Here are few classes:

public class MyItem : ParamOut
{
    public string Code
    {
        get { return _quoteCode; }
    }

    public InnerItem[] Skus
    {
        get { return _skus; }
    }

    public PriceSummary Price
    {
        get { return _price; }
    }

    public override string ToString()
    {
        return string.Concat("Code=", Code, "; SKUs=[", Skus != null ? "{" + string.Join("},{", Array.ConvertAll(Skus, item => item.ToString())) + "}" : "", "]"
            , "; Price={", Price.ToString(), "}", base.ToString()
            );
    }

    ...
}

public abstract class ParamOut
{
    public override string ToString()
    {
        return null;
    }

    public string ErrorMessage { get; set; }
}

Here is calling functionality:

{
    MyItem item = new MyItem{ ErrorMessage = "Bla-bla-bla" };
    string text = item.ToString();
}

I am getting NullReference exception inside of 'ToString()' method (each property of item variable is null).

Question:

Q1. what overload of string.Concat will be called in this case? I have 9 parameters, so I guess one of the following:

public static string Concat(params Object[] args)

or

public static string Concat(params string[] values)

But which of them?

Q2. Why exception is generated? Shouldn't 'null' be converted into something like 'null' or "" (empty string)?

Thanks a lot!