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CLS and Non-CLS Exceptions
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All programming languages for the CLR must support the throwing of Exceptionderived objects because the Common Language Specification (CLS) mandates this . However, the CLR actually allows an instance of any type to be thrown, and some programming languages will allow code to throw non CLS-compliant exception objects such as a String, Int32, or DateTime . The C# compiler allows code to throw only Exception-derived objects, whereas code written in some other languages allow code to throw Exception-derived objects as well as objects that are not derived from Exception . Many programmers are not aware that the CLR allows any object to be thrown to report an exception . Most developers believe that only Exception-derived objects can be thrown . Prior to version 2 .0 of the CLR, when programmers wrote catch blocks to catch exceptions, they were catching CLS-compliant exceptions only . If a C# method
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20 Exceptions and State Management
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called a method written in another language, and that method threw a non CLScompliant exception, the C# code would not catch this exception at all, leading to some security vulnerabilities . In version 2 .0 of the CLR, Microsoft introduced a new RuntimeWrappedException class (defined in the System.Runtime.CompilerServices namespace) . This class is derived from Exception, so it is a CLS-compliant exception type . The RuntimeWrappedException class contains a private field of type Object (which can be accessed by using RuntimeWrappedException s WrappedException read-only property) . In version 2 .0 of the CLR, when a non CLS-compliant exception is thrown, the CLR automatically constructs an instance of the RuntimeWrappedException class and initializes its private field to refer to the object that was actually thrown . In effect, the CLR now turns all non CLS-compliant exceptions into CLS-compliant exceptions . Any code that now catches an Exception type will catch non CLS-compliant exceptions, which fixes the potential security vulnerability problem . Although the C# compiler allows developers to throw Exception-derived objects only, prior to C# version 2 .0, the C# compiler did allow developers to catch non CLS-compliant exceptions by using code like this:
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private void SomeMethod() { try { // Put code requiring graceful recovery and/or cleanup operations here... } catch (Exception e) { // Before C# 2.0, this block catches CLS-compliant exceptions only // Now, this block catches CLS- & non CLS-compliant exceptions throw; // Re-throws whatever got caught } catch { // In all versions of C#, this block catches CLS- & non CLS-compliant exceptions throw; // Re-throws whatever got caught } }
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Now, some developers were aware that the CLR supports both CLS- and non CLScompliant exceptions, and these developers might have written the two catch blocks (shown above) in order to catch both kinds of exceptions . If the above code is recompiled for CLR 2 .0 or later, the second catch block will never execute, and the C# compiler will indicate this by issuing a warning: "CS1058: A previous catch clause
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already catches all exceptions. All non-exceptions thrown will be wrapped in a System.Runtime.CompilerServices.RuntimeWrappedException."
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There are two ways for developers to migrate code from a version of the .NET Framework prior to version 2 .0 . First, you can merge the code from the two catch blocks into a single catch block and delete one of the catch blocks . This is the recommended approach . Alternatively, you can tell the CLR that the code in your assembly
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wants to play by the old rules . That is, tell the CLR that your catch (Exception) blocks should not catch an instance of the new RuntimeWrappedException class . And instead, the CLR should unwrap the non CLS-compliant object and call your code only if you have a catch block that doesn t specify any type at all . You tell the CLR that you want the old behavior by applying an instance of the RuntimeCompatibilityAttribute to your assembly like this:
using System.Runtime.CompilerServices; [assembly:RuntimeCompatibility(WrapNonExceptionThrows = false)]
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