Runtime: StringBuilder.AppendJoin (appending lists to StringBuilder)

Created on 24 Sep 2015 · 55Comments · Source: dotnet/runtime

Background

Let's say you want to build a string that contains a separated list (e.g. list of filenames, messages, or anything else).

If it is the only thing you want, you can use string.Join. However what if you want to append something else? If you Concat with the result of string.Join, that means that you unnecessarily allocated an intermediate joined string, which can be rather large (depending on the original collection).

On the other hand, StringBuilder would require something like:

var builder = new StringBuilder("Names: ");
for (var i = 0; i < names.Length; i++) {
    if (i > 0) { // this becomes even more involved if `names` is `IEnumerable`
        builder.Append(", ");
    }
    builder.Append(names[i]);
}

That can be abstracted into an extension method, however I believe that extension would be general and useful enough to benefit from being in NetFX.

API Design

(updated based on API review, second pass -- see https://github.com/dotnet/corefx/issues/3419#issuecomment-273254581)

public class StringBuilder {
    public StringBuilder AppendJoin(string separator, params object[] values);
    public StringBuilder AppendJoin<T>(string separator, IEnumerable<T> values);
    public StringBuilder AppendJoin(string separator, params string[] value);

    public StringBuilder AppendJoin(chat separator, params object[] values);
    public StringBuilder AppendJoin<T>(char separator, IEnumerable<T> values);
    public StringBuilder AppendJoin(char separator, params string[] value);

    // string.Join overloads NOT taken:
    //public StringBuilder AppendJoin(string separator, IEnumerable<string> values);
    //public StringBuilder AppendJoin(string separator, string[] values, int startIndex, int count);
    //public StringBuilder AppendJoin(char separator, IEnumerable<string> values);
    //public StringBuilder AppendJoin(char separator, string[] values, int startIndex, int count);
    // REJECTED in the first API review round:
    //public StringBuilder AppendJoin<T>(string separator, T[] values, int startIndex, int count);
    //public StringBuilder AppendJoin<T>(string separator, params T[] values);
    //public StringBuilder AppendJoin<T>(char separator, params T[] values);
}

Note: Motivation for the API approval update is described in https://github.com/dotnet/corefx/issues/3419#issuecomment-273254581 -- adjusting to follow proven working form of 'string.Join', correctly working with most-used overload calls - see usage example in https://github.com/dotnet/corefx/issues/5552#issuecomment-268395367.