66 lines
1.2 KiB
Markdown
66 lines
1.2 KiB
Markdown
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---
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title: Exceptions
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localeTitle: 例外
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---
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# 例外
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例外是程序运行时发生的意外错误，例如尝试访问不存在的文件。如果不处理，它将停止程序。
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## 例
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如果我们尝试读取不存在的文件的文本：
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```
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using System.IO;
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string content = File.ReadAllText(@"C:\DoesNotExist.txt");
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```
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将引发`FileNotFoundException` 。
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其他一些常见例外：
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* `IndexOutofRangeException` ：尝试访问具有无效索引的数组。
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* `NullReferenceException` ：尝试使用未分配的引用变量。
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* `DivideByZeroException` ：尝试除以0。
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## 最佳实践
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### 使用try / catch / finally块
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```
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try
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{
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var client = new WebClient();
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var resultData = client.DownloadString("http://github.com");
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}
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catch (Exception ex)
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{
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//code for handling exceptions
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}
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finally
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{
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//this code is always executed, does not matter if an exception is thrown or not
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}
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```
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### 处理条件可能存在的例外情况
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代替
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```
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try
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{
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conn.Close();
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}
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catch (Exception ex)
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{
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//code for handling exceptions.
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}
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```
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试试这个
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```
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if (conn.State != ConnectionState.Closed)
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{
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conn.Close();
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}
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```
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