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As we mentioned at the start of this section, we will not spend much time discussing these changes, since they leverage concepts and features we have seen before. Let s move on to editing a tree node s label. 15.5.2 SUPPORTING LABEL EDITS Tree nodes can be edited in a manner similar to list items. There is a BeginEdit method in the TreeNode class to initiate a label edit programmatically, and BeforeLabelEdit and AfterLabelEdit events in the TreeView class that occur before and after the user edits the label. Event handlers for these events receive the NodeLabelEditEventArgs class for the event parameter. This class is summarized in .NET Table 15.5, and is manipulated in much the same way as we saw for the LabelEditEventArgs class when handling label events for the ListView class.

This program produces the following output:

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try block Xtest, test is: 0 Xtest, test is: 1 an exception -- value is: 1

The NodeLabelEditEventArgs class represents the event data associated with the BeforeLabelEdit and AfterLabelEdit events in the TreeView class. This class is part of the System.Windows.Forms namespace, and inherits from the System.EventArgs class. CancelEdit Gets or sets whether the edit operation should be cancelled. This property can be set both before and after the node is edited. Gets the new text to assign to the label of the indicated node. Gets the TreeNode object being edited.

A try block can be localized to a function When this is the case, each time the function is entered, the exception handling relative to that function is reset For example, examine this program

In our application, we will permit nodes to be edited using the menuEditLabel menu item, or by pressing the F2 key when a tree node is selected and the tree view has the focus. The following table details the steps required for this change:

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#include <iostream> using namespace std; // A try/catch can be inside a function other than main() void Xhandler(int test) { try{ if(test) throw test; } catch(int i) { cout << "Caught Exception #: " << i << '\n'; } } int main() { cout << "Start\n"; Xhandler(1); Xhandler(2); Xhandler(0); Xhandler(3); cout << "End"; return 0; }

This program displays this output:

SUPPORT EDITING OF TREE NODE LABELS Action 1 Set the LabelEdit property for the TreeView control to true in the MainForm.cs [Design] window. Handle the KeyDown event for the TreeView control to initiate a label edit when the F2 key is pressed in the tree control. Result Tree node labels may now be edited.

25:

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private void treeViewMain_KeyDown (object sender, System.Windows. Forms.KeyEventArgs e) { if (e.KeyCode == Keys.F2) { if (treeViewMain.SelectedNode != null) { treeViewMain.SelectedNode.BeginEdit(); e.Handled = true; } } } private void menuEdit_Popup (object sender, System.EventArgs e) { if (treeViewMain.Focused) { menuEditLabel.Enabled = (treeViewMain.SelectedNode != null); menuEditLabel.Text = "&Node"; } else // assume ListView has focus { menuEditLabel.Enabled = (listViewMain.SelectedItems.Count > 0); if (this._albumsShown) menuEditLabel.Text = "&Name"; else menuEditLabel.Text = "&Caption"; } }

As you can see, three exceptions are thrown After each exception, the function returns When the function is called again, the exception handling is reset It is important to understand that the code associated with a catch statement will be executed only if it catches an exception Otherwise, execution simply bypasses the catch statement

Update the menuEdit_Popup event handler to use the text Node for the menuEditLabel menu when the TreeView has the focus.

An exception can be of any type, including class types that you create Actually, in real-world programs, most exceptions will be class types rather than built-in types Perhaps the most common reason that you will want to define a class type for an exception is to create an object that describes the error that occurred This information can be used by the exception handler to help it process the error The following example demonstrates this

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// Catching class type exceptions #include <iostream> #include <cstring> using namespace std; class MyException { public: char str_what[80]; int what; MyException() { *str_what = 0; what = 0; } MyException(char *s, int e) { strcpy(str_what, s); what = e; } }; int main() { int i; try { cout << "Enter a positive number: "; cin >> i; if(i<0) C++

SUPPORT EDITING OF TREE NODE LABELS (continued) Action 4 Update the menuEditLabel_Click event handler to edit the appropriate item based on the current focus. Result

throw MyException("Not Positive", i); } catch (MyException e) { // catch an error cout << estr_what << ": "; cout << ewhat << "\n"; } return 0; }