Linq Query Group By and Selecting First Items

Question

I have a String array kinda like this      icon  category  tool String    subButtonData   new String            graphics gui brushsizeplus icon    Draw    DrawBrushPlus          graphics gui brushsizeminus icon    Draw    DrawBrushMinus          graphics gui freedraw icon    Draw    DrawFree          graphics gui linedraw icon    Draw    DrawLine          graphics gui rectangledraw icon    Draw    DrawRectangle          graphics gui ellipsedraw icon    Draw    DrawEllipse          graphics gui brushsizeplus icon    Brusher    BrusherBrushPlus          graphics gui brushsizeminus icon    Brusher    BrusherBrushMinus          graphics gui brushsizeplus icon    Text    TextBrushPlus          graphics gui brushsizeminus icon    Text    TextBrushMinus         Then I populate a List lt Button gt  with my Button Type named mainButtons  This is how I query for grouping for Category   var categories   from b in mainButtons                  group b by b category into g                  select new   Category   g Key  Buttons   g      How can I select the first item of each group in my main List   without iterating each and adding to another List

User · Answer

var results   list GroupBy x   gt  x Category               Select g   gt  g OrderBy x   gt  x SortByProp  FirstOrDefault       For those wondering how to do this for groups that are not necessarily sorted correctly  here s an expansion of this answer that uses method syntax to customize the sort order of each group and hence get the desired record from each   Note  If you re using LINQ-to-Entities you will get a runtime exception if you use First   instead of FirstOrDefault   here as the former can only be used as a final query operation

User · Answer

var result   list GroupBy x   gt  x Category  Select x   gt  x First

User · Answer

See LINQ  How to get the latest last record with a group by clause  var firstItemsInGroup   from b in mainButtons                  group b by b category into g select g First      I assume that mainButtons are already sorted correctly   If you need to specify custom sort order  use OrderBy override with Comparer   var firstsByCompareInGroups   from p in rows         group p by p ID into grp         select grp OrderBy a   gt  a  new CompareRows    First      See an example in my post  Select First Row In Group using Custom Comparer

User · Answer

First of all  I wouldn t use a multi-dimensional array  Only ever seen bad things come of it   Set up your variable like this   IEnumerable lt IEnumerable lt string gt  gt  data   new         new                                 etc          Then you d simply go   var firsts   data Select x   gt  x FirstOrDefault    Where x   gt  x    null      The Where makes sure it prunes any nulls if you have an empty list as an item inside   Alternatively you can implement it as   string       new         new                           new                               etc          This could be used similarly to a  x y  array but it s used like this   x  y

[c#] Linq Query Group By and Selecting First Items

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