Why Task.CurrentId returns Nullable<int> instead of int?

sorry if it's obvious question. i'm curious, why task.currentid returns nullable<int> (reference type) instead of int (value type)?

task.currentid returns task id (positive integer) if current thread belongs task, , returns null otherwise.

using nullable<int> creates overhead (garbage collection);. why not returns -1 when current thread not belong task?

thanks in advance.

edit: nullable<int> struct not class, not have problem garbage collection.

because -1 valid id, other negative numbers.

null the error return value.

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let's talk mvvm: http://social.msdn.microsoft.com/forums/en-us/wpf/thread/b1a8bf14-4acd-4d77-9df8-bdb95b02dbe2 please mark post helpfull , answers respectively.

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