Solve real coding problems
In this article, I’ll explain how to solve the Mark and Toys algorithm problem on HackerRank. Problem statement: Given a fixed budget and a list of item prices. What is the max number of items you can purchase? You can only buy each item once. Note: This is the Mark and Toys problem from HackerRank. … Read more
You can get all types from an assembly by doing a reflection-only load. This allows you to read type info from metadata without running the typical errors associated with fully loading an assembly. The way to do a reflection-only load is different in .NET Framework and .NET Core. I’ll show examples of how to do … Read more
You can use the is operator to check if an object is a certain type. Here’s an example: You can also use the is operator to declare a variable of the target type, like this: Note: The employee object is only available in the if block, but IntelliSense shows it out of its scope. If … Read more
Access modifiers are used to hide class members (methods/properties/fields) from other code. When you define a class/method/property/field, you put an access modifier on it. In C#, there are four main access modifiers: Access modifiers are enforced at compile time. When you try to use a class member that you can’t access, you get the CS0122 … Read more
The key to unit testing an ApiController class is to mock out all of its dependencies, including the controller’s HttpContext property, like this: If the controller method you’re testing uses anything from the HttpContext, then you’ll want to swap in your own value. Otherwise HttpContext will be null and you’ll get a NullReferenceException. Fortunately Microsoft … Read more
You can convert a string to a DateTime (parse) with one of these methods: I’ll show examples of using both methods for parsing various DateTime formats. Using DateTime.Parse() DateTime.Parse() can handle parsing a wide variety of culture-specific standard DateTime formats and the ISO-8601 format (which is the default date format for JSON serialization). By default, … Read more
The nameof() operator outputs the name of the class/method/property/type passed into it. Here’s an example: Note: nameof() was added in C# 6. nameof() eliminates duplication The DRY principle – Don’t Repeat Yourself – warns us against having duplication in the code. Whenever information or code is duplicated, it’s possible to change something in one spot … Read more
When you have a public method that isn’t null checking its parameters, then you’ll get the CA1062 code analysis warning. This is part of the Nullable Reference Types feature. For example, the following code isn’t null checking the movieRepository parameter: This results in the CA1062 code analysis warning: CA1062 In externally visible method ‘void StreamingService.LogMovies(MovieRepository … Read more
I’ll show examples of using the pattern matching operators – ‘not’, ‘and’, ‘or‘ – with the is operator. These are nice syntax sugar that make conditional logic a bit easier to read. Note: ‘not’, ‘and’, ‘or’ were added in C# 9. ‘not’ operator Here’s an example of the not operator: This is checking if the … Read more
Problem When you’re using WinForms and click a DataGridView column header, you get an exception like this: Index was out of range. Must be non-negative and less than the size of the collection. This problem is caused by the column header “row” triggering a click event (such as CellContentClick) with event args containing RowIndex of … Read more
Nested conditionals are a code smell because they make the code harder to understand. They tend to grow more and more complicated over time because developers will keep adding conditions and more levels of nesting. The deeper the nesting, the more time it’ll eventually take to refactor it. Code Smell: Nested conditionals.Definition: A conditional block … Read more
SqlDataReader returns a DBNull object when a column is null. This isn’t the same as a C# null. You can check if the column is null by comparing it with DBNull.Value or by using SqlDataReader.IsDBNull(). Here’s an example showing these two ways of checking if a column is null: After checking if the column is … Read more
.NET Core enables you to write cross-platform C# code. Sometimes you’ll want to do things differently depending on the OS you’re running in. You can use RuntimeInformation.IsOSPlatform() to figure out which OS you’re in. Here’s an example of using this to see if you’re running in Windows: Example – Loading a different native library depending … Read more
Given an IP range as a starting IP address and an ending IP address (as strings, like from user input or a config file), you can check if the IP range is valid by doing the following steps: Here’s an example. Let’s say you’re given starting IP “192.168.0.1” and ending “192.168.0.11”. The following table shows … Read more
When you merge two dictionaries, you can either merge them in-place, or create a new dictionary and copy the values over to it. The following extension method does an in-place merge of two dictionaries. It loops through items in the right dictionary, adding them to the left dictionary. When duplicate keys exist, it’s keeping the … Read more
You can’t use nullable bools (bool?) exactly like regular bools, because they aren’t the same thing. When you try to use them like regular bools, you run into compiler errors and runtime exceptions. Instead, you have to explicitly compare the nullable bool with true/false. Here’s an example of checking if a nullable bool is true … Read more
Problem Let’s say you’re initializing a ComboBox like this: And you get the following exception: System.ArgumentOutOfRangeException: ‘InvalidArgument=Value of ‘0’ is not valid for ‘SelectedIndex’. (Parameter ‘value’)Actual value was 0.’ You’re getting this exception because the DataSource you’re binding to is empty. Solution Are you expecting there to always be data? If you’re expecting there to … Read more
This article shows code for converting a hex string to a byte array, unit tests, and a speed comparison. First, this diagram shows the algorithm for converting a hex string to a byte array. To convert a hex string to a byte array, you need to loop through the hex string and convert two characters … Read more
The simplest way to check if a directory is empty is by calling Directory.EnumerateFileSystemEntries(). If this doesn’t return anything, then the directory doesn’t have any files or subdirectories (folders), which means it’s empty. Here’s an example: Another situation to consider is when a directory has no files but might have empty subdirectories (folders). You can … Read more
You can use Convert.ChangeType() to convert from a string to any type, like this: Normally you’d call the specific type converter method, such as when you’re converting a string to an int. However, sometimes it makes sense to use the generalized type converter method – Convert.ChangeType() – instead of hardcoding the calls to specific type … Read more
Problem When you call .First() on an empty IEnumerable, you get the following exception: System.InvalidOperationException: Sequence contains no elements Solution Option 1 – Use .FirstOrDefault() instead of .First() When the IEnumerable is empty, .FirstOrDefault() returns the default value for the type. For reference types this returns null. For value types this returns 0 or that … Read more
Here’s how to handle the DataGridViewButtonColumn button click event: In this article I’ll show a step-by-step example of how to handle the button click. Example DataGridView with a button column When I click the button I want it to say Hi to the person. 1 – Set the DataSource to BindingList<Person> 2 – Add ClickHandler(Person … Read more
If you’re working on a Windows Forms project and need to know which forms are open, use: This gives you an IEnumerable collection of form objects that are currently open. You can lookup a form by name, by type, or loop through the list. Example scenarios There are many scenarios where you’d want to know … Read more
Problem The following exception is thrown when you try to get a value from a dictionary using a key that doesn’t exist in the dictionary: KeyNotFoundException: ‘The given key was not present in the dictionary.’ Consider the following the example of initializing a dictionary with a few key/value pairs, and then trying to access non-existent … Read more
Relative paths are resolved relative to the current working directory. When you’re running a Windows Service, the default working directory is C:\Windows\system32 or C:\Windows\SysWOW64. Therefore relative paths are resolved from these system folders, which can lead to problems when read/writing files. Here are the most common problems you’ll run into: System.IO.DirectoryNotFoundException: Could not find a … Read more