why isn't Move.NullMove a constant

public string GetBestMove(Board board, int depth = 3, Move prvmove = Move.NullMove)
I want the previous move made for each depth , but its giving the following error
Default parameter value for prvmove must be a compile time constant?

[ryanheath]

You could try pass default instead
public string GetBestMove(Board board, int depth = 3, Move prvmove = default)
if you are lucky default could translate into the same state as Move.NullMove (it is not strictly the same!)

If that doesn't work, then save yourself the headache, remove the default value and pass Move.NullMove where appropiate.