# Methods and Parameters — C#/.NET

Source: https://www.skillbyai.com/en/dotnet/c-methods

> Define methods with optional, named, ref, out, in and params parameters.

## How arguments are passed

Methods take parameters **by value** by default: the method gets a copy of a value type, or a copy of the reference for a reference type (so it can modify the object, but not make the caller's variable point elsewhere). Modifiers change that. **`ref`** passes the variable itself, so the method can reassign it. **`out`** is like `ref` but the method must assign it, used by the `TryParse` pattern. **`in`** passes by reference read-only, useful for large structs. **`params`** accepts a variable number of arguments (C# 13 extended it to collection types such as `params ReadOnlySpan<T>`). Parameters can have **default values**, and callers can use **named arguments** for clarity. Short methods can be **expression-bodied** (`=>`), and **local functions** declared inside a method keep helper logic close to where it is used.

## Common parameter patterns

`TryParse` with `out` avoids exceptions for expected bad input.

```cs
if (int.TryParse("42", out int value))
{
    Console.WriteLine(value + 1);
}

void Swap(ref int a, ref int b) => (a, b) = (b, a);
int x = 1, y = 2;
Swap(ref x, ref y);   // x = 2, y = 1

decimal Total(decimal price, int qty = 1, decimal discount = 0m) =>
    price * qty * (1 - discount);

var t = Total(499m, discount: 0.1m);   // named argument skips qty

int Sum(params int[] numbers)
{
    int total = 0;
    foreach (var n in numbers) total += n;
    return total;
}
Console.WriteLine(Sum(1, 2, 3));
```

## Photocopy versus the original file

Passing by value hands the method a photocopy: scribbles do not reach your original. `ref` hands over the original file itself, so whatever the method writes, you see.

**Quiz:** Which modifier requires the called method to assign the parameter before returning?

- [ ] ref
- [x] out
- [ ] in
- [ ] params

*Answer:* out. out parameters must be definitely assigned by the method, as in TryParse.