Interlocked.Decrement Method (Int64%)
Decrements the specified 64-bit signed integer variable and stores the result, as an atomic operation.
Assembly: mscorlib (in mscorlib.dll)
The 64-bit overloads of the Increment, Decrement, and Add methods are truly atomic only on systems where a System.IntPtr is 64 bits long. On other systems, these methods are atomic with respect to each other, but not with respect to other means of accessing the data. Thus, to be thread safe on 32-bit systems, any access to a 64-bit value must be made through the members of the Interlocked class.
Silverlight for Windows Phone64-bit members of the Interlocked class are present but not supported.
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.