文字列を指定した文字数で分割（C#版）　※非LINQ

UnitTest1.cs

using System;
using System.Linq;

using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace UnitTestProject1
{
  public static class StringExtensions
  {
    public static string[] Nyan(this string value, int size)
    {
      return new[] { value.Substring(0, size), value.Substring(size) };
    }
  }

  [TestClass]
  public class UnitTest1
  {
    [TestMethod]
    public void TestMethod1()
    {
      string[] result = "0123456789".Nyan(3);
      Assert.AreEqual("012", result[0]);
      Assert.AreEqual("3456789", result[1]);
    }
  }
}