Linq查询分组和选择第一项

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<Button> 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?)

Answer 1

var result = list.GroupBy(x => x.Category).Select(x => x.First())

Answer 2

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 => a, new CompareRows()).First();

See an example in my post "Select First Row In Group using Custom Comparer "

Answer 3

var results = list.GroupBy(x => x.Category)
            .Select(g => g.OrderBy(x => 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.

Answer 4

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<IEnumerable<string>> data = new[] {
    new[]{"...", "...", "..."},
    ... etc ...
};

Then you'd simply go:

var firsts = data.Select(x => x.FirstOrDefault()).Where(x => 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]