Methods make input, output and responsibility visible

First, build a mental model

A method is a small contract: give it these inputs, receive this output, and know which side effects it may perform. Its local names belong to that call.

1. The signature states parameter types and return type. A void method has no return value for the caller to use.
2. Normal parameters receive values. Reassigning a copied int parameter does not reassign the caller's variable.
3. A return statement hands a result back and ends that path through the method.
4. Prefer explicit inputs over hidden mutable globals. ref and out are explicit alternative parameter mechanisms, not the default.

static int AddBonus(int score, int bonus)
{
    score += bonus;
    return score;
}

int score = 10;
int preview = AddBonus(score, 2);
Console.WriteLine(score);
Console.WriteLine(preview);

1. Call

   The method receives copies of 10 and 2.

2. Local calculation

   Its local score becomes 12 and is returned.

3. Caller

   preview receives 12, while the caller's score remains 10.