If separator is null, an empty string (string.Empty) is used instead. If any member of values is null, an empty string is used instead.

string.Join(string, IEnumerable<string>) is a convenience method that lets you concatenate each element in an IEnumerable(Of String) collection without first converting the elements to a string array. It is particularly useful with Language-Integrated Query (LINQ) query expressions. The following example passes a List(Of String) object that contains either the uppercase or lowercase letters of the alphabet to a lambda expression that selects letters that are equal to or greater than a particular letter (which, in the example, is "M"). The IEnumerable(Of String) collection returned by the System.Linq.Enumerable.Where``1(IEnumerable<``0>, Func<``0, bool>) method is passed to the string.Join(string, IEnumerable<string>) method to display the result as a single string.

code reference: System.String.Join#4