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: make a 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 copied and passed to the method
// 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 to the method
// 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");