string concatination in c sharp

using System.Linq;

static void Main(string[] args)
{
    string mixed = Interleave("RAHUL", "MISHRA");
}

public static string Interleave(string s1, string s2)
{
    StringBuilder sb = new StringBuilder();
    s1.Zip(s2, (c1, c2) => sb.Append(c1).Append(c2)).Count();   // .Count() to force the Lazy evaluation!
    if (s1.Length > s2.Length)
    {
        sb.Append(s1.Substring(s2.Length));
    }
    else if (s2.Length > s1.Length)
    {
        sb.Append(s2.Substring(s1.Length));
    }
    return sb.ToString();
}

public static string GetString(string string1,string string2)
      {
          char[] str1 = string1.ToCharArray();
          char[] str2 = string2.ToCharArray();

          string finalString = "";
          int length = str1.Length > str2.Length ? str2.Length : str1.Length;
          for (int i = 0; i < length ; i++)
          {
              finalString = finalString + str1[i] + str2[i];
          }
          if (str1.Length > str2.Length)
          {
              int difference = str1.Length - str2.Length;
              for (int i = 0; i < difference; i++)
              {
                  char rem = str1[str2.Length + i];
                  finalString = finalString + rem;
              }

          }
          else
          {
              int difference = str2.Length - str1.Length;
              for (int i = 0; i < difference; i++)
              {
                  char rem = str2[str1.Length + i];
                  finalString = finalString + rem;
              }
          }
          return finalString;
      }

string finalString = GetString("RAHUL", "MISHRA");