IMHO for consistency empty entry vector should return {0,0}... This special case precludes the use of a direct algorithm STL

System.NullReferenceException : Object reference not set to an instance of an object.
at Kata.CountPositivesSumNegatives(Int32[] input)
at SumTests.CountPositivesSumNegatives_InputNull()

I keep getting this error, new to C# and no clue why

The last one.

A similar case is explained in the kata's description. Read it again.

At the end of the 2nd year there will be: 
1070 + 1070 * 0.02 + 50 => 1141 inhabitants (** number of inhabitants is an integer **)

50 !== 51

.