that is all😊❗ Easy, right?