reference type parameter question

Question

I learnt the reference type parameter passing is just a copy of the reference. If you set the passed in refernece parameter point to another object inside the called method, the orginal reference will not change.

I have a test method to test the reference type parameter passing. A refTest(SystemSwEvent systemSwEvent) method is called from that test method with a valid SystemSwEvent type object. Inside the refTest() method, the processEvScanDataAvailable(EvScanDataAvaialble systemSwEvent) method is called. Inside the processEvScanDataAvailable(EvScanDataAvaialble systemSwEvent) method, I set the passed in reference parameter to null. I expect the parameter in refTest() should not be changed. But that is not true. It will be changed to null momentarily. why? enter image description here

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I just filed a connect issue on this, if you want to vote for it: connect.microsoft.com/VisualStudio/feedback/details/662071/… — Reed Copsey
– Reed Copsey, Commented Apr 15, 2011 at 20:49

Ondrej Tucny · Accepted Answer · 2011-04-15 20:30:25Z

4

The debugger recognizes the name in the current context and shows the value. It's just a coincidence that you pointed your cursor at a place that actually triggered the currently executing method (the current context).

Also note you can use the Call Stack tool to inspect the parameters of caller methods.

edited Apr 15, 2011 at 20:30

answered Apr 15, 2011 at 20:24

Ondrej Tucny

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Reed Copsey · Accepted Answer · 2011-04-15 20:22:13Z

2

why?

I suspect this is a debugger issue, and not representative of what's actually occurring inside the CLR.

Try using different variable names for your arguments, and this behavior will go away.

answered Apr 15, 2011 at 20:22

Reed Copsey

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BrokenGlass · Accepted Answer · 2011-04-15 20:22:28Z

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Object references are by default (if you don't qualify them with ref or out) passed by value, so the method is receiving a copy of the object reference - setting that copy to null doesn't change the original object reference, so what you claim to see is impossible and most likely you are just misinterpreting what you see in the debugger.

answered Apr 15, 2011 at 20:22

BrokenGlass

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4 Comments

Tejs Over a year ago

Reference types are always passed by reference, regardless of ref or out.

Jeff Mercado Over a year ago

@Tejs: But the references themselves are passed by value.

BrokenGlass Over a year ago

@Tejs: The object reference itself is passed by value, note that I said reference and not object.

5YrsLaterDBA Over a year ago

what's your mean about "misinterpreting"? yes, I cannot believe what I see but it is always like that in the debugger.

cwharris · Accepted Answer · 2011-04-15 20:30:40Z

0

The debugger is getting confused because your parameter and variable have the same name. If you change the name of your parameter, you will notice that that debugger no longer gives information about the variable being passed into the method, but only gives information on the variable inside the method.

Simply change the name of that parameter, and you will no longer have this issue.

answered Apr 15, 2011 at 20:30

cwharris

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