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The ToDictionary() LINQ method can be used to generate a Dictionary<TKey, TElement> collection based on a given IEnumerable<T> source.

IEnumerable<User> users = GetUsers();
Dictionary<int, User> usersById = users.ToDictionary(x => x.Id);

In this example, the single argument passed to ToDictionary is of type Func<TSource, TKey>, which returns the key for each element.

This is a concise way to perform the following operation:

Dictionary<int, User> usersById = new Dictionary<int User>();
foreach (User u in users) 
  usersById.Add(u.Id, u);

You can also pass a second parameter to the ToDictionary method, which is of type Func<TSource, TElement> and returns the Value to be added for each entry.

IEnumerable<User> users = GetUsers();
Dictionary<int, string> userNamesById = users.ToDictionary(x => x.Id, x => x.Name);

It is also possible to specify the IComparer that is used to compare key values. This can be useful when the key is a string and you want it to match case-insensitive.

IEnumerable<User> users = GetUsers();
Dictionary<string, User> usersByCaseInsenstiveName = users.ToDictionary(x => x.Name, StringComparer.InvariantCultureIgnoreCase);

var user1 = usersByCaseInsenstiveName["john"];
var user2 = usersByCaseInsenstiveName["JOHN"];
user1 == user2; // Returns true

Note: the ToDictionary method requires all keys to be unique, there must be no duplicate keys. If there are, then an exception is thrown: ArgumentException: An item with the same key has already been added. If you have a scenario where you know that you will have multiple elements with the same key, then you are better off using ToLookup instead.

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