I'm trying to combine <code>List<string> strings</code> using <code>string.Join(",", strings)</code> but everything I'm reading says I should do: <code>string.Join(",", strings.ToArray())</code> Is there a certain reason I have to/should use <code>.ToArray()</code>?

Probably because everything you're reading was written for an earlier version of the framework. The <code>string.Join(string, IEnumerable<string>)</code> method was added in version 4.

Why is ToArray() used when using string.Join with a List<string>?

I'm trying to combine List<string> strings using string.Join(",", strings) but everything I'm reading says I should do:

string.Join(",", strings.ToArray())

Is there a certain reason I have to/should use .ToArray()?

string.Join only started accepting IEnumerable<string> (and indeed a generic overload) as of .NET 4. Presumably you're looking at code (or instructions) written with .NET 3.5 or earlier in mind. Compare the overloads:

string.Join in .NET 3.5
string.Join in .NET 4

Probably because everything you're reading was written for an earlier version of the framework. The string.Join(string, IEnumerable<string>) method was added in version 4.