Creating a comma separated list from IList<string> or IEnumerable<string>

.NET 4+

IList<string> strings = new List<string>{"1","2","testing"};
string joined = string.Join(",", strings);

Detail & Pre .Net 4.0 Solutions

IEnumerable<string> can be converted into a string array very easily with LINQ (.NET 3.5):

IEnumerable<string> strings = ...;
string[] array = strings.ToArray();

It's easy enough to write the equivalent helper method if you need to:

public static T[] ToArray(IEnumerable<T> source)
{
    return new List<T>(source).ToArray();
}

Then call it like this:

IEnumerable<string> strings = ...;
string[] array = Helpers.ToArray(strings);

You can then call string.Join. Of course, you don't have to use a helper method:

// C# 3 and .NET 3.5 way:
string joined = string.Join(",", strings.ToArray());
// C# 2 and .NET 2.0 way:
string joined = string.Join(",", new List<string>(strings).ToArray());

The latter is a bit of a mouthful though :)

This is likely to be the simplest way to do it, and quite performant as well - there are other questions about exactly what the performance is like, including (but not limited to) this one.

As of .NET 4.0, there are more overloads available in string.Join, so you can actually just write:

string joined = string.Join(",", strings);

Much simpler :)

FYI, the .NET 4.0 version of string.Join() has some extra overloads, that work with IEnumerable instead of just arrays, including one that can deal with any type T:

public static string Join(string separator, IEnumerable<string> values)
public static string Join<T>(string separator, IEnumerable<T> values)

The easiest way I can see to do this is using the LINQ Aggregate method:

string commaSeparatedList = input.Aggregate((a, x) => a + ", " + x)