Question

I have a wcf 3.5 rest service which gets a dataset from database, i want to sent this data as xml or json according to the mime type defined in client script.

Answer 1

You could build your operation url(either xml or Json) to http request.
for e.g

makeCall(doXML)
makeCall(doJson)

and use Post method to service.svc/operation

on your service contracts build 2 different method with WebInvoke attribute to response with appropriate format.

[WebInvoke(ResponseFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.Wrapped)
        DataContractResult doXML(object parameters);

[WebInvoke(ResponseFormat = WebMessageFormat.Xml, BodyStyle = WebMessageBodyStyle.Wrapped)]
        DataContractResult doJson(object parameters);

for .NET 3.5 and enable use of non-ASP.NET client, you need to configure the address endpoint.

for detail, you could refer MSDN.