Solve real coding problems
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
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
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
I’ll show examples of the XML documentation warnings and how to fix them. CS1573 Parameter ‘X’ has no matching param tag in the XML comment (but other parameters do) Warning CS1573 means there’s a missing <param> tag for one of the method parameters. This usually happens when you add a new parameter (or rename one … Read more
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
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
Problem When you deserialize JSON, 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 of a trailing comma: … 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 a request with invalid data (boxOfficeMillions is outside … Read more
Postman is a great tool for web development. You can send a request to any web API, look at the response details, and reuse the the request later (which saves lots of effort!). This is useful for testing your own web API, or for when you’re integrating with a third-party API and want to try … Read more
You can use GROUP BY to group rows based on one or more columns. This produces one row per group in the query results. Each group contains the grouping columns and the result(s) of any aggregate function(s) (COUNT/MIN/MAX/AVG/SUM) you want to use on the group. Let’s say you have the following table and you want … Read more
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
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
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
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
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
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
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
Problem You’re using a DataGridView and you can’t edit one or more of the columns. When you click a cell, it just highlights and doesn’t enter editing mode. Check if your grid, column, or bound property are readonly. If any of these are readonly, you won’t be able to edit the values in the column. … Read more
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
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
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
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
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
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
