HOW TO：明確擲回例外狀況
您可以使用 throw 陳述式，明確擲回例外狀況。 您也可以使用 throw 陳述式，再次將已攔截的例外狀況擲回。 加入資訊至例外狀況 (在偵錯時被重新擲回以提供更多資訊)，是良好的程式設計方式。
下列程式碼範例使用 Try/Catch 區塊來攔截可能的 FileNotFoundException。 Try 區塊之後的是會攔截 FileNotFoundException，且如果沒找到資料檔案就將訊息寫入至主控台的 Catch 區塊。 下一個陳述式是 Throw 陳述式，它會擲回新的 FileNotFoundException 並加入文字資訊至例外狀況。
範例
Option Strict On
Imports System
Imports System.IO
Public Class ProcessFile
Public Shared Sub Main()
Dim fs As FileStream = Nothing
Try
'Opens a text file.
fs = New FileStream("c:\temp\data.txt", FileMode.Open)
Dim sr As New StreamReader(fs)
Dim line As String
'A value is read from the file and output to the console.
line = sr.ReadLine()
Console.WriteLine(line)
Catch e As FileNotFoundException
Console.WriteLine("[Data File Missing] {0}", e)
Throw New FileNotFoundException("[data.txt not in c:\temp directory]", e)
Finally
If fs IsNot Nothing Then fs.Close
End Try
End Sub
End Class
using System;
using System.IO;
public class ProcessFile
{
public static void Main()
{
FileStream fs = null;
try
{
//Opens a text tile.
fs = new FileStream(@"C:\temp\data.txt", FileMode.Open);
StreamReader sr = new StreamReader(fs);
string line;
//A value is read from the file and output to the console.
line = sr.ReadLine();
Console.WriteLine(line);
}
catch(FileNotFoundException e)
{
Console.WriteLine("[Data File Missing] {0}", e);
throw new FileNotFoundException(@"[data.txt not in c:\temp directory]",e);
}
finally
{
if (fs != null)
fs.Close();
}
}
}