The ToString() throws a NullReferenceException when the QuadraticObjectiveFunction is instantiated without using the Expression constructor (i.e. instantiated using matrices and vectors) because the Dictionaries are never initialised.

I'm open to suggestions on what should be shown but I would propose, say, "6-dimensional Quadratic Objective Function" (clearly we cannot enumerate the whole problem if it is a 1000x1000 matrix!

Thinking about it, perhaps it wouldn't be so bad to do away with those dictionaries altogether. The info seems mostly redundant as one can quite easily back out a string representation from knowledge of the underlying matrix and vector (ignoring zeros). That would make GH-819 easier but let me know if you'd like to keep them.

thanks,
Alex