请用C#语言实现，最小公共字符串

class Program
{
public Program(string str1,string str2)
{
StringBuilder sb = new StringBuilder();
sb.Append(str1);
sb.Append(str2);
for (int i = 0; i < sb.Length; i++)
{
for (int j = i + 1; j < sb.Length; j++)
{
if (sb[i] == sb[j])
{
sb.Remove(j, 1);
}
}
}
Console.WriteLine("最小公共字符串为：");
Console.WriteLine(sb.ToString());
}

static void Main(string[] args)
{
Console.WriteLine("请输入第一个字符串:");
string str1 = Console.ReadLine();
Console.WriteLine("请输入第二个字符串:");
string str2 = Console.ReadLine();

new Program(str1,str2);
Console.ReadLine();
}
}