Are arrays or lists passed by default by reference in c#?

Do they? Or to speed up my program should I pass them by reference?

Solution 1:

The reference is passed by value.

Arrays in .NET are object on the heap, so you have a reference. That reference is passed by value, meaning that changes to the contents of the array will be seen by the caller, but reassigning the array won't:

void Foo(int[] data) {
    data[0] = 1; // caller sees this
}
void Bar(int[] data) {
    data = new int[20]; // but not this
}

If you add the ref modifier, the reference is passed by reference - and the caller would see either change above.

Solution 2:

They are passed by value (as are all parameters that are neither ref nor out), but the value is a reference to the object, so they are effectively passed by reference.