Parameter modifiers

ref

Pass a parameter by reference.

// value type var i = 10; Method0(i); // 10 Method1(ref i); // 11 void Method0(int i) => i++; // pass the value type parameter by value (copy) void Method1(ref int i) => i++; // pass the value type parameter by reference // reference type var item = new Item("init name"); Method2(item); // init name Method3(ref item); // new name 3 // pass the reference type parameter by value: the reference is passed by value (copy) // if the parameter is overwritten (the copy of the reference is overwritten), it doesn't affect the initial object void Method2(Item item) => item = new("new name 2"); // pass the reference type parameter by reference: the reference is passed by reference // if the parameter is overwritten (the initial reference is overwritten), it does affect the initial object void Method3(ref Item item) => item = new("new name 3");

ref readonly

Pass a parameter by reference and make it readonly.
Useful when you have a big value type and you want to pass only its reference for performances reason, but you don't want it to be overwritten.

var bigStruct = new BigStruct("name"); Method(ref bigStruct); void Method(ref readonly BigStruct bigStruct) { bigStruct = new BigStruct("new name"); // you can't assign a readonly variable bigStruct.Name = "new name"; // you can modify the object properties } int[] array = [ 1, 2, 3, 4 ]; var bigStruct = new BigStruct(array); ref readonly int element = ref bigStruct.GetElement(2); // 3 element = 5; // you can't assign a readonly variable ref readonly int GetElement(int index) => ref data[index];

If the variable is a readonly variable, you must use the in modifier while calling the method.

in

Create a temporary variable for the argument and pass the argument by readonly reference.

var i = 10; Method(in i); // in is optional in th calling method void Method6(in int i) // readonly copy of the initial parameter { i = 5; // you can't assign a readonly variable }

out

Allow to initialize the parameter in the method.

Method5(out var j); // 5 void Method5(out int i) => i = 5;

params

A parameter which allows a comma-separated list of arguments or a collection of arguments.

Method(1, 2, 3); int[] a = [1, 2, 3]; Method(a); void Method(params int[] integers) { }