C# checkbox return invalid value

I having below code but I don't understand why when I check the check box , the access string is not change to "Y".

string access1 = "N";

    void checkvalue(CheckBox chkbox, string access)
    {
        if (chkbox.Checked)
        {
            access = "Y";
        }
        else
        {
            access = "N";
        }
    }

private void checkBox1_CheckedChanged(object sender, EventArgs e) { checkvalue(checkBox1, access1); }