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];

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

Method(1, 2, 3);

void Method(params int[] integers)
{ }