Solve real coding problems
Articles about ASP.NET Core web development.
I wrote about how to add custom validation attributes. These are used for model validation on the server-side. You can also use these for client-side validation, which I’ll show in this article. 1 – Implement IClientModelValidator The first step is to implement the IClientModelValidator interface in the custom validation attribute class. This has a single … Read more
When you send a request to ASP.NET with a JSON body, you get the following exception: System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $ | LineNumber: 0 | BytePositionInLine: 1. You can’t deserialize JSON to a string. You’re either using a string parameter with [FromBody] or your model has a string … Read more
ASP.NET Core gives you a chance to gracefully shutdown your background services (such as cleaning up and/or finishing in-progress work). By default, you’re given a grand total of 5 seconds to shutdown all background services. If you need more time than this, there are a few ways to control the graceful shutdown time, which I’ll … Read more
Manually validating a model can mean a few different things. It depends on what you’re trying to do exactly. Are you trying to validate a model object against its validation attributes? Use TryValidateModel(). Are you trying to do validation logic manually (instead of using validation attributes)? You can add errors to ModelState in that case. … Read more
There are two ways to receive CSV data in a web API: In this article, I’ll show examples of both of these approaches. I’ll be using the CsvHelper library to parse CSV data into model objects and then do model validation. Note: To use CsvHelper, install the CsvHelper package (Install-Package CsvHelper). Or use whichever parser … Read more
In this article, I’ll show how to unit test a custom InputFormatter. The main thing to test is the output of the ReadRequestBodyAsync() method. To test this, you have to pass in an InputFormatterContext object containing the request body. As an example, I’ll show how to unit test the following ReadRequestBodyAync() method: Note: This is … Read more
Input formatters are used to deserialize the request body to a model object (which is then passed into an action method). There are built-in input formatters for handling JSON and XML. You can add your own input formatter when you want to customize request body deserialization. There are two scenarios where a custom InputFormatter would … Read more
Receiving requests with XML content is straightforward. You have to register the built-in XML InputFormatter (otherwise you get 415 – Unsupported Media Type errors). When a request comes in with an XML Content-Type (such as application/xml), it uses this InputFormatter to deserialize the request body XML. In this article, I’ll show step-by-step how to receive … Read more
When a request comes in and your action method has parameters, the framework tries to find the appropriate InputFormatter to handle deserializing the request data. There’s no built-in text/plain InputFormatter though, so when you send a request with text/plain content, it fails with a 415 – Unsupported Media Type error response. In this article, I’ll … Read more
When you have multiple parameters on an action method that are bound to the request body (implicitly or explicitly), you get the following fatal exception upon starting the web API: System.InvalidOperationException: Action ‘RecipeController.Post’ has more than one parameter that was specified or inferred as bound from request body. Only one parameter per action may be … Read more
There are four basic ways to receive parameters in an ASP.NET Core Web API: query strings, path parameters, request body, and request headers. I’ll show examples of these below. Query string parameters Let’s say you have two query string keys: name and servings. To add these query string parameters, add method parameters with names that … Read more
Action filters can be added globally, per-controller, and for specific actions. When you add filters globally or per-controller, it applies the filter to multiple actions. This is convenient, but what if you want to add the filter for *most* actions, but not all of them? One way to do that is by disabling the filter … Read more
The simplest way to log requests/responses is to use the HTTP Logging middleware (added in v6). This is configurable, so you can make it suit your needs. If you need more control, you can add your own middleware instead. To use the HTTP Logging middleware, call UseHttpLogging() in your initialization code: Then add Microsoft.AspNetCore.HttpLogging.HttpLoggingMiddleware to … Read more
There are two ways to get request headers: When a request comes in, the framework loads request headers into the Request.Headers dictionary. You can use this just like any other dictionary. Here’s an example of using TryGetValue() to check if a request header exists and get its value: Note: To just check if a header … Read more
There are many built-in model validation attributes available – such as [Required] and [Range] – which you can use to handle most validation scenarios. When these aren’t sufficient, you can create a custom validation attribute with your own validation logic. I’ll show an example of how to do that. 1 – Subclass ValidationAttribute and implement … Read more
Problem When you send a request to an endpoint, you get the following error response: Note: If you’re using Swagger, you may see this in the UI as a generic error: “Failed to load API definition … response status is 500.” If you look in the ASP.NET Core Web Server output in Visual Studio, you … Read more
It’s always a good idea to validate data coming into your web API. There are two steps you can do to guard against invalid data: Here’s an example of using model validation attributes: When a request comes in, the framework does two things: Let’s say you send in the following request invalid data (boxOfficeMillions is … Read more
A redirect response contains a redirect status code (3xx) and a redirect URL in the Location header. The redirect URL can be an absolute or relative path. It’s up to the client to follow the redirect properly. Here’s an example of returning a redirect response using the Redirect() helper method: Note: The redirect helper methods … Read more
When the client posts a file in a multipart/form-data request, it’s loaded into an IFormFile object. This contains file information (such as the file name) and exposes the file content as a stream. This allows you to save the file or process it however you want to. You can access the IFormFile object through Request.Form.Files: … Read more
The simplest way to return a 500 response is to use the Problem() helper method, like this: This produces the following response: The ControllerBase class has many helper methods like Problem() that simplify returning responses. These have several optional parameters that you can use to customize the response. Internally, these helper methods return result objects. … Read more
The ASP.NET Core framework automatically parses query strings (i.e. ?name=Dune&year=2021) into HttpContext.Request.Query and maps the query string values to action parameters (if you’ve added them). You can get the mapped query string values by adding action parameters, like this: Or you can use HttpContext.Request.Query directly (which is useful in many scenarios): This can either return … Read more
To unit test an action filter, you have to pass in an action filter context object (which requires a lot of setup). Action filter methods are void, so you have to verify the behavior by inspecting the context object (or dependencies, like a logger, if you are injecting those). Here’s an example of doing the … Read more
Action filters allow you to look at requests right before they are routed to an action method (and responses right after they are returned from the action method). The simplest way to add your own action filter in ASP.NET Core is to subclass ActionFilterAttribute and then override the appropriate methods depending on if you want … Read more
To get posted form data in an API Controller (using the [ApiController] attribute) in ASP.NET Core, use parameters with the [FromForm] attribute. A request to this endpoint would look like this: The form data is a string of key-value pairs (ex: location=United+States). The framework tries to map the form data to parameters by matching the … Read more
I was reading about logging in ASP.NET when I came across this statement about logging to the database: When logging to SQL Server, don’t do so directly. Instead, add log messages to an in-memory queue and have a background worker dequeue and insert data to SQL Server. Paraphrased from Microsoft – No asynchronous logger methods … Read more
