最后:是否保证在任何情况下都可以调用它?

Reliability Best Practices

Stephen Toub: Keep Your Code Running with the Reliability Features of the .NET Framework

CriticalFinalizerObject SafeHandle

finally

this was possible finally Thread.Abort

finally

IDisposable someDisposableObject = null;
IDisposable someOtherDisposableObject = null;

try
{
    someDisposableObject = GetDisposableObject();

    throw new Exception("Holy crap, something bad happened.");

    someOtherDisposableObject = GetOtherDisposableObject();
}
finally
{
    // This will throw a NullReferenceException...
    someOtherDisposableObject.Dispose();

    // ...so this actually won't run.
    someDisposableObject.Dispose();
}