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Creation QR Code JIS X 0510 in Visual C# Lesson 2: Using Common Reference Types

Lesson 2: Using Common Reference Types
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Catch ex As Exception MessageBox.Show(ex.Message) Finally tr.Close() End Try Catch ex As Exception MessageBox.Show(ex.Message) End Try // C# try { TextReader tr = new StreamReader(locationTextBox.Text); try { displayTextBox.Text = tr.ReadToEnd(); } catch (Exception ex) { MessageBox.Show(ex.Message); } finally { tr.Close(); } } catch (Exception ex) { MessageBox.Show(ex.Message); }
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3. Run your application. First verify that it can successfully display a text file. Then provide an invalid filename, and verify that a message box appears when an invalid filename is provided. 4. Next add overloaded exception handling to catch System.IO.FileNotFoundException and System.UnauthorizedAccessException. The following code sample demonstrates this:
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' VB Try Dim tr As TextReader = New StreamReader(locationTextBox.Text) Try displayTextBox.Text = tr.ReadToEnd Catch ex As Exception MessageBox.Show(ex.Message) Finally tr.Close() End Try Catch ex As System.IO.FileNotFoundException MessageBox.Show("Sorry, the file does not exist.") Catch ex As System.UnauthorizedAccessException MessageBox.Show("Sorry, you lack sufficient privileges.") Catch ex As Exception MessageBox.Show(ex.Message) End Try
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Framework Fundamentals
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// C# try { TextReader tr = new StreamReader(locationTextBox.Text); try { displayTextBox.Text = tr.ReadToEnd(); } catch (Exception ex) { MessageBox.Show(ex.Message); } finally { tr.Close(); } } catch (System.IO.FileNotFoundException ex) { MessageBox.Show("Sorry, the file does not exist."); } catch (System.UnauthorizedAccessException ex) { MessageBox.Show("Sorry, you lack sufficient privileges."); } catch (Exception ex) { MessageBox.Show(ex.Message); }
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5. Run your application again, and verify that it provides your new error message if an invalid filename is provided.
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Lesson Summary
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Reference types contain the address of data rather than the actual data. When you copy a value type, a second copy of the value is created. When you copy a reference type, only the pointer is copied. Therefore, if you copy a reference type and then modify the copy, both the copy and the original variables are changed. The .NET Framework includes a large number of built-in reference types that you can use directly or use to build your own custom types. Strings are immutable; use the StringBuilder class to create a string dynamically. Use streams to read from and write to files, memory, and the network. Use the Catch clause within Try blocks to filter exceptions by type. Close and dispose of nonmemory resources in the Finally clause of a Try block.
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Lesson Review
You can use the following questions to test your knowledge of the information in Lesson 2, Using Common Reference Types. The questions are also available on the companion CD if you prefer to review them in electronic form.
Lesson 2: Using Common Reference Types
NOTE Answers
Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
1. Which of the following are reference types (Choose all that apply.) A. Types declared Nullable B. String C. Exception D. All types derived from System.Object 2. What is the correct order for Catch clauses when handling different exception types A. Order from most general to most specific. B. Order from most likely to least likely to occur. C. Order from most specific to most general. D. Order from least likely to most likely to occur. 3. When should you use the StringBuilder class instead of the String class A. When building a string from shorter strings. B. When working with text data longer than 256 bytes. C. When you want to search and replace the contents of a string. D. When a string is a value type. 4. Why should you close and dispose of resources in a Finally block instead of a Catch block A. It keeps you from having to repeat the operation in each Catch. B. Finally blocks run whether or not an exception occurs. C. The compiler throws an error if resources are not disposed of in the Finally block. D. You cannot dispose of resources in a Catch block.
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