Question

I need to create a WCF Service that will have a download file function. This WCF will be consumed by a Delphi application.

The problem: The files that will be downloaded are very large and may cause memory problems on Delphi side. So far, I have this code:

[OperationContract]
byte[] DownloadFile(string filePath);

But this code will cause the client app to hold all data in memory which can be an issue.

I have read that WCF is capable of streaming data as you can read at: How to: Enable Streaming

But I have a question regarding this piece of code cut from MSDN:

[OperationContract]
Stream GetStream(string data);

On the client side I want to pass a TFileStream to the function. By using TFileStream every byte read will go directly to the disk. But the function RETURNS a stream and what I want will not be possible since the stream will not a parameter to the function.

How can I download a file from a WCF service directly to the disk?

Answer 1

I have found that relying on “built-in” streaming capability in WCF when working with other (non-.NET) clients is a big source for strange problems…

Basically we solve this kind of scenario by defining:

[OperationContract]
string DownloadFile(string filePath);

The method generates a HTTP(S) url and returns it…

This way any http-capable client can work with the data in a robust fashion…

BEWARE that this makes the server a bit more complicated since you now need to have some mechanism to generate (and serve HTTP GET on) URLs (security, “globally” unique, only usable for a limited time etc.).

BUT the big advantage is that any client out there (mobile or some strange embedded device or whatever you might encounter) will be able to implement this scheme as long as it has http-support available (Delphi has some very good http-client options).