C# params Keyword with Collections

Last modified April 19, 2025

This tutorial explores how to use the params keyword in C# with various collection types like List<T>, Span<T>, and IEnumerable<T>.

In C# 13, the params keyword has been extended to support additional collection types beyond arrays, enabling more flexible and efficient method parameter handling.

Understanding the params Keyword in C#

The params keyword allows a method to accept a variable number of arguments, which are treated as a collection inside the method. With C# 13, params now supports types like List<T>, Span<T>, IEnumerable<T>, and others, in addition to arrays.

Key characteristics of params:

- It must be the last parameter in the method signature.

- Only one params parameter is allowed per method.

- It supports specific collection types like List<T> or Span<T>.

- It allows zero or more arguments to be passed.

Using params with Array

This example demonstrates params with an array to calculate the average.

Program.cs

double Average(params int[] numbers) { return numbers.Length > 0 ? numbers.Average() : 0; }

int[] values = { 4, 8, 12 }; Console.WriteLine(Average(1, 2, 3)); Console.WriteLine(Average(values));

The Average method computes the average of a variable number of integers or an array passed directly.

$ dotnet run 2 8

Using params with List<T>

This example uses params with a List<int> to sum numbers.

Program.cs

int SumNumbers(params List<int> numbers) { return numbers.Sum(); }

Console.WriteLine(SumNumbers(1, 2, 3)); Console.WriteLine(SumNumbers(10, 20, 30, 40)); Console.WriteLine(SumNumbers());

The SumNumbers method accepts a variable number of integers as a List<int> and returns their sum.

$ dotnet run 6 100 0

Using params with Span<T>

This example uses params with a Span<string> to concatenate strings.

Program.cs

string ConcatStrings(params Span<string> words) { return string.Join(" “, words.ToArray()); }

Console.WriteLine(ConcatStrings(“Hello”)); Console.WriteLine(ConcatStrings(“C#”, “is”, “awesome”)); Console.WriteLine(ConcatStrings());

The ConcatStrings method joins strings from a Span<string>, handling any number of inputs.

$ dotnet run Hello C# is awesome

Using params with IEnumerable<T>

This example combines a fixed parameter with a params IEnumerable<string> parameter.

Program.cs

void PrintItems(string category, params IEnumerable<string> items) { Console.WriteLine($“Category: {category}”); foreach (var item in items) { Console.WriteLine($” - {item}"); } }

PrintItems(“Fruits”, new[] { “Apple”, “Banana”, “Orange” }); PrintItems(“Tools”, “Hammer”, “Screwdriver”);

The PrintItems method takes a category and a variable number of items as an IEnumerable<string>, printing them in a list.

$ dotnet run Category: Fruits

• Apple
• Banana
• Orange Category: Tools
• Hammer
• Screwdriver

Using params with ReadOnlySpan<T>

This example demonstrates params with a ReadOnlySpan<object>.

Program.cs

void DisplayValues(params ReadOnlySpan<object> values) { foreach (var value in values) { Console.WriteLine($“Value: {value}”); } }

DisplayValues(42, “Hello”, 3.14, true); DisplayValues(“Test”, 100);

The DisplayValues method accepts a variable number of objects as a ReadOnlySpan<object>, displaying each one.

$ dotnet run Value: 42 Value: Hello Value: 3.14 Value: True Value: Test Value: 100

Passing a Collection to params

This example shows passing a List<int> to a params List<int> parameter.

Program.cs

int MaxValue(params List<int> numbers) { return numbers.Any() ? numbers.Max() : 0; }

List values = [5, 2, 8, 1, 9]; Console.WriteLine(MaxValue(1, 2, 3)); Console.WriteLine(MaxValue(values));

The MaxValue method finds the maximum value from a variable number of integers or a List<int> passed directly.

$ dotnet run 3 9

Best Practices

When using params with collections in C#:

- Choose the appropriate collection type (e.g., Span<T> for performance).

- Handle empty collections gracefully in method logic.

- Avoid overloading with params to prevent ambiguity.

- Use specific types over object for type safety.

Source

params keyword - language reference

This tutorial has demonstrated how to use the params keyword in C# with various collection types for flexible method parameters.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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