Why is string.Join(string, object[]) special?

Question

I was looking at the following expressions:

string.Join(",", new object[] { null, "StackOverflow" })
string.Join(",", new string[] { null, "StackOverflow" })
string.Join(",", new object[] { string.Empty, "StackOverflow" })
string.Join(",", new string[] { string.Empty, "StackOverflow" })

I would have thought they would return the same value:

,StackOverflow

However, the first expression actually returns string.Empty. This is actually defined behavior:

If the first element of values is null, the Join(String, Object[]) method does not concatenate the elements in values but instead returns String.Empty. A number of workarounds for this issue are available. The easiest is to assign a value of String.Empty to the first element of the array, as the following example shows.

Does anyone know the reason behind this inconsistency?

Answer 1

In your favorite decompiler you can see that the code for the method is

public static string Join(string separator, params object[] values)
{
  if (values == null)
    throw new ArgumentNullException("values");
  if (values.Length == 0 || values[0] == null)
    return string.Empty;
  if (separator == null)
    separator = string.Empty;
  StringBuilder sb = StringBuilderCache.Acquire(16);
  string str1 = values[0].ToString();
  if (str1 != null)
    sb.Append(str1);
  for (int index = 1; index < values.Length; ++index)
  {
    sb.Append(separator);
    if (values[index] != null)
    {
      string str2 = values[index].ToString();
      if (str2 != null)
        sb.Append(str2);
    }
  }
  return StringBuilderCache.GetStringAndRelease(sb);
}

The part responsible for the special behavior is

  if (values.Length == 0 || values[0] == null)
    return string.Empty;

but we can see a few lines below

  string str1 = values[0].ToString();
  if (str1 != null)
    sb.Append(str1);

It seems quite strange to me to return on values[0] == null but to handle values[0].ToString() == null. Combined with the wording on MSDN ("issue", "workaround"), the fact that this overload is relatively new (.NET 4.0) and the fact the the other Joins have a different implementation that accept null as the first element, it looks like a bug to me and not really an intended exception.

Of course, this is only my interpretation and not a definitive answer...