Saving data of array

I have one question,i made a class with some strings,integers and array of another class that has some more strings and floats.
They both have some complex operations under but its not relevant.

Users would create their account and Class A(that has array of other class) and add it to one static array of mine.They can create as many as they like.I save everything with BinaryFormatter under their_username.dat inside class that has that array and 2 strings carrying some stuff for program.

Will this be safe for work,or should i find another method and doing all of this?Will array ever brake?