Simple .NET/ASP.NET PDF document editor web control SDK

isEmpty() ) return fileSaveAs(); else return saveFile( currentFilename ); } The final option needed to make the dialog behave as expected is to let the user save the file from the warning dialog shown when a modified document is being closed The new implementation of the isSafeToClose method is shown in Listing 8-26, in which the lines containing the actual changes are highlighted The first change is the addition of the Save option to the warning dialog using the QMessageBox::Save enumerated value The other change consists of a case for handling the Save button If the button is pressed, a call is made to fileSave If the file is not saved (that is, false is returned), the close event is aborted This makes it impossible for the user to lose a document without actually having chosen to do so (or experiencing some sort of power failure) Listing 8-26.

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string myString = "Indexing"; char theThirdCharacter = myString[2]; Console.WriteLine(theThirdCharacter);

If you execute that code in a console application, you ll see:

d

Source code for checking whether to close a document bool SdiWindow::isSafeToClose() { if( isWindowModified() ) { switch( QMessageBox::warning( this, tr("SDI"), tr("The document has unsaved changes\n" "Do you want to save it before it is closed "), QMessageBox::Save | QMessageBox::Discard | QMessageBox::Cancel ) ) { case QMessageBox::Cancel: return false; case QMessageBox::Save: return fileSave();.

What if we try to use the indexer to assign a value (i.e., to replace the character at that location in the string) as in Example 10-50

Well, that doesn t compile. We get an error:

Once a string has been created, it is immutable. You can t slice it up into substrings, trim characters off it, add characters to it, or replace one character or substring with another. What I hear you ask. Then how are we supposed to do our string processing Don t worry, you can still do all of those things, but they don t affect the original string copies (of the relevant pieces) are made instead. Why did the designers of the .NET Framework make strings immutable All that copying is surely going to be an overhead. Well, yes, it is, and sometimes you need to be aware of it. That being said, there are balancing performance improvements when dealing with unchanging strings. The framework can store a single instance of a string and then any variables that reference that particular sequence of characters can reference the same instance. This can actually save on allocations and reduce your working set. And in multithreaded scenarios, the fact that strings never change means it s safe to use them

When you run the wiki application, you will see a screen like in Figure 7-10.

without the cross-thread coordination that is required when accessing modifiable data. As usual, performance considerations are largely a compromise between the competing needs of various possible scenarios. In our view, an overridingly persuasive argument for immutability relates to the safe use of strings as keys. Consider the code in Example 10-51.

default: return true; } } return true; } Adding these saving and loading capabilities fits well into the SDI structure presented earlier. By confirming that the document actually has been saved (by using the return value from all methods involved), you can build a waterproof protection, making it impossible to close an unsaved document without confirming to do so.

string myKey = "TheUniqueKey"; Dictionary<string, object> myDictionary = new Dictionary<string, object>(); myDictionary.Add(myKey, new object()); // Imagine you could do this... myKey[2] = 'o';

Remember, a string is a reference type, so the myKey variable references a string object which is initialized to "TheUniqueKey" When we add our object to the dictionary, we pass a reference to that same string object, which the dictionary will use as a key If you cast your mind back to 9, you ll remember that the dictionary relies on the hash code for the key object when storing dictionary entries, which can then be disambiguated (if necessary) by the actual value of the key itself Now, imagine that we could modify the original string object, using the reference we hold in that myKey variable One characteristic of a (useful!) hash algorithm is that its output changes for any change in the original data So all of a sudden our key s hash code has changed The hash for "TheUniqueKey" is different from the one for "ThoUnique Key".