why does datatable.Clear, clears the other datatable as well?

Question

DataTable dttemp = new DataTable();
dttemp=orderedTable;
dttemp.Rows.Clear();

for some reason the orderedTable is being cleared as well! does anyone know why this is happening?

Answer 1

By the time you've executed this line:

dttemp = orderedTable;

there's only actually one DataTable which is relevant. The DataTable you created in the first line of code is eligible for garbage collection - nothing's taking any notice of it. The line quoted here doesn't make a copy of the object - it makes a copy of the reference, so that dttemp and orderedTable refer to the same DataTable.

DataTable is a reference type. It's absolutely vital that you understand how reference types work in C# - otherwise the whole language will have baffling results for you. I have an article which you may find useful along those lines.

To put your code into context, imagine that a DataTable is actually a house. You give your home address to two different people. One goes and paints the house red... and then the other visits the house. Obviously he'll find it's red.

The important thing is that you didn't give each person "your house" - you just gave them a way of getting to your house. That's how reference types work in C#.

The value of dttemp isn't a DataTable - it's a reference to a DataTable. Changes made to that object via any other variables will still be seen via dttemp.

Answer 2

You have two variables (dttemp and orderedTable) which, at the time when you call Clear(), point at the same object. Thus a change to the object pointed to by dttemp is also a change to the object pointed to by orderedTable.

The table you create with new DataTable() is simply discarded onto the heap (waiting to be garbage-collected) when you assign dttemp=orderedTable;