Another Linqification question

Question

Say I have a List of objects, and the object has a string property. I want to get a single comma-separated list of the value of each string property of each object in the list. Here's 1 way to do it (sans linq)

StringBuilder result = new StringBuilder()
foreach(myObject obj in myList)
{
  result.Append(obj.TheString);
  result.Append(", ");
}
// then trim the trailing ", " and call ToString() on result, etc, etc...

Here's my first shot at linqification. Is there a better way?

string result = string.Join(", ", myList.Select(myObj => myObj.TheString).ToArray());

That's one line of code, but it doesn't look very efficient to me -- iterate the list just to build an array, just to iterate the array and build a string... whew!

Is there a better way?

Answer 1

If you want efficient, use Enumerable.Aggregate with StringBuilder:

string result = myList.Aggregate(new StringBuilder(),
                                 (sb, o) => sb.Append(o.TheString).Append(", "))
                      .ToString();

The original problem is that String.Join wants an array. In .NET 4, there will be an overload that takes IEnumerable<string> (and I expect it will be implemented like above).

Answer 2

I like this extension method for joining strings. It's basically the same technique you are using, but wrapped in an extension method. I wouldn't recommend it for large sets since efficiency was not the goal. The benefit to me is expressiveness (very linqy) and convenient for small sets:

[Test]
public void Should_make_comma_delimited_list()
{
    var colors = new List<HasColor>
    {
        new HasColor { Color = "red" },
        new HasColor { Color = "green" },
        new HasColor { Color = "blue" }
    };

    var result = colors.Implode(x => x.Color, ", ");

    Assert.That(result, Is.EqualTo("red, green, blue"));
}

public class HasColor
{
    public string Color { get; set; }
}

public static class LinqExtensions
{
    public static string Implode<T>(this IEnumerable<T> list, Func<T, string> func, string separator)
    {
        return string.Join(separator, list.Select(func).ToArray());
    }
}

Answer 3

Here's another way (result is a StringBuilder):

myList.ForEach(
    myObj => 
    { 
        if (result.Length != 0) 
            result.Append(","); 
        result.Append(myObj.TheString); 
    }
    );

Answer 4

Use string.Join, it's good enough.

Optimize when profiler will tell you to do so.

Readability of the version with StringBuilder is poor and you are still getting your trailing comma.