Why does a leading null value in an array cause string.Join() to return empty string?

Question

The following test passes as written. Why? (i.e., why does a leading null value in an array cause string.Join() to return empty string?)

[TestMethod]
public void Test_string_Join()
{
    object[] values0 = { "foo", 3, null };
    var text0 = string.Join(", ", values0);
    Console.WriteLine("text0 = " + text0);
    AssertX.AreEqual("foo, 3, ", text0); // works as expected

    object[] values1 = { null, "foo", 3, null };
    var text1 = string.Join(", ", values1);
    Console.WriteLine("text1 = " + text1);
    AssertX.AreEqual("", text1); // does NOT work as expected, why empty?
}

Because NUL is the signal for end of string in NET and much of Windows — Ňɏssa Pøngjǣrdenlarp
– Ňɏssa Pøngjǣrdenlarp, Commented Apr 20, 2018 at 2:08
@Plutonix Nope. A null in the middle of the array doesn't return empty string. — user202729
– user202729, Commented Apr 20, 2018 at 2:11
Because the documentation says that. "If separator is null or if any element of values other than the first element is null, an empty string (String.Empty) is used instead. [...] 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." — user202729
– user202729, Commented Apr 20, 2018 at 2:15
@user202729 I was looking at the docs for String.Join(String, String[]). Wow!! String.Join(String, Object[]) and String.Join(String, String[]) are NOT symmetric in this regard!! — dthal
– dthal, Commented Apr 20, 2018 at 2:28

d219 · Accepted Answer · 2018-04-20 15:16:25Z

2

The question is about string.Join() for an "array". It turns out that Microsoft is violating the principle of least astonishment and String.Join(String, **Object[]**) behaves differently from String.Join(String, **String**[]) wrt a leading null. The object[] overload has the issue; the string overload behaves as one would expect.

I believe Microsoft is confessing as much when they refer to the "issue" and provide a "workaround" in the object[] overload documentation (link provided by @user202729).

I did not expect the docs for object[] overload to be different than the string[] overload (which I read thoroughly).

edited Apr 20, 2018 at 15:16

d219

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answered Apr 20, 2018 at 13:19

dthal

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