System.AccessViolationException class

This article provides supplementary remarks to the reference documentation for this API.

An access violation occurs in unmanaged or unsafe code when the code attempts to read or write to memory that has not been allocated, or to which it does not have access. This usually occurs because a pointer has a bad value. Not all reads or writes through bad pointers lead to access violations, so an access violation usually indicates that several reads or writes have occurred through bad pointers, and that memory might be corrupted. Thus, access violations almost always indicate serious programming errors. An AccessViolationException clearly identifies these serious errors.

In programs consisting entirely of verifiable managed code, all references are either valid or null, and access violations are impossible. Any operation that attempts to reference a null reference in verifiable code throws a NullReferenceException exception. An AccessViolationException occurs only when verifiable managed code interacts with unmanaged code or with unsafe managed code.

Troubleshoot AccessViolationException exceptions

An AccessViolationException exception can occur only in unsafe managed code or when verifiable managed code interacts with unmanaged code:

• An access violation that occurs in unsafe managed code can be expressed as either a NullReferenceException exception or an AccessViolationException exception, depending on the platform.
• An access violation in unmanaged code that bubbles up to managed code is always wrapped in an AccessViolationException exception.

In either case, you can identify and correct the cause of the AccessViolationException exception as follows:

• Make sure that the memory that you're attempting to access has been allocated. An AccessViolationException exception is always thrown by an attempt to access protected memory—that is, to access memory that's not allocated or that's not owned by a process.

  Automatic memory management is one of the services that the .NET runtime provides. If managed code provides the same functionality as your unmanaged code, you may wish to move to managed code to take advantage of this functionality. For more information, see Automatic Memory Management.

• Make sure that the memory that you're attempting to access has not been corrupted. If several read or write operations have occurred through bad pointers, memory might be corrupted. This typically occurs when reading or writing to addresses outside of a predefined buffer.

AccessViolationException and try/catch blocks

AccessViolationException exceptions thrown by the .NET runtime aren't handled by the catch statement in a structured exception handler if the exception occurs outside of the memory reserved by the runtime. To handle such an AccessViolationException exception, apply the HandleProcessCorruptedStateExceptionsAttribute attribute to the method in which the exception is thrown. This change does not affect AccessViolationException exceptions thrown by user code, which can continue to be caught by a catch statement.