asp.net C#如何实现if验证不区分大小写

string a="验证码字符串"
string b="要输入的字符串";  
if (a.Equals(b, StringComparison.OrdinalIgnoreCase))
        {
            Response.Write("<script>alert('验证码输入成功')</script>");
        }
        else
        {
            Response.Write("<script>alert('验证码输入失败')</script>");
        }