Parallel.StringBuilder

Parallel.StringBuilder.cs

using System.Linq;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        var result = new StringBuilder();
        Parallel.ForEach(Enumerable.Range(1, 1000000), () => new StringBuilder(), (x, option, sb) =>
        {
            sb.Append(x);
            return sb;
        }, sb =>
        {
            lock (result)
            {
                result.Append(sb.ToString());
            }
        });
    }
}