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Each dictionary key maps to exactly one value. When you want multiple values per key, you can use a dictionary of lists. Here’s an example of creating a dictionary of lists, where each key maps to multiple ints: Notice that you can add to the dictionary like normal. The difference with a dictionary of lists … Read more
You can use dictionaries with tuples as the keys or values. I’ll show examples below. Dictionary with tuple as key Tuples have two or more fields, so using a tuple as the key is a simple way to have compound keys in a dictionary. Here’s an example of creating a dictionary with a named tuple … Read more
The simplest way to make a copy of a dictionary is to use the copy constructor. You pass the dictionary you want to copy to the new dictionary’s constructor, like this: Note: If you’re using a non-default comparer, you’ll want to pass it to the copy constructor (i.e. new Dictionary(d, d.Comparer)). The copy constructor handles … Read more
The simplest way to convert a dictionary to a list is by using the Linq ToList() method, like this: When used on a dictionary, ToList() returns a list of KeyValuePair objects. This example outputs the following: Dictionary keys to list If you want a list of the dictionary’s keys, use ToList() on Dictionary.Keys, like this: … Read more
I’ll show examples of how to deserialize JSON to dictionaries in different scenarios. Deserializing to Dictionary<string, string> When your JSON object has properties with simple string values (as opposed to nested objects), you can deserialize to Dictionary<string, string>. Here’s an example. Consider the following simple JSON object: Note: In all examples, I’ll show the pretty-printed … Read more
When you want to convert a dictionary to a JSON string, you can use the built-in JsonSerializer (from System.Text.Json) to serialize the dictionary. Here’s an example: Note: This is passing in WriteIndented=true to pretty print the JSON string for readability. This serializes the dictionary to a JSON string with the following format: I’ll show more … Read more
Dictionaries contain key/value pairs. The keys must be unique, but the values can be repeated many times. When you want to find duplicate values, the simplest option is to use Linq methods GroupBy() and Where(), like this: Note: I initialized the dictionary with a few duplicate values. This groups the dictionary key/value pairs by value. … Read more
Dictionaries have keys mapped to values, which enables you to efficiently lookup values by key. But you can also do a reverse lookup: get the key associated with a value. The simplest option is to use FirstOrDefault(), but that’s only a good idea if you know the value exists for sure. Instead, the best option … Read more
Normally you’d check if a dictionary contains a key or get a value by key. But you can also check if the dictionary contains a specific value by using Dictionary.ContainsValue(). Here’s an example: Dictionary.ContainsValue() returns true if the value was found and otherwise returns false. In this example, I initialized the dictionary with a single … Read more
When you want to check if a key exists, use Dictionary.ContainsKey(). This returns true if the key exists and otherwise returns false. Here’s an example: This initializes the dictionary with a few items and then checks if one of the key exists. This outputs: Usually you’ll want to do something based on if the key … Read more
After you add a key/value pair to a dictionary, you can update the value for the key by using the indexer syntax (i.e. dictionary[key]=value), like this: This outputs the updated value for the ‘Bob’ key: The indexer syntax inserts if the key doesn’t exist -or- updates the value if the key already exists. This means … Read more
Problem Dictionaries require keys to be unique. When you try to add a key/value to a dictionary and the key already exists, you get the following exception: System.ArgumentException: An item with the same key has already been added. This can happen when you use Dictionary.Add() or when you initialize a dictionary using the “curly brace” … Read more
Dictionaries contain key/value pairs. When you want to remove one or more items from a dictionary, you can use one of the following methods: I’ll show examples below. Remove item by key Use Dictionary.Remove() to remove an item based on its key. If the key exists, it removes the key/value pair from the dictionary and … Read more
Dictionaries store key/value pairs. When you want to get the value for a key, you can use the indexer syntax and specify the key, like this: Note: In order to show how to get a value, I had to initialize the dictionary with key/value pairs. This returns the value associated with the “Bob” key and … Read more
You can declare a dictionary and initialize it with values by using the collection initializer syntax. Within the initializer block, you have two options for adding key/value pairs to the dictionary: I’ll show examples of both options below. Note: If you have a list of items already, you can convert the list to a dictionary … Read more
The simplest way to add a key/value pair to a dictionary is by using Dictionary.Add(), like this: If the key already exists, Dictionary.Add() throws an ArgumentException because the key must be unique. There are a few other ways to add to a dictionary in different scenarios, which I’ll explain below. Add or update key/value in … Read more
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 loop through a dictionary and directly change its values. 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: Note: All examples shown initialize the dictionary with a small number of key/value pairs for readability. This outputs … Read more
The simplest way to loop through a dictionary is with a foreach loop. Here’s an example of initializing a dictionary with values and then looping through it: 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, … 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
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
When you’re working with a JSON array, you can deserialize it to a list like this: This deserializes all of the objects in the JSON array into a List<Movie>. You can use this list object like usual. Note: All examples will use System.Collections.Generic and System.Text.Json. I’ll exclude the using statements for brevity. Example – JSON … Read more