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What are params, in, out, and ref keywords in C#?

Anubhav Sharma 498 19 Jun 2025

What are params, in, out, and ref keywords in C# ?

c# c#
Updated 23 Jun 2025
Anubhav Sharma
Anubhav Sharma

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1 Answers

Ponu Maurya
Ponu Maurya 23 Jun 2025
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In C#, the keywords params, in, out, and ref are used to control how arguments are passed to methods. They allow more flexible method signatures, especially when dealing with dynamic argument counts or modifying variables inside a method. Here's a breakdown of each:

params – Variable Number of Arguments

The params keyword allows you to pass a variable number of arguments of a specified type to a method. It must be the last parameter in the method signature, and only one params parameter is allowed.

Example:

public void PrintNumbers(params int[] numbers)
{
    foreach (int number in numbers)
    {
        Console.WriteLine(number);
    }
}

You can call it like:

PrintNumbers(1, 2, 3, 4);

Use Case: Simplifies calling methods with a list of values without explicitly creating an array.

ref – Pass by Reference (Input/Output)

The ref keyword is used when a method needs to read and modify the caller's variable. The caller must initialize the variable before passing it.

Example:

public void DoubleValue(ref int number)
{
    number *= 2;
}

int x = 10;
DoubleValue(ref x);  // x is now 20

Use Case: When you want the method to change the original value passed from the caller.

out – Output Parameter (Only for Returning)

The out keyword is used to return multiple values from a method. Unlike ref, the variable does not need to be initialized before being passed. However, the method must assign a value before returning.

Example:

public void GetDetails(out string name, out int age)
{
    name = "Alice";
    age = 30;
}

string name;
int age;
GetDetails(out name, out age);  // name = "Alice", age = 30

Use Case: Useful when you want to return more than one value from a method, especially before Tuple or ValueTuple became popular.

in – Pass by Reference (Read-Only)

Introduced in C# 7.2, the in keyword passes arguments by reference, but they are read-only inside the method. It improves performance for large value types by avoiding copies, while still preventing modification.

Example:

public void ShowPoint(in Point p)
{
    Console.WriteLine($"X: {p.X}, Y: {p.Y}");
    // p.X = 5; // ❌ Not allowed
}

Use Case: Best used with large structs to avoid copying but ensure immutability.

Summary Table

Keyword Passed By Needs Initialization? Can Modify in Method? Can Return Value?
params Value Optional No No
ref Reference Yes Yes Yes
out Reference No Yes (must assign) Yes
in Reference Yes No No

When to Use What?

  • Use params for flexible argument counts.
  • Use ref to read and modify a variable.
  • Use out to return multiple results.
  • Use in for performance on large structs with read-only intent.
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