In this exercise, you will write a console application that displays whether objects are value or reference types.

1. Using Visual Studio, create a new console application project. Name the project List-Value-Types.

2. Create instances of the following classes:

   • SByte

   • Byte

   • Int16

   • Int32

   • Int64

   • String

   • Exception

   The following code demonstrates this:

   ' VBDim a As SByte = 0Dim b As Byte = 0Dim c As Int16 = 0Dim d As Int32 = 0Dim e As Int64 = 0Dim s As String = ""Dim ex As Exception = New Exception
   
   // C#SByte a = 0;Byte b =0;Int16 c = 0;Int32 d = 0;Int64 e = 0;string s = "";Exception ex = new Exception();

3. Add each of the instances to a new object array, as the following code demonstrates:

   ' VBDim types As Object() = {a, b, c, d, e, s, ex}
   
   // C#object[] types = { a, b, c, d, e, s, ex };
   

4. Within a foreach loop, check the object.GetType().IsValueType property to determine whether the type is a value type. Display each type name and whether it is a value type or a reference type, as the following code demonstrates:

   ' VBFor Each o As Object In types    Dim type As String    If o.GetType.IsValueType Then        type = "Value type"    Else        type = "Reference Type"    End If    Console.WriteLine("{0}: {1}", o.GetType, type)Next
   
   // C#foreach ( object o in types ){    string type;    if (o.GetType().IsValueType)        type = "Value type";    else        type = "Reference Type";
   
       Console.WriteLine("{0}: {1}", o.GetType(), type);}

5. Run the console application, and verify that each type matches your understanding.

在这里练习里面，你将编写一个函数来排序字符串.

1. Using Visual Studio, create a new console application project. Name the project SortString.

2. Define a string. Then use the String.Split method to separate the string into an array of words. The following code demonstrates this:

   ' VBDim s As String = "Microsoft .NET Framework 2.0 Application Development Foundation"Dim sa As String() = s.Split(" ")
   
   // C#string s = "Microsoft .NET Framework 2.0 Application Development Foundation";string[] sa = s.Split(' ');

3. Call the Array.Sort method to sort the array of words, as the following code demonstrates:

   ' VBArray.Sort(sa)
   
   // C#Array.Sort(sa);

4. Call the String.Join method to convert the array of words back into a single string, and then write the string to the console. The following code sample demonstrates this:

   ' VBs = String.Join(" ", sa)Console.WriteLine(s)
   
   // C#s = string.Join(" ", sa);Console.WriteLine(s);

5. Run the console application, and verify that it works correctly.

Consider a scenario（情节） in which a coworker wrote a simple Windows Forms application to view text files. However, users complain that it is very temperamental（喜怒无常的）. If the user mistypes the filename or if the file is not available for any reason, the application fails with an unhandled exception error. You must add exception handling to the application to display friendly error messages to users if a file is not available.

1. Copy the Chapter01\Lesson2-ViewFile folder from the companion CD to your hard disk, and open either the C# version or the Visual Basic .NET version of the ViewFile project.

2. Exceptions occur when users attempt to view a file. Therefore, edit the code that runs for the showButton.Click event. Add code to catch any type of exception that occurs, and display the error in a dialog box to the user. If an exception occurs after the TextReader object is initialized, you should close it whether or not an exception occurs. You will need two nested Try blocks: one to catch exceptions during the TextReader initialization, and a second one to catch exceptions when the file is read. The following code sample demonstrates this:

   ' VBTry    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 TryCatch 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); }

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:

   ' VBTry    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 TryCatch 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
   
   // 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); }

5. Run your application again, and verify that it provides your new error message if an invalid filename is provided.