i want to create random alphanumeric string but it gives my an error?

i got some erros on this sentence:

char randomChar = validchars(idx);

private void metroButton1_Click(object sender, EventArgs e)
{
    if (connected == true)
    {
        //*** Declaration of variables
        string baseCID = null;
        string validchars = null;
        StringBuilder sb = new StringBuilder();
        dynamic randomString = sb.ToString();

        //*** Assign values to variables
        baseCID = "00000001008";
        validchars = "ABCDEF1234567890";

        //*** Generate random string
        Random rand = new Random();
        for (int i = 1; i <= 21; i++)
        {
            int idx = rand.Next(0, validchars.Length);
            char randomChar = validchars(idx);//this givs me an error
            sb.Append(randomChar);
        }

        metroTextBox1.Text = baseCID + sb.ToString();
    }
    else
    {
        MessageBox.Show("You are Not Connected!", "Connect Please");
    }
}