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How to: Catch a non-CLS Exception

Some .NET languages, including C++/CLI, allow objects to throw exceptions that do not derive from Exception. Such exceptions are called non-CLS exceptions or non-Exceptions. In Visual C# you cannot throw non-CLS exceptions, but you can catch them in two ways:

  • Within a catch (Exception e) block as a RuntimeWrappedException.

    By default, a Visual C# assembly catches non-CLS exceptions as wrapped exceptions. Use this method if you need access to the original exception, which can be accessed through the WrappedException property. The procedure later in this topic explains how to catch exceptions in this manner.

  • Within a general catch block (a catch block without an exception type specified) that is put after a catch (Exception) or catch (Exception e) block.

    Use this method when you want to perform some action (such as writing to a log file) in response to non-CLS exceptions, and you do not need access to the exception information. By default the common language runtime wraps all exceptions. To disable this behavior, add this assembly-level attribute to your code, typically in the AssemblyInfo.cs file: [assembly: RuntimeCompatibilityAttribute(WrapNonExceptionThrows = false)].

To catch a non-CLS exception

  1. Within a catch(Exception e) block, use the as keyword to test whether e can be cast to a RuntimeWrappedException.

  2. Access the original exception through the WrappedException property.

The following example shows how to catch a non-CLS exception that was thrown from a class library written in C++/CLR. Note that in this example, the Visual C# client code knows in advance that the exception type being thrown is a System.String. You can cast the WrappedException property back its original type as long as that type is accessible from your code.

// Class library written in C++/CLR.
   ThrowNonCLS.Class1 myClass = new ThrowNonCLS.Class1();

   try
   {
    // throws gcnew System::String(
    // "I do not derive from System.Exception!");
    myClass.TestThrow(); 
   }
   

   catch (Exception e)
   {
    RuntimeWrappedException rwe = e as RuntimeWrappedException;
    if (rwe != null)    
    {
      String s = rwe.WrappedException as String;
      if (s != null)
      {
        Console.WriteLine(s);
      }
    }
    else
    {
       // Handle other System.Exception types.
    }
   }           

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