这是示例代码。

System.IO.MemoryStream ms = new System.IO.MemoryStream();
System.IO.StreamWriter writer = new System.IO.StreamWriter(ms);
writer.Write("Hello its my sample file");
writer.Flush();
writer.Dispose();
ms.Position = 0;

System.Net.Mime.ContentType ct = new System.Net.Mime.ContentType(System.Net.Mime.MediaTypeNames.Text.Plain);
System.Net.Mail.Attachment attach = new System.Net.Mail.Attachment(ms, ct);
attach.ContentDisposition.FileName = "myFile.txt";

// I guess you know how to send email with an attachment
// after sending email
ms.Close();

编辑1

您可以通过System.Net.Mime.MimeTypeNames指定其他文件类型，例如
System.Net.Mime.MediaTypeNames.Application.Pdf

根据 Mime类型， 您需要在FileName中指定实例扩展名"myFile.pdf"