Errata

The PrintPortfolio method of the StockPortfolio class indicates that the field "_stocks" has a method named "DisplayStocks" associated with it. The type specified for "_stocks" does not have such a method. Trying to invoke the method causes an error.

public void PrintPortfolio(string title)
{
Console.WriteLine(title);
_stocks.DisplayStocks();
}

Unnecessary if check. Since the last two items the if...else...if chain return the same value, and we end on an else, then we can get rid of the final else if. It doesn't matter if that final self if passes or fails, the method still returns a -1. So, just get rid of it and save a bit of computing power while you're at it.

A reference type can be of type Square and have a null value. Saying that a parameter needs to be of type Square, when the problem is that it has a null value, will not help your fellow developers when they use your code. You should check the firstSquare and secondSqure objects for null values before you try to cast them into objects of type Square.

There's no null check. If this were a private method, then you could get away from not having a null check because developers using this class wouldn't have a way to pass in a null value to it. But, it's a public method, so anyone can call this method and pass in a null value.