How Can I Group a List of Objects by a Shared Property Using LINQ?

Group a list of objects using LINQ

In object-oriented programming, it is often necessary to group objects according to sharing conditions. This can be done efficiently using LINQ (Language Integrated Query). Below we'll explore how to group a list of objects into a new grouped list that contains multiple object lists.

Question:

Consider a class representing a user:

<code class="language-csharp">public class User
{
    public int UserID { get; set; }
    public string UserName { get; set; }
    public int GroupID { get; set; }
}</code>

Suppose we have a list of users:

<code class="language-csharp">List<User> userList = new List<User>();
userList.Add(new User { UserID = 1, UserName = "UserOne", GroupID = 1 });
userList.Add(new User { UserID = 2, UserName = "UserTwo", GroupID = 1 });
userList.Add(new User { UserID = 3, UserName = "UserThree", GroupID = 2 });
userList.Add(new User { UserID = 4, UserName = "UserFour", GroupID = 1 });
userList.Add(new User { UserID = 5, UserName = "UserFive", GroupID = 3 });
userList.Add(new User { UserID = 6, UserName = "UserSix", GroupID = 3 });</code>

Our goal is to group users by their GroupID, resulting in a list of users grouped by group.

Solution:

LINQ provides a powerful grouping mechanism using the GroupBy method. The following code snippet demonstrates how to achieve the desired grouping:

<code class="language-csharp">var groupedUserList = userList
    .GroupBy(u => u.GroupID)
    .Select(grp => grp.ToList())
    .ToList();</code>

Code breakdown:

1. GroupBy: This is the core of the grouping operation. It groups users by their GroupID, generating a list of groups where each group represents users who belong to the same group.
2. Select: Since the output of GroupBy is a list of groups (denoted as IGrouping<int, User>), we need to select users from each group. We use ToList method in Select to convert each group into a list of users.
3. ToList: Finally, we use ToList to convert the list of user lists back into a list.

groupedUserList now contains the required grouping: a list of user lists, where each inner list represents a user that belongs to the same group.

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