qr code c# mvc Lesson 2: Using Common Reference Types in Visual C#

Draw QR-Code in Visual C# Lesson 2: Using Common Reference Types

Lesson 2: Using Common Reference Types
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' Read and display a text file Dim sr As StreamReader = New StreamReader("text.txt") Console.WriteLine(sr.ReadToEnd) sr.Close // C# // Create and write to a text file StreamWriter sw = new StreamWriter("text.txt"); sw.WriteLine("Hello, World!"); sw.Close(); // Read and display a text file StreamReader sr = new StreamReader("text.txt"); Console.WriteLine(sr.ReadToEnd()); sr.Close();
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MORE INFO Streams
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For more information about streams, refer to 2, Input/Output (I/O).
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How to Throw and Catch Exceptions
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Exceptions are unexpected events that interrupt normal execution of an assembly. For example, if your assembly is reading a large text file from a removable disk and the user removes the disk, the runtime will throw an exception. This makes sense because there is no way your assembly could continue running. Exceptions should never cause your assembly to fail completely. Instead, you should plan for exceptions to occur, catch them, and respond to the event. In the preceding example, you could notify the user that the file was not available, and then await further instructions from the user. The following simplified code, which requires the System.IO namespace, demonstrates this:
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' VB Try Dim sr As StreamReader = New StreamReader("C:\boot.ini") Console.WriteLine(sr.ReadToEnd) Catch ex As Exception ' If there are any problems reading the file, display an error message Console.WriteLine("Error reading file: " + ex.Message) End Try // C# try { StreamReader sr = new StreamReader(@"C:\boot.ini"); Console.WriteLine(sr.ReadToEnd()); }
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catch (Exception ex) { // If there are any problems reading the file, display an error message Console.WriteLine("Error reading file: " + ex.Message); }
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In the preceding example, if any type of error occurs including a file not found error, insufficient privileges error, or an error during the reading of the file processing continues within the Catch block. If no problems occur, the runtime skips the Catch block. The base Exception class is very useful and contains an error message and other application data. In addition to the base Exception class, the Framework defines hundreds of exception classes to describe different types of events, all derived from System.SystemException. Additionally, you can define your own exceptions when you need to describe an event in more detail than allowed by the standard exception classes by deriving from System.ApplicationException. Having multiple exception classes allows you to respond differently to different types of errors. The runtime will execute only the first Catch block with a matching exception type, however, so order Catch blocks from the most-specific to the least-specific. The following code sample displays different error messages for a file not found error, an insufficient privileges error, and any other type of error that might occur:
' VB Try Dim sr As StreamReader = New StreamReader("text.txt") Console.WriteLine(sr.ReadToEnd) Catch ex As System.IO.FileNotFoundException Console.WriteLine("The file could not be found.") Catch ex As System.UnauthorizedAccessException Console.WriteLine("You do not have sufficient permissions.") Catch ex As Exception Console.WriteLine("Error reading file: " + ex.Message) End Try
This process is sometimes called filtering exceptions. Exception handling also supports a Finally block. The Finally block runs after the Try block and any Catch blocks have finished executing, whether or not an exception was thrown. Therefore, you should use a Finally block to close any streams or clean up any other objects that might be left open if an exception occurs. The following code sample closes the StreamReader object whether or not an exception occurs:
' VB Dim sr As StreamReader = New StreamReader("text.txt") Try Console.WriteLine(sr.ReadToEnd)
Lesson 2: Using Common Reference Types
Catch ex As Exception ' If there are any problems reading the file, display an error message Console.WriteLine("Error reading file: " + ex.Message) Finally ' Close the StreamReader, whether or not an exception occurred sr.Close End Try // C# StreamReader sr = new StreamReader("text.txt"); try { Console.WriteLine(sr.ReadToEnd()); } catch (Exception ex) { // If there are any problems reading the file, display an error message Console.WriteLine("Error reading file: " + ex.Message); } finally { // Close the StreamReader, whether or not an exception occurred sr.Close(); }
Notice that the StreamReader declaration was moved outside the Try block in the preceding example. This is necessary because the Finally block cannot access variables that are declared within the Try block. This makes sense because depending on where an exception occurred, variable declarations within the Try block might not yet have been executed. To catch exceptions that occur both during and after the StreamReader declaration, use nested Try/Catch/Finally blocks. Typically, all code except for simple variable declarations should occur within Try blocks. Robust error handling improves the user experience when problems occur and greatly simplifies debugging problems. However, exception handling does incur a slight performance penalty. To conserve space and focus on specific topics, sample code within this book will typically not include exception handling.
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