Why float.NaN != double.NaN ?

while float.PositiveInfinity == double.PositiveInfinity and float.NegativeInfinity == double.NegativeInfinity are equal.

EXAMPLE:

bool PosInfinity = (float.PositiveInfinity == double.PositiveInfinity); //true
bool NegInfinity = (float.NegativeInfinity == double.NegativeInfinity); //true

bool isNanEqual = (float.NaN == double.NaN);  //false, WHY?
c# .net nan
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