Number5.cs

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

using SimpleExpressionEvaluator;

namespace ConsoleApplication1
{
    internal static class Program
    {
        private const int Target = 100;
        private static readonly int[] Digits = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        private static readonly string[] Ops = { "", "+", "-" };

        private static void Main()
        {
            int solutionCount = (int)Math.Pow(Ops.Length, Digits.Length - 1);
            Parallel.For(0, solutionCount, solutionIndex =>
            {
                ExpressionEvaluator e = new ExpressionEvaluator();
                StringBuilder sb = new StringBuilder();
                sb.Append(Digits[0]);
                for (int i = 1; i < Digits.Length; i++)
                {
                    sb.Append(Ops[solutionIndex % Ops.Length]);
                    solutionIndex /= Ops.Length;
                    sb.Append(Digits[i]);
                }

                string exp = sb.ToString();
                decimal result = e.Evaluate(exp);
                if (result == Target)
                {
                    Console.WriteLine(exp);
                }
            });
        }
    }
}

Requires: https://www.nuget.org/packages/ExpressionEvaluator/