StringBuilder pool C#

FixedStringBuilderPool.cs

using System.Text;
using System.Threading;

namespace XLog.Formatters
{
    internal static class FixedStringBuilderPool
    {
        private const int BuildersCount = 10;
        private const int BuilderLength = 1000;
        private static readonly StringBuilder[] Builders;
        private static int _index = -1;

        static FixedStringBuilderPool()
        {
            Builders = new StringBuilder[BuildersCount];
            for (int i = 0; i < BuildersCount; i++)
            {
                Builders[i] = new StringBuilder(BuilderLength);
            }
        }

        public static StringBuilder Get(out int num)
        {
            // Race condition here. :(

            int index;  
            StringBuilder builder;
            do
            {
                index = Interlocked.Increment(ref _index) % BuildersCount;
            } while ((builder = Interlocked.CompareExchange(ref Builders[index], null, null)) == null);

            num = index;
            return builder;
        }

        public static void Return(int num, StringBuilder sb)
        {
            sb.Clear();
            Builders[num] = sb;
        }
    }
}