Interview Question: Compare two strings and return a third string containing only the letters that appear in both.

gistfile1.cs

string firstString = "Hello Ken";
string secondString = "Hello Samuel";

StringBuilder sb = new StringBuilder();
if ((!string.IsNullOrEmpty(firstString)) && (!string.IsNullOrEmpty(secondString)))
{
    foreach (char iChar in firstString)
    {
        if (!sb.ToString().Contains(iChar) && secondString.Contains(iChar))
        {
            sb.Append(iChar);
        }
    }
}

Console.Write(sb.ToString());