just changed the above code like as follows and it worked!
SqlConnection con = new SqlConnection(main.ConnString);
SqlCommand cmd1 = new SqlCommand();
cmd1.CommandText = selectstatement;
cmd1.Connection = con;
con.Open();
SqlDataReader r1 = cmd1.ExecuteReader();
using (var compressedFileStream = new MemoryStream())
{
using (var zipArchive = new ZipArchive(compressedFileStream, ZipArchiveMode.Create, false))
{
foreach (var caseAttachmentModel in r1)
{
var zipEntry = zipArchive.CreateEntry((string)r1["filename"]);
using (var originalFileStream = new MemoryStream((byte[])r1["stored_content"]))
using (var zipEntryStream = zipEntry.Open())
{
originalFileStream.CopyTo(zipEntryStream);
}
}
}
sendOutZIP(compressedFileStream.ToArray(), "FileName.zip");
}