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All LINQ based methods follow one of two similar patterns. They take an enumerable sequence. They return either a different sequence, or a single value. The consistency of the shape enables you to extend LINQ by writing methods with a similar shape. In fact, the .NET libraries gained new methods in many .NET releases since LINQ was first introduced. In this article, you see examples of extending LINQ by writing your own methods that follow the same pattern.
Add custom methods for LINQ queries
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.
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.
Beginning in C# 14, you can declare an extension block to contain multiple extension members. You declare an extension block with the keyword extension followed by the receiver parameter in parentheses. The following code example shows how to create an extension method called Median in an extension block. The method computes a median for a sequence of numbers of type double.
extension(IEnumerable<double>? source)
{
public double Median()
{
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 can also add the this modifier to a static method to declare an extension method. The following code shows the equivalent Median extension method:
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 either 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($"double: Median = {query}");
// This code produces the following output:
// double: Median = 4.85
You can overload your aggregate method so that it accepts sequences of various types. The standard approach is to create an overload for each type. Another approach is to create an overload that takes a generic type and convert it to a specific type by using a delegate. You can also combine both approaches.
You can create a specific overload for each type that you want to support. The following code example shows an overload of the Median method for the int type.
// int overload
public static double Median(this IEnumerable<int> source) =>
(from number in source select (double)number).Median();
You can now call the Median overloads for both integer and double types, as shown in the following code:
double[] numbers1 = [1.9, 2, 8, 4, 5.7, 6, 7.2, 0];
var query1 = numbers1.Median();
Console.WriteLine($"double: Median = {query1}");
int[] numbers2 = [1, 2, 3, 4, 5];
var query2 = numbers2.Median();
Console.WriteLine($"int: Median = {query2}");
// This code produces the following output:
// double: Median = 4.85
// int: Median = 3
You can also create an overload that accepts a generic sequence of objects. This overload takes a delegate as a parameter and uses it to convert a sequence of objects of a generic type to a specific type.
The following code shows an overload of the Median method that takes the Func<T,TResult> delegate as a parameter. This delegate takes an object of generic type T and returns an object of type double.
// generic overload
public static double Median<T>(
this IEnumerable<T> numbers, Func<T, double> selector) =>
(from num in numbers select selector(num)).Median();
You can now call the Median method for a sequence of objects of any type. If the type doesn't have its own method overload, you have to pass a delegate parameter. In C#, you can use a lambda expression for this purpose. Also, in Visual Basic only, if you use the Aggregate or Group By clause instead of the method call, you can pass any value or expression that is in the scope this clause.
The following example code shows how to call the Median method for an array of integers and an array of strings. For strings, the median for the lengths of strings in the array is calculated. The example shows how to pass the Func<T,TResult> delegate parameter to the Median method for each case.
int[] numbers3 = [1, 2, 3, 4, 5];
/*
You can use the num => num lambda expression as a parameter for the Median method
so that the compiler will implicitly convert its value to double.
If there is no implicit conversion, the compiler will display an error message.
*/
var query3 = numbers3.Median(num => num);
Console.WriteLine($"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
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
Each example shown in this article has a different receiver. That means each method must be declared in a different extension block that specifies the unique receiver. The following code example shows a single static class with three different extension blocks, each of which contains one of the methods defined in this article:
public static class EnumerableExtension
{
extension(IEnumerable<double>? source)
{
public double Median()
{
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];
}
}
}
extension(IEnumerable<int> source)
{
public double Median() =>
(from number in source select (double)number).Median();
}
extension<T>(IEnumerable<T> source)
{
public double Median(Func<T, double> selector) =>
(from num in source select selector(num)).Median();
public IEnumerable<T> AlternateElements()
{
int index = 0;
foreach (T element in source)
{
if (index % 2 == 0)
{
yield return element;
}
index++;
}
}
}
}
The final extension block declares a generic extension block. The type parameter for the receiver is declared on the extension itself.
The preceding example declares one extension member in each extension block. In most cases, you create multiple extension members for the same receiver. In those cases, you should declare the extensions for those members in a single extension block.
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