This rule raises an issue when a generic exception (such as Exception, SystemException, ApplicationException, IndexOutOfRangeException, NullReferenceException, OutOfMemoryException or ExecutionEngineException) is thrown.

Why is this an issue?

Throwing general exceptions such as Exception, SystemException and ApplicationException will have a negative impact on any code trying to catch these exceptions.

From a consumer perspective, it is generally a best practice to only catch exceptions you intend to handle. Other exceptions should ideally not be caught and let to propagate up the stack trace so that they can be dealt with appropriately. When a generic exception is thrown, it forces consumers to catch exceptions they do not intend to handle, which they then have to re-throw.

Besides, when working with a generic type of exception, the only way to distinguish between multiple exceptions is to check their message, which is error-prone and difficult to maintain. Legitimate exceptions may be unintentionally silenced and errors may be hidden.

For instance, if an exception such as StackOverflowException is caught and not re-thrown, it may prevent the program from terminating gracefully.

When throwing an exception, it is therefore recommended to throw the most specific exception possible so that it can be handled intentionally by consumers.

Additionally, some reserved exceptions should not be thrown manually. Exceptions such as IndexOutOfRangeException, NullReferenceException, OutOfMemoryException or ExecutionEngineException will be thrown automatically by the runtime when the corresponding error occurs. Many of them indicate serious errors, which the application may not be able to recover from. It is therefore recommended to avoid throwing them as well as using them as base classes.

How to fix it

To fix this issue, make sure to throw specific exceptions that are relevant to the context in which they arise. It is recommended to either:

• Throw a subtype of Exception when one matches. For instance ArgumentException could be raised when an unexpected argument is provided to a function.
• Define a custom exception type that derives from Exception or one of its subclasses.

Code examples

Noncompliant code example

Public Sub DoSomething(obj As Object)
  If obj Is Nothing Then
    ' Noncompliant
    Throw New NullReferenceException("obj") ' Noncompliant: This reserved exception should not be thrown manually
  End If
  ' ...
End Sub

Compliant solution

Public Sub DoSomething(obj As Object)
  If obj Is Nothing Then
    Throw New ArgumentNullException("obj") ' Compliant: this is a specific and non-reserved exception type
  End If
  ' ...
End Sub

Resources

Standards

• CWE - CWE-397 Declaration of Throws for Generic Exception