90 lines
1.7 KiB
Markdown
90 lines
1.7 KiB
Markdown
---
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title: Exceptions
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---
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# Exceptions
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An exception is an unexpected error that occurs while a program is running, such as an attempt to access a file that does not exist. It will stop the program if not handled.
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## Example
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If we try to read the text of a file that does not exist:
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```csharp
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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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A `FileNotFoundException` will be raised.
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Some other common exceptions:
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* `IndexOutofRangeException`: Attempted to access an array with an invalid index.
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* `NullReferenceException`: Attempted to use an unassigned reference variable.
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* `DivideByZeroException`: Attempted to divide by 0.
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## Get The Message Inside An Exception
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Whenever you use the generic `Exception` catch all handler, you can grab the message of what caused the error each time. It is very simple to do so:
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```csharp
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try
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{
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string content = File.ReadAllText(@"C:\DoesNotExist.txt");
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}
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catch (Exception ex)
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{
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string message = "";
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if (ex.InnerException != null) {
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message = ex.InnerException.Message;
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}
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else {
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message = ex.Message;
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}
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Console.WriteLine(message);
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}
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```
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## Best Practices
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### Use try/catch/finally Blocks
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```csharp
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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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### Handle Possible Exceptions With Conditions
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Instead of
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```csharp
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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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Try this
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```csharp
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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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