String.Join dropping null values returning less items than number of Elements in the Array

Question

Given this method, The resulting string created drops the consecutive null values. Is th ?? being used wrong below? It's behaving as if it is concatentating all consectively indexed null values before returning

 public static IEnumerable<string> ToCsv<T>(string separator, IEnumerable<T> objectlist)
        {
            FieldInfo[] fields = typeof(T).GetFields();
            PropertyInfo[] properties = typeof(T).GetProperties();
            yield return String.Join(separator, fields.Select(f => f.Name).Union(properties.Select(p => p.Name)).ToArray());
            foreach (var o in objectlist)
            {
                var pp = properties.Select(
                    p => (p.GetValue(o, null) ?? String.Empty));

                var val = string.Join(separator, fields.Select(f => (f.GetValue(o)).ToString() )
                    .Union(pp).ToArray());
                ;
                yield return val;
            }
        }

Here is my array

[0]"0001020003"
[1]"Bob Jones"
[2] NULL
[3] NULL
[4] "Longboat"

With that array the joined string created is...

"0001020003,Bob Jones,,Longboat"

Answer 1

Enumerable.Union excludes duplicates, so you are only getting a single blank instance returned.

Try using Concat instead.