Comma “izing” a list of items

Question

Given a list of strings, what is the best method for concatenating these strings into a comma separated list with no comma at the end. (VB.NET or C#) (Using either StringBuilder or String Concat.)

Dim strResult As String = ""
Dim lstItems As New List(Of String)
lstItems.Add("Hello")
lstItems.Add("World")
For Each strItem As String In lstItems
    If strResult.Length > 0 Then
        strResult = strResult & ", "
    End If
    strResult = strResult & strItem
Next
MessageBox.Show(strResult)

Answer 1

Like this:

lstItems.ToConcatenatedString(s => s, ", ")

If you want to ignore empty strings as in your example:

lstItems
    .Where(s => s.Length > 0)
    .ToConcatenatedString(s => s, ", ")

The most popular custom aggregate function in my toolbox. I use it every day:

public static class EnumerableExtensions
{

    /// <summary>
    /// Creates a string from the sequence by concatenating the result
    /// of the specified string selector function for each element.
    /// </summary>
    public static string ToConcatenatedString<T>(
        this IEnumerable<T> source,
        Func<T, string> stringSelector)
    {
        return EnumerableExtensions.ToConcatenatedString(source, stringSelector, String.Empty);
    }

    /// <summary>
    /// Creates a string from the sequence by concatenating the result
    /// of the specified string selector function for each element.
    /// </summary>
    /// <param name="separator">The string which separates each concatenated item.</param>
    public static string ToConcatenatedString<T>(
        this IEnumerable<T> source,
        Func<T, string> stringSelector,
        string separator)
    {
        var b = new StringBuilder();
        bool needsSeparator = false; // don't use for first item

        foreach (var item in source)
        {
            if (needsSeparator)
                b.Append(separator);

            b.Append(stringSelector(item));
            needsSeparator = true;
        }

        return b.ToString();
    }
}

Answer 2

There are several ways to do this, but they're basically variations on a theme.

Pseudocode:

For Each Item In Collection:
  Add Item To String
  If Not Last Item, Add Comma

A different way that I like a little better is something like this:

For Each Item In Collection:
  If Not First Item, Add Comma
  Add Item To String

Edit: The reason I like the second way of doing it is that each item stands on its own. Using the first approach, if you modified your logic later so that a subsequent item might not get added, you could end up with a stray comma at the end of the string unless you also made your test in the previous item more intelligent, which is dumb.

Answer 3

Dim Result As String
Dim Items As New List(Of String)
Items.Add("Hello")
Items.Add("World")

Result = String.Join(",", Items.ToArray())
MessageBox.Show(Result)

If you're really concerned about empty strings, use this join function:

Function Join(ByVal delimiter As String, ByVal items As IEnumerable(Of String), Optional ByVal IgnoreEmptyEntries As Boolean = True) As String
    Dim delim As String = ""
    Dim result As New Text.StringBuilder("")

    For Each item As String In items
        If Not IgnoreEmptyEntries OrElse Not String.IsNullOrEmpty(item) Then
            result.Append(delim).Append(item)
            delim = delimiter
        End If
    Next
    Return result.ToString()
End Function

Answer 4

Thanks for all of the responses.

It appears that the "proper" answer depends on the context in which the comma separated list is being built. I don't have a neat list of items to use (had to use something for an example...), but I do have an array whose items may or may not be added to the comma seperated list depending on various conditions.

So I chose something to the effect of

strResult = ""
strSeparator = ""
for i as integer = 0 to arrItems.Length - 1
  if arrItems(i) <> "test" and arrItems(i) <> "point" then
    strResult = strResult & strSeparator & arrItem(i)
    strSeparator = ", "
  end if
next

As usual, there are numerous methods of doing this. I don't know that any one method deserves more praise or promotion than another. Some are more useful in certain contexts while others satisfy the requirements of different contexts.

Again, thanks to all for your input.

BTW, the original post with the "off the top of my head" code sample is not filtering zero length items, instead it is waiting for the result string to become greater than zero length before adding the comma. Probably not very efficient but I have'nt tested it. Again, it was off the top of my head.