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How to add custom methods for LINQ queries (C#)

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You extend the set of methods that you use for LINQ queries by adding extension methods to the IEnumerable<T> interface. For example, in addition to the standard average or maximum operations, you create a custom aggregate method to compute a single value from a sequence of values. You also create a method that works as a custom filter or a specific data transform for a sequence of values and returns a new sequence. Examples of such methods are Distinct, Skip, and Reverse.

When you extend the IEnumerable<T> interface, you can apply your custom methods to any enumerable collection. For more information, see Extension Methods.

Add an aggregate method

An aggregate method computes a single value from a set of values. LINQ provides several aggregate methods, including Average, Min, and Max. You can create your own aggregate method by adding an extension method to the IEnumerable<T> interface.

The following code example shows how to create an extension method called Median to compute a median for a sequence of numbers of type double.

public static class EnumerableExtension
{
    public static double Median(this IEnumerable<double>? source)
    {
        if (source is null || !source.Any())
        {
            throw new InvalidOperationException("Cannot compute median for a null or empty set.");
        }

        var sortedList =
            source.OrderBy(number => number).ToList();

        int itemIndex = sortedList.Count / 2;

        if (sortedList.Count % 2 == 0)
        {
            // Even number of items.
            return (sortedList[itemIndex] + sortedList[itemIndex - 1]) / 2;
        }
        else
        {
            // Odd number of items.
            return sortedList[itemIndex];
        }
    }
}

You call this extension method for any enumerable collection in the same way you call other aggregate methods from the IEnumerable<T> interface.

The following code example shows how to use the Median method for an array of type double.

double[] numbers = [1.9, 2, 8, 4, 5.7, 6, 7.2, 0];
var query = numbers.Median();

Console.WriteLine($"double: Median = {query}");
// This code produces the following output:
//     double: Median = 4.85

Overload an aggregate method to accept various types

You can overload your aggregate method so that it accepts sequences of various types. The standard approach is to create an overload for each type. Another approach is to create an overload that will take a generic type and convert it to a specific type by using a delegate. You can also combine both approaches.

Create an overload for each type

You can create a specific overload for each type that you want to support. The following code example shows an overload of the Median method for the int type.

// int overload
public static double Median(this IEnumerable<int> source) =>
    (from number in source select (double)number).Median();

You can now call the Median overloads for both integer and double types, as shown in the following code:

double[] numbers1 = [1.9, 2, 8, 4, 5.7, 6, 7.2, 0];
var query1 = numbers1.Median();

Console.WriteLine($"double: Median = {query1}");

int[] numbers2 = [1, 2, 3, 4, 5];
var query2 = numbers2.Median();

Console.WriteLine($"int: Median = {query2}");
// This code produces the following output:
//     double: Median = 4.85
//     int: Median = 3

Create a generic overload

You can also create an overload that accepts a sequence of generic objects. This overload takes a delegate as a parameter and uses it to convert a sequence of objects of a generic type to a specific type.

The following code shows an overload of the Median method that takes the Func<T,TResult> delegate as a parameter. This delegate takes an object of generic type T and returns an object of type double.

// generic overload
public static double Median<T>(
    this IEnumerable<T> numbers, Func<T, double> selector) =>
    (from num in numbers select selector(num)).Median();

You can now call the Median method for a sequence of objects of any type. If the type doesn't have its own method overload, you have to pass a delegate parameter. In C#, you can use a lambda expression for this purpose. Also, in Visual Basic only, if you use the Aggregate or Group By clause instead of the method call, you can pass any value or expression that is in the scope this clause.

The following example code shows how to call the Median method for an array of integers and an array of strings. For strings, the median for the lengths of strings in the array is calculated. The example shows how to pass the Func<T,TResult> delegate parameter to the Median method for each case.

int[] numbers3 = [1, 2, 3, 4, 5];

/*
    You can use the num => num lambda expression as a parameter for the Median method
    so that the compiler will implicitly convert its value to double.
    If there is no implicit conversion, the compiler will display an error message.
*/
var query3 = numbers3.Median(num => num);

Console.WriteLine($"int: Median = {query3}");

string[] numbers4 = ["one", "two", "three", "four", "five"];

// With the generic overload, you can also use numeric properties of objects.
var query4 = numbers4.Median(str => str.Length);

Console.WriteLine($"string: Median = {query4}");
// This code produces the following output:
//     int: Median = 3
//     string: Median = 4

Add a method that returns a sequence

You can extend the IEnumerable<T> interface with a custom query method that returns a sequence of values. In this case, the method must return a collection of type IEnumerable<T>. Such methods can be used to apply filters or data transforms to a sequence of values.

The following example shows how to create an extension method named AlternateElements that returns every other element in a collection, starting from the first element.

// Extension method for the IEnumerable<T> interface.
// The method returns every other element of a sequence.
public static IEnumerable<T> AlternateElements<T>(this IEnumerable<T> source)
{
    int index = 0;
    foreach (T element in source)
    {
        if (index % 2 == 0)
        {
            yield return element;
        }

        index++;
    }
}

You can call this extension method for any enumerable collection just as you would call other methods from the IEnumerable<T> interface, as shown in the following code:

string[] strings = ["a", "b", "c", "d", "e"];

var query5 = strings.AlternateElements();

foreach (var element in query5)
{
    Console.WriteLine(element);
}
// This code produces the following output:
//     a
//     c
//     e

See also