C# – How to get the status code when using HttpClient

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When you use HttpClient to make requests, you can directly get the status code from the HttpResponseMessage object, like this: The main reason for checking the status code is to determine if the request was successful and then reacting to error status codes (usually by throwing an exception). The HttpResponseMessage class has two helpers that … Read more

C# – Get subclass properties with reflection

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When you use reflection to get properties, you can get just the subclass properties by using BindingFlags.DeclaredOnly (this causes it to exclude inherited properties). Here’s an example: Note: Use GetType() if you have an object. Use typeof() if you have a class. The code outputs just the subclass properties (from the Driver subclass): Get base … Read more

C# – How to read problem details JSON with HttpClient

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Problem details (RFC7807) is a standardized error response format that has a Content-Type of application/problem+json, an error response code (i.e. 400 – Bad Request), and has a response body that looks like this: This can be extended to include any number of properties. The example shown above comes from the default way ASP.NET Core returns … Read more

C# – How to unit test a model validation attribute

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You can unit test a validation attribute by creating an instance of it and then testing the two methods: In this article, I’ll show examples of unit testing these methods in a custom validation attribute and in a built-in validation attribute (i.e. [Range]). Unit testing a custom validation attribute Consider the following custom validation attribute … Read more

ASP.NET Core – Create a custom model validation attribute

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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

C# – JSON object contains a trailing comma at the end which is not supported

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Problem When you deserialize JSON with a “trailing comma”, you get the following error: The JSON object contains a trailing comma at the end which is not supported in this mode. Change the reader options. JSON properties are separated with commas. A trailing comma is one that has no properties after it. Here’s an example … Read more

ASP.NET Core – API model validation attributes

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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 a request with invalid data (boxOfficeMillions is outside … Read more

C# – Deserialize JSON using different property names

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When JSON property names and class property names are different, and you can’t just change the names to match, you have three options: These options affect both deserialization and serialization. Let’s say you have the following JSON with camel-cased property names: Here’s an example of using the JsonPropertyName attribute: Note: The Newtonsoft equivalent is [JsonProperty(“title”)] … Read more

C# – Deserialize JSON with a specific constructor

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When your class has multiple constructors, you can use the JsonConstructor attribute to specify which constructor to use during deserialization. Here’s an example. The Person class has two constructors. I put the JsonConstructor attribute on one of the constructors: Note: JsonConstructor for System.Text.Json was added in .NET 5. Now deserialize a JSON string to the … Read more

Visual Studio – How to use conditional breakpoints

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Breakpoints cause execution to pause when you’re running the debugger. This is referred to as breaking, and it allows you to look at the current state of things for debugging purposes. In some cases, you may want to use a conditional breakpoint to only break execution when certain conditions are met (ex: break when name … Read more

Redirect a request in ASP.NET Core

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Redirecting a request means returning a response with redirect status code (3xx) and a redirect URL in the Location header. The client uses this info to follow the redirect (which means making a request to the redirect URL). You can redirect by using a helper method (from ControllerBase) or returning a RedirectResult. I’ll show several … Read more

Postman – Follow redirects with the original HTTP method

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When you send a request with Postman and get a 301/302 redirect response, it follows the redirect with a forced GET. This means it sends a GET request to the redirect URL and doesn’t include the original HTTP method or Content-Type. This can result in two unexpected errors: Here’s what this looks like in Postman … Read more

C# – Call a constructor from another constructor

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To call one constructor from another one, you have to use the constructor chaining syntax, like this: This means when you use the Person(string name) constructor, it’ll first call the Person(string name, string birthDate) constructor. If the constructor is in a base class, use base() instead of this(): Employee subclasses Person. So calling base(name) here … Read more

C# – How to read the Description attribute

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You can use the Description attribute to describe types and type members (properties, methods). One of the most common use cases is providing a user-friendly string for enum values. Here’s an example of using the Description attribute with an enum: To read the Description attribute, use reflection and do the following steps: This can be … Read more

C# – Handling redirects with HttpClient

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HttpClient handles redirects automatically. When you send a request, if the response contains a redirect status code (3xx) and redirect location, then it’ll send a new request to the redirect location. You can turn off this auto-redirect behavior by passing in an HttpClientHandler with AllowAutoRedirect=false. This prevents it from following redirects automatically, and allows you … Read more

The required library hostfxr.dll could not be found

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Problem You are trying to run a .NET executable and you get the following error: A fatal error occurred. The required library hostfxr.dll could not be found.If this is a self-contained application, that library should exist in [C:\MyApp].If this is a framework-dependent application, install the runtime in the global location [C:\Program Files\dotnet] or use the … Read more

Categories C#

Error: Host is not allowed to connect to this MySQL server

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Problem When you try to connect to MySQL remotely, you get the following error: Host <hostname or IP> is not allowed to connect to this MySQL server In MySQL, hosts are allowed (whitelisted) per user. So this error means the user you’re trying to connect with doesn’t have your remote host whitelisted. You’re likely trying … Read more

C# – Connect to a MySQL database

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The simplest way to connect to a MySQL database in a .NET project is to use the MySql.Data package (from Oracle). It provides classes that implement the standard ADO.NET interfaces (such as IDbConnection). First, add the MySql.Data package to your project (this is using View > Other Windows > Package Manager Console): Now use the … Read more

C# – How to use SortedSet

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When you have a collection of elements that you’re continuing to add to, and need to keep the objects in sorted order at all times, you can use SortedSet. Internally, it uses a tree data structure to keep elements in sorted order (O(log n) insertion). This is far more efficient than repeatedly sorting a list … Read more

C# – Deserialize JSON to a derived type

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The simplest way to deserialize JSON to a derived type is to put the type name in the JSON string. Then during deserialization, match the type name property against a set of known derived types and deserialize to the target type. System.Text.Json doesn’t have this functionality out of the box. That’s because there’s a known … Read more