Question

I have the following extension method that takes a List and converts it to a comma separated string:

    static public string ToCsv(this List<string> lst)
    {
        const string SEPARATOR = ", ";
        string csv = string.Empty;

        foreach (var item in lst)
            csv += item + SEPARATOR;

        // remove the trailing separator
        if (csv.Length > 0)
            csv = csv.Remove(csv.Length - SEPARATOR.Length);

        return csv;
    }

I want to do something analogous but apply it to a List (instead of List of String) , however, the compiler can’t resolve for T:

    static public string ToCsv(this List<T> lst)
    {
        const string SEPARATOR = ", ";
        string csv = string.Empty;

        foreach (var item in lst)
            csv += item.ToString() + SEPARATOR;

        // remove the trailing separator
        if (csv.Length > 0)
            csv = csv.Remove(csv.Length - SEPARATOR.Length);

        return csv;
    }

What am I missing?

Answer 1

First, the method declaration should be:

public static string ToCsv<T>(this List<T> list) { // }

Note that the method must be parameterized; this is the <T> after the name of the method.

Second, don’t reinvent the wheel. Just use String.Join:

public static string ToCsv<T>(this IEnumerable<T> source, string separator) {
    return String.Join(separator, source.Select(x => x.ToString()).ToArray());
}

public static string ToCsv<T>(this IEnumerable<T> source) {
    return source.ToCsv(", ");
}

Note that I’ve gone hog wild and generalized the method further by accepting an IEnumerable<T> instead of a List<T>.

In .NET 4.0 you will be able to say:

public static string ToCsv<T>(this IEnumerable<T> source, string separator) {
    return String.Join(separator, source.Select(x => x.ToString());
}

public static string ToCsv<T>(this IEnumerable<T> source) {
    return source.ToCsv(", ");
}

That is, we do not require to convert the result of source.Select(x => x.ToString()) to an array.

Finally, for an interesting blog post on this topic, see Eric Lippert’s post Comma Quibbling.