Callback methods normally return JSON or XML string based on the return type of the Callback method. However, you can also return raw binary (or string) data from your service by returning a few special result types from your Callback methods.

Direct access GET URLs look like this: http://localhost/Ajax/AutoCompleteHandler.ashx?Method=GetImage&imageFile=sailbig.jpg

or you can set up a RouteUrl on the method a use a custom route instead.

You specify the method to call in the Callback handler along with any of the named parameters each specified as a querystring parameter or POST parameter. Using this approach you can call Callback methods that return binary data types in these formats.

Special Return Types Supported

The following special return types are supported:

• Stream
  Any arbitrary data can be returned as stream as long as you can force it into stream format. This allows you to return almost any kind of format such as bitmaps, binary files, strings etc. as long as the format supports turning into a Stream type.

[CallbackMethod(ContentType="image/png")]
public Stream GetStockHistoryGraph(string symbol)
{
    StockServer server = new StockServer();
    byte[] img = server.GetStockHistoryGraph(new[] { symbol },
                                                "History for " + symbol.ToUpper(), 
                                                450, 300, 2);

    MemoryStream ms = new MemoryStream(img);
    return ms;
}

• byte[]
  Like Streams you can also return raw byte data using the byte[] data type:

[CallbackMethod(ContentType="image/png")]
public byte[] GetStockHistoryGraph(string symbol)
{
    StockServer server = new StockServer();
    byte[] img = server.GetStockHistoryGraph(new[] { symbol },
                                                "History for " + symbol.ToUpper(), 
                                                450, 300, 2);

    return img;
}

• Bitmap
  Bitmap results are very common and you can return the Bitmap type from your Callback method to have the image rendered as a binary bitmap.

[CallbackMethod(ContentType="image/png")]
public Bitmap GetImage(string imageFile)
{
    Bitmap img = new Bitmap(HttpContext.Current.Server.MapPath("~/images/" + imageFile));            
    return img;
}

Note that you can set the content type in the attribute which is recommended. If not provided Callback Handler will use the Bitmap's internal ImageFormat (RawFormat) if set properly (typically when you load from file on disk).

• Raw String Results
  You can also return raw string results rather than a JSON or XML serialized string, by using the ReturnAsRawString CallbackMethod attribute value. When this parameter is specified a string result type is not serialized but returned as is. This can be useful to feed back custom HTML or return plain string content to a JavaScript client that doesn't require deserialization first.

[CallbackMethod(ReturnAsRawString=true,
                ContentType="text/plain; charset=UTF-8")]
public string GetCities()
{
    var searchFor = HttpContext.Current.Request.QueryString["q"];

    busCustomer customer = new busCustomer();
    var res = (from cust in customer.Context.nw_Customers
                where cust.City.Contains(searchFor)
                orderby cust.City
                select new { City = cust.City, cust.CustomerID }).Distinct();

    StringBuilder sb = new StringBuilder();
    foreach (var city in res)
    {
        sb.AppendLine(city.City + "|" + city.CustomerID);
    }

    return sb.ToString();
}

The content type is optional. The default content type for raw string results if not specified is text/html (default Response.ContentType). You can also override the content type in code with HttpContext.Current.Response.ContentType.