Exceptions

All exceptions derive from System.Exception.

Catch Exceptions

try { … }
catch (exception e) { // Catches a specific exception.  Stores it in e.
	// e.StackTrace contains the current call stack, file name, and line number where the exception was thrown.
	// e.Message contains the string describing the exception.
	// e.GetType() contains the type of the exception.
}
catch { … }	// Catches any exception.

catch { } // Use an empty catch block to catch errors, ignore them, and continue running.

finally { … } // This block is always executed, whether or not an exception is caught.

Notes:

• Order catch blocks from most specific to least specific. ○ Example: DirectoryNotFoundException –> FileNotFoundException –> IOException
• If you catch System.Exception, rethrow it at the end of the catch block.

Exception Filters

Exception Filters allow you to catch an exception only when a condition is true:

catch (Exception e) when (condition) { … }

Throw Exceptions

throw new ExceptionType("Exception message");

Rethrow Exceptions

If an exception is caught and you want to rethrow it up the stack use throw.

• If you throw ex to rethrow, the stack trace is lost.

If a catch block does nothing but rethrow exceptions, it is useless. Remove it.

To wrap a caught exception in another with more information, throw a new exception and pass the caught exception as the innerException parameter:

catch (IOException ex) {
	…
	throw new InvalidOperationException(message: "This thing failed.", innerException: ex)
}

Throwing Exceptions

Throw These Exceptions

• ArgumentException
• ArgumentNullException ○ Includes a second parameter, ParamName, that should be set to the name of the argument that caused the exception to be thrown. ○ In a property setter, ParamName should be set to value.
• ArgumentOutOfRangeException
• FormatException
• InvalidOperationException – If the object is in an inappropriate state.
• NotSupportedException

Do Not Throw These Exceptions

Don’t throw or derive from:

• AccessViolationException
• ApplicationException
• ComException
• ExecutionEngineException
• Exception (why?)
• IndexOutOfRangeException
• NullReferenceException
• OutOfMemoryException
• StackOverflowException
• SystemException

Create a Custom Exception [ Documentation]

Custom exception classes can be created. They should have at least 4 constructors:

class CustomException : Exception {
	public CustomException() : base() { }
	
	// This constructor sets the message property only:
	public CustomException(string message) : base(message) { }
	
	// This constructor sets the message property and InnerException:
	public CustomException(string message, Exception inner) : base(message, inner) { }
	
	// This constructor creates serialization, which is needed when an exception propagates from a remote server to the client:
	public CustomException(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) : base(info, context) { }
}

throw new CustomException("Exception message");

Common Exception Classes