C# 遍历检测是否有重复字段

private void button1_Click(object sender, EventArgs e)
{
string oldphone = textBox5.Text.TrimEnd(',');
string exphone = oldphone + ",";              //为了防止到最后一次循环length为0
string newphone = "";
if (oldphone.IndexOf(',') < 0)
{
textBox1.Text = oldphone;
}
else
{
for (int i = 0; i < oldphone.Split(',').Length; i++)
{
string a = exphone.Substring(0, exphone.IndexOf(','));
string b = exphone.Substring(a.Length + 1, exphone.Length - a.Length - 1);
if (!b.Contains(a)) //检测是否有重复字段出现！
{
newphone += a + ",";
}
exphone = exphone.Substring(exphone.IndexOf(',') + 1, exphone.Length - exphone.IndexOf(',') - 1);
textBox1.Text = newphone.TrimStart(',').TrimEnd(',');
}
}
}