使用Microsoft EWS发送带附件邮件后无法删除附件文件的问题求助

使用Microsoft EWS发送带PDF附件的邮件后，尝试删除该PDF文件时Windows提示“拒绝访问路径'C:\IIS_PAGES\wwwroot\FILES\report.pdf'”。此前使用.NET标准SMTP处理程序时，调用Dispose()方法解决过同类问题，但EWS中没有对应的Dispose或Clear方法，该如何解决？

原代码示例：

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);
service.Credentials = new WebCredentials("secret", "data", "ad");
 
service.TraceEnabled = true;
service.TraceFlags = TraceFlags.All;

service.Url = new Uri("https://mail.example.com/EWS/exchange.asmx");

EmailMessage email = new EmailMessage(service);
email.ToRecipients.Add(receiver);
email.Subject = "Test";
email.Attachments.AddFileAttachment(file);        
email.Body = "test";
email.Send();
File.Delete(file);

问题出在AddFileAttachment方法会持续锁定原文件，直到EWS相关资源释放。以下是几种可靠的解决方法：

• 内存流中转附件：先将文件内容读取到内存流，再添加为附件。这样EWS操作的是内存中的数据，不会锁定原文件，发送完成后可直接删除原文件。
• 使用using语句释放资源：ExchangeService和EmailMessage均实现了IDisposable接口，用using包裹可确保资源被及时释放，解除文件锁定。
• 延迟删除（临时方案）：若前两种方法无效，可短暂延迟后再执行删除操作，给EWS足够时间释放文件句柄，但不推荐作为长期解决方案。

内存流中转方式 #

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1);
service.Credentials = new WebCredentials("secret", "data", "ad");
service.TraceEnabled = true;
service.TraceFlags = TraceFlags.All;
service.Url = new Uri("https://mail.example.com/EWS/exchange.asmx");

EmailMessage email = new EmailMessage(service);
email.ToRecipients.Add(receiver);
email.Subject = "Test";
email.Body = "test";

// 读取文件到内存流，避免锁定原文件
using (var fileStream = new FileStream(file, FileMode.Open, FileAccess.Read))
{
    var memoryStream = new MemoryStream();
    fileStream.CopyTo(memoryStream);
    memoryStream.Position = 0;
    // 添加内存流作为附件，指定原文件名
    email.Attachments.AddFileAttachment(Path.GetFileName(file), memoryStream);
}

email.Send();
// 此时文件已无锁定，可正常删除
File.Delete(file);

Using释放资源方式 #

using (ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2013_SP1))
{
    service.Credentials = new WebCredentials("secret", "data", "ad");
    service.TraceEnabled = true;
    service.TraceFlags = TraceFlags.All;
    service.Url = new Uri("https://mail.example.com/EWS/exchange.asmx");

    using (EmailMessage email = new EmailMessage(service))
    {
        email.ToRecipients.Add(receiver);
        email.Subject = "Test";
        email.Attachments.AddFileAttachment(file);        
        email.Body = "test";
        email.Send();
    }
}
// 资源已释放，文件锁定解除
File.Delete(file);

内容的提问来源于stack exchange，提问作者Hakuryuu