Using Linq to concatenate strings

Question

What is the most efficient way to write the old-school:

StringBuilder sb = new StringBuilder();
if (strings.Count > 0)
{
    foreach (string s in strings)
    {
        sb.Append(s + ", ");
    }
    sb.Remove(sb.Length - 2, 2);
}
return sb.ToString();

...in Linq?

Answer 1

Real example from my code:

return selected.Select(query => query.Name).Aggregate((a, b) => a + ", " + b);

A query is an object that has a Name property which is a string, and I want the names of all the queries on the selected list, separated by commas.

Answer 2

Use aggregate queries like this:

string[] words = { "one", "two", "three" };
var res = words.Aggregate((current, next) => current + ", " + next);
Console.WriteLine(res);

This outputs:

one, two, three

An aggregate is a function that takes a collection of values and returns a scalar value. Examples from T-SQL include min, max, and sum. Both VB and C# have support for aggregates. Both VB and C# support aggregates as extension methods. Using the dot-notation, one simply calls a method on an IEnumerable object.

Remember that aggregate queries are executed immediately.

http://msdn.microsoft.com/en-us/library/bb386914.aspx

Because this does not use a StringBuilder it will have horrible performance for very long sequences.

Answer 3

There are various alternative answers at this previous question - which admittedly was targeting an integer array as the source, but received generalised answers.

Answer 4

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

---

In .Net 4, there's a new overload for string.Join that accepts IEnumerable<string>. The code would then look like:

return string.Join(", ", strings);

Answer 5

Lots of choices here. You can use LINQ and a StringBuilder so you get the performance too like so:

StringBuilder builder = new StringBuilder();
List<string> MyList = new List<string>() {"one","two","three"};

MyList.ForEach(w => builder.Append(builder.Length > 0 ? ", " + w : w));
return builder.ToString();

Answer 6

quick performance data for the stingbuilder vs Select case over 3000 elements:

unit test Duration (seconds) LINQ_SELECT 00:00:01.8012535
LINQ_StringBuilder 00:00:00.0036644

    [TestMethod()]
    public void LINQ_StringBuilder()
    {
        IList<int> ints = new List<int>();
        for (int i = 0; i < 3000;i++ )
        {
            ints.Add(i);
        }
        StringBuilder idString = new StringBuilder();
        foreach (int id in ints)
        {
            idString.Append(id + ", ");
        }
    }
    [TestMethod()]
    public void LINQ_SELECT()
    {
        IList<int> ints = new List<int>();
        for (int i = 0; i < 3000; i++)
        {
            ints.Add(i);
        }
        string ids = ints.Select(query => query.ToString()).Aggregate((a, b) => a + ", " + b);
    }

Answer 7

You can use StringBuilder in Aggregate:

  List<string> strings = new List<string>() { "one", "two", "three" };

  StringBuilder sb = strings
    .Select(s => s)
    .Aggregate(new StringBuilder(), (ag, n) => ag.Append(n).Append(", "));

  if (sb.Length > 0) { sb.Remove(sb.Length - 2, 2); }

  Console.WriteLine(sb.ToString());

(The Select is in there just to show you can do more LINQ stuff.)