//默认返回 json GlobalConfiguration.Configuration.Formatters.XmlFormatter.SupportedMediaTypes.Clear();

          //  GlobalConfiguration.Configuration.Formatters.JsonFormatter.MediaTypeMappings.Add(new QueryStringMapping("datatype", "json", "application/json"));  

            //返回格式选择 datatype 可以替换为任何参数 //GlobalConfiguration.Configuration.Formatters.XmlFormatter.MediaTypeMappings.Add(              //    new QueryStringMapping("datatype", "xml", "application/xml")); 

using System.IO;  
/// <summary>  
/// WebApi返回图片  
/// </summary>  
public HttpResponseMessage GetQrCode()  
{  
    var imgPath = @"D:\ITdosCom\Images\itdos.jpg";  
    //从图片中读取byte  
    var imgByte = File.ReadAllBytes(imgPath);  
    //从图片中读取流  
    var imgStream = new MemoryStream(File.ReadAllBytes(imgPath));  
    var resp = new HttpResponseMessage(HttpStatusCode.OK)  
    {  
        Content = new ByteArrayContent(imgByte)  
        //或者  
        //Content = new StreamContent(stream)  
    };  
    resp.Content.Headers.ContentType = new MediaTypeHeaderValue("image/jpg");  
    return resp;  
}  
/// <summary>  
/// WebApi返回json数据  
/// </summary>  
public HttpResponseMessage GetQrCode()  
{  
    var jsonStr = "{\"IsSuccess\":true,\"Data\":\"www.itdos.com\"}";  
    var result = new HttpResponseMessage(HttpStatusCode.OK)  
                    {  
                        Content = new StringContent(jsonStr, Encoding.UTF8, "text/json")  
                    };  
    return result;  
}  
/// <summary>  
/// WebApi返回字符串  
/// </summary>  
public HttpResponseMessage GetQrCode()  
{  
    var str = "IT大师www.itdos.com";  
    var result = new HttpResponseMessage(HttpStatusCode.OK)  
                    {  
                        Content = new StringContent(str, Encoding.UTF8, "text/plain")  
                    };  
    return result;  
}