Select Multiple Fields from List in Linq

Question

In ASP NET C  I have a struct   public struct Data       public int item1      public int item2      public int category id      public string category name      and I have a List of those   I want to select category id and category name  running a DISTINCT and finally an ORDERBY on category name   Here s what I have now   List lt Data gt  listObject   getData    string   catNames   listObject                      Select i  gt  i category name                       Distinct                        OrderByDescending s   gt  s                       ToArray      This obviously just gets the category name   My question is  how do I get multiple fields  and what data structure will I store this in  not a string      EDIT  Using a list of structs is not set in stone   If it would be advisable to change my backing data structure to make selects easier  I ll be writing a lot of these  then I d gladly take recommendations

User · Answer

You can make it a KeyValuePair  so it will return a  IEnumerable lt KeyValuePair lt string  string gt  gt    So  it will be like this    Select i   gt  new KeyValuePair lt string  string gt  i category id  i category name    Distinct

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Given List lt MyType1 gt  internalUsers and List lt MyType2 gt  externalUsers  based on the shared key of an email address     For C  7 0   var matches         from i in internalUsers     join e in externalUsers     on i EmailAddress ToUpperInvariant   equals e Email ToUpperInvariant       select  internalUser i  externalUser e    ToList     Which gives you matches as a List lt  MyType1  MyType2  gt   From there you can compare them if you wish  var internal in external   matches Select m   gt  m internalUser  ToList    var external in internal   matches Select m   gt  m externalUser  ToList     var internal notIn external   internalUsers Except internal in external  ToList    var external notIn internal   externalUsers Except external in internal  ToList     internal in external and internal notIn external will be of type List lt MyType1 gt   external in internal and external notIn internal will be of type List lt MyType2 gt   For versions of C  prior to 7 0  var matches         from i in internalUsers     join e in externalUsers     on i EmailAddress ToUpperInvariant   equals e Email ToUpperInvariant       select new Tuple lt MyType1  MyType2 gt  i  e    ToList     Which gives you matches as a List lt Tuple lt MyType1  MyType2 gt  gt   From there you can compare them if you wish  var internal in external   matches Select m   gt  m Item1  ToList    var external in internal   matches Select m   gt  m Item2  ToList     var internal notIn external   internalUsers Except internal in external  ToList    var external notIn internal   externalUsers Except external in internal  ToList     internal in external and internal notIn external will be of type List lt MyType1 gt   external in internal and external notIn internal will be of type List lt MyType2 gt

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public class Student       public string Name   set  get        public int ID   set  get       class Program     static void Main string   args                Student   students                     new Student   Name  zoyeb    ID 1           new Student   Name  Siddiq    ID 2           new Student   Name  sam    ID 3           new Student   Name  james    ID 4           new Student   Name  sonia    ID 5                      var studentCollection   from s in students select new   s ID   s Name            foreach  var student in studentCollection                        Console WriteLine student Name               Console WriteLine student ID

User · Answer

This is task for which anonymous types are very well suited  You can return objects of a type that is created automatically by the compiler  inferred from usage   The syntax is of this form   new   Property1   value1  Property2   value2          For your case  try something like the following   var listObject   getData    var catNames   listObject Select i   gt      new   CatName   i category name  Item1   i item1  Item2   i item2         Distinct   OrderByDescending s   gt  s  ToArray

User · Answer

You could use an anonymous type    Select i   gt  new   i name  i category name      The compiler will generate the code for a class with name and category name properties and returns instances of that class  You can also manually specify property names   i   gt  new   Id   i category id  Name   i category name     You can have arbitrary number of properties

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var selectedCategories       from value in          from data in listObject         orderby data category name descending         select new   ID   data category id  Name   data category name        group value by value Name into g     select g First     foreach  var category in selectedCategories  Console WriteLine category     Edit  Made it more LINQ-ey

User · Answer

Anonymous types allow you to select arbitrary fields into data structures that are strongly typed later on in your code   var cats   listObject      Select i   gt  new   i category id  i category name         Distinct        OrderByDescending i   gt  i category name       ToArray      Since you  apparently  need to store it for later use  you could use the GroupBy operator   Data   cats   listObject      GroupBy i   gt  new   i category id  i category name         OrderByDescending g   gt  g Key category name       Select g   gt  g First         ToArray

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from i in list  select new   i category id  i category name      Distinct     OrderBy i   gt  i category name

User · Answer

var result   listObject Select  i   gt  new  i category name  i category id       This uses anonymous types so you must the var keyword  since the resulting type of the expression is not known in advance

User · Answer

You can select multiple fields using linq Select as shown above in various examples this will return as an Anonymous Type  If you want to avoid this anonymous type here is the simple trick   var items   listObject Select f   gt  new List lt int gt      f Item1  f Item2    SelectMany item   gt  item  Distinct      I think this solves your problem

[c#] Select Multiple Fields from List in Linq

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