GroupBy one or multiple fields C# with Example

Lets assume we have some Film model: 
public class Film { 
public string Title { get; set; } 
public string Category { get; set; } 
public int Year { get; set; } 
} 
Group by Category property: 
foreach (var grp in films.GroupBy(f => f.Category)) { 
var groupCategory = grp.Key; 
var numberOfFilmsInCategory = grp.Count(); 
 

} 
Group by Category and Year: 
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(); 
}