Am I using passing my argument correctly with ref?

Question

SMSCOMMS SMSEngine = new SMSCOMMS("COM6");

The code doesn't seem to take my argument of COM6 as a valid ref string.How can I solve this?

public class SMSCOMMS { public SMSCOMMS(ref string COMMPORT) { SMSPort = new SerialPort(); SMSPort.PortName = COMMPORT; SMSPort.BaudRate = 9600; SMSPort.Parity = Parity.None; SMSPort.DataBits = 8; SMSPort.StopBits = StopBits.One; SMSPort.Handshake = Handshake.RequestToSend; SMSPort.DtrEnable = true; SMSPort.RtsEnable = true; SMSPort.NewLine = System.Environment.NewLine; ReadThread = new Thread( new System.Threading.ThreadStart(ReadPort)); }

Matthew Flaschen · Accepted Answer · 2010-06-14 05:39:51Z

You can't pass a temporary with ref, because the called method must be able to assign to the caller's variable. Why are you using it to begin with? You never assign to COMMPORT.

Why not just:

public SMSCOMMS(string COMMPORT)

Matthew Flaschen · Accepted Answer · 2010-06-14 06:21:07Z

2

There's no need to pass a ref param unless you're intending to modify the actual variable the caller's passed. Since you can't modify a string literal (it's constant, by definition), it's not valid for passing by reference.

edited Jun 14, 2010 at 6:21

Matthew Flaschen

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answered Jun 14, 2010 at 5:47

cHao

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3 Comments

Matthew Flaschen Over a year ago

Wrong. You can use ref with strings, and there are cases when you want to. If the method assigns to the ref variable, the caller now has a reference to a different string in their own variable.

cHao Over a year ago

I didn't say string -- i said string literal. If you know of a way to modify a string literal, let the .net people know about it, cause it's probably a bug. By definition, literals are constant. "a" will never be anything but "a". Though it could be replaced by another string, that is not the same thing as modifying the string itself.

cHao Over a year ago

Add to that, the code in question obviously was passing a string literal, not a variable (read: there's no variable to assign to), which i thought was obvious, and you have the reason stuff broke.

jrista · Accepted Answer · 2010-06-14 05:49:14Z

You can only use ref when you are passing something that has a usable reference. That means you have to declare a variable first, then pass that variable by ref:

string comm = "COM6"; SMSCOMMS SMSEngine = new SMSCOMMS(ref comm);

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