dictionary-trygetvalue

C# – Dictionary with multiple values per key

by

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

C# – Check if key exists in dictionary

by

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

C# – Update value in a dictionary

by

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

C# – Get value from dictionary

by

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

C# – Add to a dictionary

by

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

ASP.NET Core – How to get request headers

by

There are two ways to get request headers: When a request comes in, the framework loads request headers into the Request.Headers dictionary. You can use this just like any other dictionary. Here’s an example of using TryGetValue() to check if a request header exists and get its value: Note: To just check if a header … Read more

C# – How to read problem details JSON with HttpClient

by

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# – Map query results to multiple objects with Dapper

by

When you’re querying joined tables, you can map each row to multiple objects by using the multi mapping feature in Dapper. To multi map, you have to provide Dapper with the following: In this article, I’ll show examples of multi mapping. Note: If you don’t specify the split column, it’ll use the default of “Id”. … Read more

C# – How to send a file with HttpClient

by

In order to send a file in a request with HttpClient, add the file into a MultipartFormDataContent object, and send this object as the request content. Here’s an example: This sends the following multipart/form-data POST request: In this article, I’ll explain a few details about MultipartFormDataContent, and show a few other file-sending scenarios. MultipartFormDataContent Add() … Read more

KeyNotFoundException: The given key was not present in the dictionary

by

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