Why modify an Array of objects don't change my objects?

Question

I have DataRow from grid and I need to modify few columns in one row. So I put all my columns in Array, and tried to modify them but it doesn't work as I wish. I need an explanation for that.

My goal is to get all columns in specific order in Array or some collection, and then modify them etc. I think I am now creating some new objects which reference to something else than my column. Maybe I should try to store in collection some references? Using ref should is best option?

DataRow dr = rows[i] as DataRow;
dr["size"] = 5000; // => size is 5000;
ChangeSize(dr); // => size is 6000;

ChangeSize body

private void ChangeSize(DataRow dataRow)
{
             dataRow["size"] = 6000; // => size is 6000
             Object[] arrayOfColumns= { dataRow["size"], ... };
             arrayOfColumns[0] = 7000; // => won't change size...

}

Jon Skeet · Accepted Answer · 2013-10-08 07:54:55Z

1

You're just changing the value in the array. You happened to initialize that via dataRow["size"] but that doesn't mean there's any perpetual link between the two.

If you need changes to be reflected back to the DataRow, I suspect you should have another method to do that:

private void CopyToDataRow(Object[] source, DataRow target)
{
    target["size"] = source[0];
    // etc
}

There's no concept of triggering custom code like this whenever an array is modified - you'll need to call it at the appropriate time. (And no, ref wouldn't help you here at all.)

answered Oct 8, 2013 at 7:54

Jon Skeet

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1 Comment

boski Over a year ago

I use that method in my application. It's good to hear that ref won't help.

AlexH · Accepted Answer · 2013-10-08 07:56:07Z

1

dataRow["size"] contains an int which is a value type.

When you instanciate and intialize arrayOfColumns you get a copy of the value contained at dataRow["size"] and not a reference.

answered Oct 8, 2013 at 7:56

AlexH

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2 Comments

boski Over a year ago

Is there any way to initialize Array with ref to that object? Or I should just copy the value like Jon Skeet write below?

AlexH Over a year ago

In that case, you can't use ref. As Jon Skeet noticed, you should use an extra method to update the value

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