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Articles about working with the dictionaries (Dictionary class) in C# to store key-value pairs and look them up.
Dictionaries are unordered data structures. Key/value pairs aren’t stored in sorted order. When you want the Dictionary in sorted order, there are two simple options: I’ll show both options. Sort Dictionary with OrderBy() Use OrderBy() (from System.Linq) to sort the Dictionary by key or value. It returns the Dictionary’s KeyValuePairs in ascending sorted order. I’ll … Read more
The simplest way to filter a dictionary is by using the Linq Where() + ToDictionary() methods. Here’s an example: Note: You can use the Dictionary constructor (new Dictionary<string, int>(filterList)) instead of ToDictionary() if you prefer. This produces a new dictionary with the filtered item: Where() produces a list (actually an IEnumerable) of KeyValuePair objects. Most … Read more
In .NET 5 and above, you can directly change a dictionary’s values in a foreach loop. Here’s an example: This outputs the following: You couldn’t do this before .NET 5, because it would invalidate the enumerator and throw an exception: InvalidOperationException: Collection was modified; enumeration operation my not execute. Instead, you’d have to make the … Read more
The simplest way to get the key with the max value in a dictionary is to use the Linq MaxBy() method (added in .NET 6). This returns the key/value pair with the max value. Here’s an example: This outputs the following: In this article, I’ll show examples of getting the max by key, min by … Read more
The simplest way to loop through a dictionary is with a foreach loop. Here’s an example: This outputs the following: The loop variable is a KeyValuePair<string, int> with Key and Value properties. Instead of using this, you can deconstruct the KeyValuePair into named variables, like this: This simplifies things and makes the loop easier to … Read more
The simplest way to add or overwrite a value in a ConcurrentDictionary is to use the indexer: If the key doesn’t exist, this adds it. If the key exists, this overwrites it. The indexer is thread-safe. The indexer is the simplest way to unconditionally add / overwrite a value. Sometimes you’ll want to use other … Read more
The simplest way to convert a list to a dictionary is to use the Linq ToDictionary() method: This loops through the list and uses the key/element selector lambdas you passed in to build the dictionary. In this article, I’ll go into details about how to use ToDictionary() and show how to deal with duplicate keys. … 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 puts items from the right dictionary into the left dictionary. When duplicate keys exist, it’s keeping the value from the … Read more
If you want a case insensitive dictionary, use: In the Dictionary constructor you can specify how keys are compared. For string keys, the default is a case sensitive comparison. To make it case insensitive, you can pass in StringComparer.InvariantCultureIgnoreCase. Example I have a table that maps users to devices. The user-to-device mapping gets cached in … 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: When the user enters a city name that isn’t in the dictionary, such as “New … Read more
