The source DataSet value is changing when changing a DataSet value which is copied from Source DataSet.

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The source DataSet value is changing when changing a DataSet value which is copied from Source DataSet.

For Eg

C#

DataSet dataset1;
DataSet dataset2;

dataset2=dataset1;

//Do some changes in dataset2
//the changes are reflecting in dataset1 also . How ?

Posted 16-Oct-11 22:44pm

vivektp

Updated 16-Oct-11 22:52pm

Mehdi Gholam

v2

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OriginalGriff · Accepted Answer · 2011-10-16T23:04:00

Ok, you need to go back to basics.

When you declare a class variable, you do not create an instance of that class, you create a variable that can hold a reference to an instance of that class. It's like the difference between a web page and a http: address - the later is a reference to the former, but it isn't the page itself.

When you create two class variables, and assign one to the other, the reference only is copied - not the content - they both refer to the same instance. Looking at in in terms of web pages again, if you have two different sites with an http: link to the same web page, then you would expect any changes to that page to be seen regardless of which site you linked from.

So, when you create a DataSet:

C#

DataSet dataset1 = new DataSet();
DataSet dataset2;
myDataAdapter.Fill(dataset1);
dataset2=dataset1;

Any changes to either dataset1 or dataset2 are reflected in the same instance of a dataset - you are just getting there by different routes.

This is a lot easier to explain with pictures!

"Thanxx
But..String is also a reference type. Why it is not happening for string
string str1="1";
string str2=str1;
str2="2";
//str1 value remains the same, ie "1".."

There are two reasons for this:
1) It does happen exactly the same in the example you give:

C#

string str1="1";

Creates a reference str1 and assigns a string "1" to it.

C#

string str2=str1;

Copies the reference to the "1" string from str1 to str2. Both variables now refer to the same string: "1"

C#

str2="2";

Creates a new string "2" and assigns the reference to str2. This does not change str1 which remains pointing at the string "1".

I told you this was easier with pictures!

2) Because strings are a special type of object - they are "immutable". What this means is that once a string is created, it cannot be changed. any method which appears to be changing a string content is in fact creating a new string and returning the reference to that - the original string remains untouched.
Don't worry about this yet - it won't affect you for a few weeks when you get a bit more advanced - the first explanation covers your example. I included this for completeness (and because someone would point it out if I didn't and I didn't want to confuse you :laugh:).

I.explore.code · Accepted Answer · 2011-10-16T23:00:00

Solution 1

It seems like assigning one reference to the other merely just tells the CLR that both this references point to the same object so the changes in one reflect the other because they are writing to the same memory location.

To copy u can instead use

C#

dataset2 = dataset1.Copy();

, which just does an explicit copy of the values and the structure on a different memory location.

Hope this helps...

Cheers

Posted 16-Oct-11 23:00pm

I.explore.code

v2

Comments

vivektp 17-Oct-11 5:05am

String is also a reference type. Why it is not happening for string
string str1="1";
string str2=str1;
str2="2";
//str1 value remains the same, ie "1".

I.explore.code 17-Oct-11 5:12am

String objects are immutable and they can also be treated as value types. So whenever you copy a string, it creates a new string in memory which is why you can retain both the values. This is one of the huge security issues as well where one is discouraged from using strings for password information, coz they can lurk around in memory for a while. StringBuilder is recommended instead, but i digress. The copying of all objects in C# works this way, they just create a new ref that points to the same memory location. Hope this helps

HarinderS · Accepted Answer · 2011-10-16T23:14:00

Solution 3

It's simply because you are copying the dataset 2 into dataset 1 by saying dataset2 = dataset1

Whatever you have coded, you are getting exactly the same.

Posted 16-Oct-11 23:14pm

HarinderS