$.validator.addMethod("maxlength", function (value, element, param) {
    return value.length <= param;
}, "输入内容不能超过 {0} 个字符");

$("#myform").validate({
    rules: {
        myinput: {
            required: true,
            maxlength: 10
        }
    }
});

$("#myform").validate({
    rules: {
        myinput: {
            required: true,
            maxlength: 10,
            validator: function (value, element, param) {
                return value.length <= param;
            }
        }
    },
    messages: {
        myinput: {
            maxlength: "输入内容不能超过 {0} 个字符"
        }
    }
});