gtk#: Tree View object always passed as call-by-reference - mono

Question

It seems as if my arguments in the method call of processing a Tree View is always done as call-by-reference.
I have a visible GTK "Tree View" control on a top level window. The data was written by the respective model.
Now I want to remove some of the columns (based on options set by the user) and pass the manipulated Tree View to an Export-Function.
In order to remove the columns only from the output, not from the GUI itself, I thought of copying the visible Tree View control into a temporary one, manipulating the temportary one and calling the export-functionality on the temporary one.
My problem is: even though I pass my origin, visible Tree View as referenc-by-value (as of my understanding), the origin will be manipulated and the removing of columns will be done on the visual Tree View.
It seem as if my arguments in the method call is always done as call-by-reference.
Code:
"treeview1" is the visual Gtk.Tree View...
I call my Export-function:
...
TreeView treeviewExport = SetExportViewAccordingToCheckboxes(treeview1);
ExportFile(treeviewExport);
...
In the method SetExportViewAccordingToCheckboxes() I just pass the global treeview1 as call-by-value, manipulate it internally and return the manipulated Tree View:
protected static TreeView SetExportViewAccordingToCheckboxes(TreeView tvSource)
{
TreeView tvRet = tvSource;
if (cbName == false)
tvRet.RemoveColumn( ... );
...
return tvRet;
}
But even though I have removed the columns from the internal Tree View "tvRet", my visual control "treeview1" lacks all the columns which were removed from "tvRet"; it looks like "treeview1" was passed as call-by-reference.
Question: why is that?
Note: I also tried with the keyword "in" which made no difference:
protected static TreeView SetExportViewAccordingToCheckboxes(in TreeView p_tvSource)

Answer 1

The problem comes here:
In the method SetExportViewAccordingToCheckboxes() I just pass the
global treeview1 as call-by-value, manipulate it internally and return
the manipulated Tree View:
protected static TreeView SetExportViewAccordingToCheckboxes(TreeView tvSource)
{
TreeView tvRet = tvSource;
if (cbName == false)
tvRet.RemoveColumn( ... );
...
return tvRet;
}
First some background. In C# terminology, value types are those that directly contain a value, while reference types are those that reference the data, instead of holding it directly.
So, int x = 5 means that you are creating the value object 5 of type integer, and storing it in x, while TreeView tree = new TreeView() means that you are creating a reference tree of type TreeView, which points to an object of the same type.
All of this means that you cannot pass an object by value, even if you want to. In the best case, you are passing the reference by value, which has no effect.
So, the next step is to copy the data, and modify the copied object instead of the original one. This is theoretically sound, but the line: TreeView tvRet = tvSource; unfortunately does not achieve that. You are creating a new reference, yes, but that reference points to the same object the original reference points to.
Now, say that we are managing objects of class Point instead of TreeView, with properties x and y.
class Point {
public int X { get; set; }
public int Y { get; set; }
}
You can create a point easily:
Point p1 = new Point { X = 5, Y = 7 };
But this does not copy it:
Point p2 = p1;
This would do:
Point p2 = new Point { X = p1.X, Y = p1.Y };
Now the original problem was that you wanted to pass a few columns to an Export() function. In that case, you only need to pass a vector of the filtered columns to the exporting function, instead of a copy of the TreeView.
void PrepareExporting()
{
var columns = new List<TreeViewColumn>();
foreach(TreeViewColumn col in this.treeView.Columns) {
if ( this.Filter( col ) ) {
columns.add( col );
}
}
this.Export( columns.ToArray() );
}
void Export(TreeViewColumn[] columns)
{
// ...
}
I think that would be easier, since it is not needed to try to achieve a pass-by-reference (impossible), nor copy the tree view.
Hope this helps.