返回一个可用的名称如1_4.txt

private string GetCanUseName(string newName,string oldName,int num)
{            
    if (!File.Exists(newName))
    {
        return newName;
    }
    else
    {
        //存在                
        string ext = Path.GetExtension(oldName);
        string nameInfo = oldName.Replace(ext, "");
        return GetCanUseName(nameInfo + "_" + (++num) + ext, oldName, ++num);
    }
}

private void button1_Click(object sender, EventArgs e)
{
    string name = "C:\123\1.txt";
    string tt = GetCanUseName(name, name, 0);
    MessageBox.Show(tt);
}