.NET REST API JSON Filtering
Style  

Be consistent in your style. If you use braces {} for single if statements then you should use them always.  

Using guard conditions help you to save horizontal space.  

NameValueCollection mapQuery = UriExtensions.ParseQueryString(requestUri);
if (mapQuery == null) { return target; }

string fields = mapQuery.Get("fields");
if (String.IsNullOrEmpty(fields)) { return target; }

IEnumerable&lt;string&gt; requiredFields = fields.Split(new char[] { ',' });
if (requiredFields == null) { return target; }  

your loop here  


otherwise your code looks good.