将SendGridMessage LoadSendGridTemplate()中的Content、Subject、Attachments等属性，分别在邮件发送前手动赋值，而不是使用模板。具体代码示例：

var sendGridMessage = new SendGridMessage();
var templateId = "your-template-id";
var substitutions = new Dictionary
{
    { "first_name", "John" },
    { "last_name", "Doe" }
};

sendGridMessage.SetTemplateId(templateId);
foreach (var substitution in substitutions)
{
    sendGridMessage.AddSubstitution(substitution.Key, substitution.Value);
}

var message = new SendGridMessage
{
    Subject = "Example Subject",
    PlainTextContent = "Example Plain Text Content"
};

var htmlContent = await sendGridClient.RequestAsync(sendGridMessage);
var attachments = sendGridMessage.Attachments.ToDictionary(a => a.FileName);

message.HtmlContent = htmlContent;

foreach (var attachment in attachments.Values)
{
    message.AddAttachment(attachment.FileName, attachment.Content, attachment.ContentType, attachment.Disposition);
}

var response = await sendGridClient.SendEmailAsync(message);