HOW TO:明確擲回例外狀況
更新:2007 年 11 月
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();
}
}
}