Linq查询分组和选择第一项

Question

我有一个像这样的字符串数组:

// 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"},
};

然后我List<Button>使用名为的Button Type 填充amainButtons

这是我查询分组的方式Category:

var categories = from b in mainButtons
                 group b by b.category into g
                 select new { Category = g.Key, Buttons = g };

如何在主列表中选择每个组的第一个项目？(没有迭代每个并添加到另一个列表？)

Answer 1

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

Answer 2

请参阅LINQ:如何使用group by子句获取最新/最后一条记录

var firstItemsInGroup = from b in mainButtons
                 group b by b.category into g
select g.First();

我假设mainButtons已经正确排序.

如果需要指定自定义排序顺序,请对Comparer使用OrderBy override.

var firstsByCompareInGroups = from p in rows
        group p by p.ID into grp
        select grp.OrderBy(a => a, new CompareRows()).First();

看到我后一个例子"使用自定义比较选择第一行集团 "

Answer 3

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

对于那些想知道如何为那些群体做到这一点并不一定正确排序,这里的扩展这个答案,使用方法的语法自定义每个组的排序顺序,从而获取每个所需的记录.

注意:如果您正在使用LINQ-to-Entities,那么如果您在此处使用First()而不是FirstOrDefault(),则会出现运行时异常,因为前者只能用作最终查询操作.

Answer 4

首先,我不会使用多维数组.只见过不好的事情.

像这样设置你的变量:

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

然后你就去:

var firsts = data.Select(x => x.FirstOrDefault()).Where(x => x != null); 

如果你有一个空列表作为一个项目,Where确保它修剪任何空值.

或者,您可以将其实现为:

string[][] = new[] {
    new[]{"...","...","..."},
    new[]{"...","...","..."},
    ... etc ...
};

这可以类似于[x,y]数组使用,但它的使用方式如下:[x][y]