[c#] Using LINQ to group a list of objects

I have an object:

public class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int GroupID { get; set; }
}

I return a list that may look like the following:

List<Customer> CustomerList = new List<Customer>();
CustomerList.Add( new Customer { ID = 1, Name = "One", GroupID = 1 } );
CustomerList.Add( new Customer { ID = 2, Name = "Two", GroupID = 1 } );
CustomerList.Add( new Customer { ID = 3, Name = "Three", GroupID = 2 } );
CustomerList.Add( new Customer { ID = 4, Name = "Four", GroupID = 1 } );
CustomerList.Add( new Customer { ID = 5, Name = "Five", GroupID = 3 } );
CustomerList.Add( new Customer { ID = 6, Name = "Six", GroupID = 3 } );

I want to return a linq query which will look like

CustomerList
  GroupID =1
    UserID = 1, UserName = "UserOne", GroupID = 1
    UserID = 2, UserName = "UserTwo", GroupID = 1
    UserID = 4, UserName = "UserFour", GroupID = 1
  GroupID =2
    UserID = 3, UserName = "UserThree", GroupID = 2
  GroupID =3
    UserID = 5, UserName = "UserFive", GroupID = 3
    UserID = 6, UserName = "UserSix",

I tried from

Using Linq to group a list of objects into a new grouped list of list of objects

code

var groupedCustomerList = CustomerList
  .GroupBy(u => u.GroupID)
  .Select(grp => grp.ToList())
  .ToList();

works but does not give the desired output.

This question is related to c# linq group-by linq-to-objects

The answer is


var groupedCustomerList = CustomerList.GroupBy(u => u.GroupID)
                                      .Select(grp =>new { GroupID =grp.Key, CustomerList = grp.ToList()})
                                      .ToList();

var result = from cx in CustomerList
         group cx by cx.GroupID into cxGroup
     orderby cxGroup.Key
     select cxGroup; 

foreach (var cxGroup in result) { 
Console.WriteLine(String.Format("GroupID = {0}", cxGroup.Key)); 
  foreach (var cx in cxGroup) { 
    Console.WriteLine(String.Format("\tUserID = {0}, UserName = {1}, GroupID = {2}", 
      new object[] { cx.ID, cx.Name, cx.GroupID })); 
  }
}

The desired result can be obtained using IGrouping, which represents a collection of objects that have a common key in this case a GroupID

 var newCustomerList = CustomerList.GroupBy(u => u.GroupID)
                                                  .Select(group => new { GroupID = group.Key, Customers = group.ToList() })
                                                  .ToList();

var groupedCustomerList = CustomerList
                         .GroupBy(u => u.GroupID, u=>{
                                                        u.Name = "User" + u.Name;
                                                        return u;
                                                     }, (key,g)=>g.ToList())
                         .ToList();

If you don't want to change the original data, you should add some method (kind of clone and modify) to your class like this:

public class Customer {
  public int ID { get; set; }
  public string Name { get; set; }
  public int GroupID { get; set; }
  public Customer CloneWithNamePrepend(string prepend){
    return new Customer(){
          ID = this.ID,
          Name = prepend + this.Name,
          GroupID = this.GroupID
     };
  }
}    
//Then
var groupedCustomerList = CustomerList
                         .GroupBy(u => u.GroupID, u=>u.CloneWithNamePrepend("User"), (key,g)=>g.ToList())
                         .ToList();

I think you may want to display the Customer differently without modifying the original data. If so you should design your class Customer differently, like this:

public class Customer {
  public int ID { get; set; }
  public string Name { get; set; }
  public int GroupID { get; set; }
  public string Prefix {get;set;}
  public string FullName {
    get { return Prefix + Name;}
  }            
}
//then to display the fullname, just get the customer.FullName; 
//You can also try adding some override of ToString() to your class

var groupedCustomerList = CustomerList
                         .GroupBy(u => {u.Prefix="User", return u.GroupID;} , (key,g)=>g.ToList())
                         .ToList();

is this what you want?

var grouped = CustomerList.GroupBy(m => m.GroupID).Select((n) => new { GroupId = n.Key, Items = n.ToList() });

Examples related to c#

How can I convert this one line of ActionScript to C#? Microsoft Advertising SDK doesn't deliverer ads How to use a global array in C#? How to correctly write async method? C# - insert values from file into two arrays Uploading into folder in FTP? Are these methods thread safe? dotnet ef not found in .NET Core 3 HTTP Error 500.30 - ANCM In-Process Start Failure Best way to "push" into C# array

Examples related to linq

Async await in linq select How to resolve Value cannot be null. Parameter name: source in linq? What does Include() do in LINQ? Selecting multiple columns with linq query and lambda expression System.Collections.Generic.List does not contain a definition for 'Select' lambda expression join multiple tables with select and where clause LINQ select one field from list of DTO objects to array The model backing the 'ApplicationDbContext' context has changed since the database was created Check if two lists are equal Why is this error, 'Sequence contains no elements', happening?

Examples related to group-by

SELECT list is not in GROUP BY clause and contains nonaggregated column .... incompatible with sql_mode=only_full_group_by Count unique values using pandas groupby Pandas group-by and sum Count unique values with pandas per groups Group dataframe and get sum AND count? Error related to only_full_group_by when executing a query in MySql Pandas sum by groupby, but exclude certain columns Using DISTINCT along with GROUP BY in SQL Server Python Pandas : group by in group by and average? How do I create a new column from the output of pandas groupby().sum()?

Examples related to linq-to-objects

Using LINQ to group a list of objects Linq select objects in list where exists IN (A,B,C) Find() vs. Where().FirstOrDefault() how to query LIST using linq How can I get LINQ to return the object which has the max value for a given property? Remove duplicates in the list using linq Sorting a list using Lambda/Linq to objects Filtering lists using LINQ Dynamic LINQ OrderBy on IEnumerable<T> / IQueryable<T>