Can I modify a passed method parameter

Question

my gut feeling says I shouldn\'t do the following. I don\'t get any warnings about it.

void test(DateTime d)
{
 d = d.AddDays(2);
//do some thing with d
 }
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Answer 1

Changes to the value of a parameter are invisible to the caller, unless it's a ref or out parameter.

That's not the case if you make a change to an reference type object referred to by a parameter. For example:

public void Foo(StringBuilder b)
{
    // Changes the value of the parameter (b) - not seen by caller
    b = new StringBuilder();
}

public void Bar(StringBuilder b)
{
    // Changes the contents of the StringBuilder referred to by b's value -
    // this will be seen by the caller
    b.Append("Hello");
}

Finally, if the parameter is passed by reference, the change is seen:

public void Baz(ref StringBuilder b)
{
    // This change *will* be seen
    b = new StringBuilder();
}

For more on this, see my article on parameter passing.

Answer 2

You can change it but the change will not go back to the caller.

If it is a ValueType -> The copy of object is sent

If it is a RefernceType -> Copy of Object reference will be sent by value. In this way properties of the object can be changed but not the reference itself - caller will not see the change anyway.

If it is sent ref -> Reference can be changed.

In C++ you can use const to prevent the change but C# does not have that. This is only to prevent the programmer by mistake try to change it - depending where the const is used.

Answer 3

If you want to be able to access the original value, use the second method.

If you don't care about the original value, you can use either one (I'd probably still use the second though).

Either way, you're not going to hurt anybody else's values (even if you re-assign the value, it won't make it back to the caller) in this case.