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Sub LoadText() 'In this sub, we check whether a file exists 'If it does, the file is loaded as a block 'of text into txtFileInfo Dim fileName As String Dim inStream As IOStreamReader fileName = "c:\data\TextFiletxt" If IOFileExists(fileName) Then 'Open the file inStream = New IOStreamReader(fileName) 'Read contents as a block and add to text box txtFileInfoText = inStreamReadToEnd() inStreamClose() Else MessageBoxShow(fileName & " does not exist") End If End Sub Listing 113: Reading text from a le using a StreamReader
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In addition to the steps of getting a le name and creating a StreamReader object, we have one other check to perform if the program is to be robust While a StreamWriter will create a new le if one does not exist to write to, a StreamReader expects the le-name passed in its constructor to be the name of an existing le, and the code will cause a crash if this is not so The File class gives us a method for checking to see if the named le exists before the StreamReader is constructed If the Exists() method of the File class (all of the methods of this class are Shared, so we do not need to create an object) returns True, we can go on and create the stream If not, a suitable error message can be displayed The ReadToEnd() method of the StreamReader class will return the entire contents of the le, which can be passed directly to the Text property of the TextBox Again, the stream is closed immediately to place the le into a safe state
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1142 Safe File Operations
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All operations that deal with bringing data into a program from some device or writing data to a device are likely to cause problems When a program tries to read from a device, the device name could be wrong, the device might not exist, or might be switched off, or the information on the device might be in a different format from what is expected; all situations that will cause the read operation to fail Similarly, an attempt to write data to a device can fail because the device is missing, or cannot be written to, or has failed in some other way Compared to moving information around inside a program, moving data to or from a storage device is fraught with danger When writing code to interact with some storage device, it is necessary to be aware of the potential pitfalls, and optimally, to deal with them when they arise Fortunately, the NET framework gives us a catch-all way of dealing with such problems; exceptions, described in- 5, are tailor-made for coping with the types of run-time error that can occur while trying to move data to or from les
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11 n Files, Streams and Serialization
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Recall that run-time error-handling is about stopping a program from crashing simply because some situation that cannot be dealt with in the normal ow of an algorithm arises For example, we might try to open a le and read data from it If it all works, the program can continue to do its nominal work If, however, the le fails because it is corrupt or because of some other physical problem, the default behaviour of a VB NET program is to exit with a terse error message If we were to deal with the unavailable le safely, we might inform the user that the expected data was not available, and try to tidy up before continuing without the expected information Generally it is a good idea to try to close a le, and perhaps even provide some default information to take the place of what we expected to read from the le In VB NET, we can use a TryCatchFinally block to provide a safety net (Listing 114)
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Try inStream = New IOStreamReader(fileName) txtFileInfoText = inStreamReadToEnd() lstFileLinesItemsAddRange(txtFileInfoLines) Catch ex As Exception MessageBoxShow(exMessage) Finally If Not inStream Is Nothing Then inStreamClose() End If End Try Listing 114: Using a TryCatchFinally block to deal with le errors
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Now, if the stream object cannot gain access to the named le for any reason, the Catch block will tell the user and the Finally block will ensure that the stream is not left open It is important to be aware that every le operation involving opening, reading or writing has the potential to cause a program to crash, and so every le operation ought to be enclosed in an exception-handling structure in this way Later in this chapter we will look at more structured les and this will introduce a whole new category of potential problems to deal with exception-handling will save us from becoming bogged down in code written solely to work around the potential problems that might occur
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