LINQ Group By on multiple Coulms

You can use the group clause in LINQ to group data by multiple columns. To do so, you can create an anonymous type that combines the values of the columns you want to group by, and then group by that anonymous type.

Here's an example of how to use group by on multiple columns in LINQ:

csharp
// create a collection of objects
List<Person> people = new List<Person>
{
    new Person { Name = "Alice", Age = 25, Gender = "Female" },
    new Person { Name = "Bob", Age = 30, Gender = "Male" },
    new Person { Name = "Charlie", Age = 25, Gender = "Male" },
    new Person { Name = "David", Age = 30, Gender = "Male" },
    new Person { Name = "Eva", Age = 25, Gender = "Female" }
};

// group people by age and gender
var groupedPeople = from p in people
                    group p by new { p.Age, p.Gender };

// iterate through each group and its elements
foreach (var group in groupedPeople)
{
    Console.WriteLine("Age: " + group.Key.Age + ", Gender: " + group.Key.Gender);
    foreach (var person in group)
    {
        Console.WriteLine("  " + person.Name);
    }
}

In this example, we first create a collection of Person objects with three properties: Name, Age, and Gender. We then use the group keyword in a LINQ query to group the people by their Age and Gender properties. To do this, we create an anonymous type that combines the Age and Gender properties, and group by that anonymous type.

We then iterate through each group and its elements using a foreach loop, printing out the Age and Gender of each group, and the names of the people in each group.

The output of this code will be:

yaml
Age: 25, Gender: Female
  Alice
  Eva
Age: 30, Gender: Male
  Bob
  David
Age: 25, Gender: Male
  Charlie

As you can see, the people have been grouped by their Age and Gender, and the output shows each combination of Age and Gender, along with the names of the people in each group.