Datarow object, why is this code working?

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after assign a row values of datatabe to a new data row object , any changes in this object affects on the data table , Why ?

have a look at this code.

C#

DataRow dr = mydataset.Tables["NewTable"].Rows[0];
dr["Name"] = TextBox2.Text;

Now if you try to debug your application you will find that the original data table ["NewTable"] has the new value "hello" instead of the old value Although we didn't return back the above data row to data table (we didn't save it )

I Just need a clarification and also need to know & understand what actually happen when i create an Object of a data row .

Posted 22-Nov-14 1:08am

Hercal

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Tomas Takac 22-Nov-14 7:20am

Of course the "NewTable" has the value, you just assigned it. If you want the original value use the overload which takes DataRowVersion[^] like this: dr["Name", DataRowVersion.original].

Hercal 22-Nov-14 7:37am

Thanks Tomas .

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OriginalGriff · Accepted Answer · 2014-11-22T01:24:00

OK, this is going to get interesting, because it's kinda fundamental to how .NET works.

DataRow is a Class - which means it's what's called a "Reference type" rather than a "Value type" as (for example) integers are.
When you create an integer variable and assign it a value:

C#

int x = 5;
int y = x;

You crate a new variable and copy the value into it: so if you then do this:

C#

x = x + 10;
Console.WriteLine("{0}:{1}", x, y);

It will print:

15:5

Because y got a copy of the content of x, not a reference to it. This is what you expect!

Reference types are different:

C#

public class MyClass
   {
   public int Val;
   }
...
MyClass x = new MyClass();
x.Val = 5;
MyClass y = x;

x.Val = x.Val + 10;
Console.WriteLine("{0}:{1}", x.Val, y.Val)

This time it prints

15,15

Because y got a copy of the content of x, which was a reference to the actual instance. So when you changed the Val via the x variable, it also changed the Val of the instance that y referred to because they are both referencing the same instance!

The same thing is happening in your code:

C#

DataRow dr = mydataset.Tables["NewTable"].Rows[0];

Createa a new variable called dr, and assigns a copy of the reference to the Table row to it - they both refer to the same area of memory, the same instance of a Row.
So then:

C#

dr["Name"] = TextBox2.Text;

Changes the one and only instance - which affects the instance in the table because it's the same one!

Does that make sense?

[edit]
There is a more detailed description of the difference here: Using struct and class - what's that all about?[^]
[/edit]