asp net qr code generator free Part IV in .NET framework

Generating QR Code JIS X 0510 in .NET framework Part IV

Part IV
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Important You need to be aware of some versioning issues here . If in version 1, a base type
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doesn t implement the IDisposable interface, it can never implement this interface in a later version . If the base type were to add the IDisposable interface in the future, all of the derived types wouldn t know to call the base type s methods, and the base type wouldn t get a chance to clean itself up properly . On the other hand, if in version 1 a base type implements the IDisposable interface, it can never remove this interface in a later version because the derived type would try to call methods that no longer exist in the base type .
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Another noteworthy part of this code is the call to GC s static SuppressFinalize method inside the Dispose method that takes a Boolean . You see, if code using a SafeHandle object explicitly calls Dispose or Close, there is no need for the object s Finalize method to execute, because if Finalize did execute, there would be an unnecessary attempt to release the resource a second time . The call to GC s SuppressFinalize turns on a bit flag associated with the object referred to by its single this parameter . When this flag is on, the CLR knows not to move this object s pointer from the finalization list to the freachable queue, preventing the object s Finalize method from being called and ensuring that the object doesn t live until the next garbage collection . Note that the SafeHandle class calls SuppressFinalize even when the object is being finalized . This has no ill effect because the object is already in the process of being finalized .
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using a Type That Implements the Dispose Pattern
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Now that you know how a type implements the dispose pattern, let s take a look at how a developer uses a type that offers the dispose pattern . Instead of talking about the SafeHandle class, let s talk about the more common System.IO.FileStream class . The FileStream class offers the ability to open a file, read bytes from the file, write bytes to the file, and close the file . When a FileStream object is constructed, the Win32 CreateFile function is called, the returned handle is saved in a SafeFileHandle object, and a reference to this object is maintained via a private field in the FileStream object . The FileStream class also offers several additional properties (such as Length, Position, CanRead) and methods (such as Read, Write, Flush) . Let s say that you want to write some code that creates a temporary file, writes some bytes to the file, and then deletes the file . You might start writing the code like this:
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using System; using System.IO; public static class Program { public static void Main() { // Create the bytes to write to the temporary file. Byte[] bytesToWrite = new Byte[] { 1, 2, 3, 4, 5 }; // Create the temporary file. FileStream fs = new FileStream("Temp.dat", FileMode.Create);
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21
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Automatic Memory Management (Garbage Collection)
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// Write the bytes to the temporary file. fs.Write(bytesToWrite, 0, bytesToWrite.Length); // Delete the temporary file. File.Delete("Temp.dat"); // Throws an IOException } }
Unfortunately, if you build and run this code, it might work, but most likely it won t . The problem is that the call to File s static Delete method requests that Windows delete a file while it is still open . And so Delete throws a System.IO.IOException exception with the following string message: "The process cannot access the file "Temp.dat" because it is being used by another process." Be aware that in some cases, the file might actually be deleted! If another thread somehow caused a garbage collection to start after the call to Write and before the call to Delete, the FileStream s SafeFileHandle field would have its Finalize method called, which would close the file and allow Delete to work . The likelihood of this situation is extremely rare, however, and therefore the previous code will fail more than 99 percent of the time . Fortunately, the FileStream class implements the dispose pattern, allowing you to modify the source code to explicitly close the file . Here s the corrected source code:
using System; using System.IO; public static class Program { public static void Main() { // Create the bytes to write to the temporary file. Byte[] bytesToWrite = new Byte[] { 1, 2, 3, 4, 5 }; // Create the temporary file. FileStream fs = new FileStream("Temp.dat", FileMode.Create); // Write the bytes to the temporary file. fs.Write(bytesToWrite, 0, bytesToWrite.Length); // Explicitly close the file when finished writing to it. fs.Dispose(); // Delete the temporary file. File.Delete("Temp.dat"); // This always works now. } }
The only difference here is that I ve added a call to FileStream s Dispose method . The Dispose method calls the Dispose method that takes a Boolean as parameter, which calls Dispose on the SafeFileHandle object, which ends up calling the Win32 CloseHandle function, which causes Windows to close the file . Now, when File s Delete method is called, Windows sees that the file isn t open and successfully deletes it .
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