GroupBy 一个或多个字段

让我们假设我们有一些电影模型：

public class Film {
    public string Title { get; set; }
    public string Category { get; set; }
    public int Year { get; set; }
}

分类属性：

foreach (var grp in films.GroupBy(f => f.Category)) {
    var groupCategory = grp.Key;
    var numberOfFilmsInCategory = grp.Count();
}

按类别和年份分组：

foreach (var grp in films.GroupBy(f => new { Category = f.Category, Year = f.Year })) {
    var groupCategory = grp.Key.Category;
    var groupYear = grp.Key.Year;
    var numberOfFilmsInCategory = grp.Count();
}