.NET 9 —LINQ — New Index Method

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With .NET 9, the new LINQ method Index (Index<TSource>(IEnumerable<TSource>) was introduced. With this method you can easily extract the implicit index of an Enumerable.

In a previous article, I demonstrated the three new LINQ methods that were added in .NET 9: CountBy, AggregateBy and Index. In this article, I’d like to focus on the Index method and demonstrate alternative ways to achieve similar results in earlier .NET versions, alongside with a benchmark to compare performance.

The Index method

The Index method returns a tuple (IEnumerable<(int Index, TSource Item)>), where the first value is the Index and the second is the Element in the collection.

To demonstrate how this method works, I created a list of cities, which will be used in the examples below:

public class City
{
    public string Name { get; set; }
    public string Country { get; set; }

    public City(string name, string country)
    {
        Name = name;
        Country = country;
    }
}

var cities = new List<City>()
{
    new City("Paris", "France"),
    new City("Berlin", "Germany"),
    new City("Madrid", "Spain"),
    new City("Rome", "Italy"),
    new City("Amsterdam", "Netherlands")
};

With the Index method, we can easy retrieve the index and the element of a list, using a foreach loop:

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