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An OData Journey in ASP.NET Web API Part 2 – Introducing Linq to Querystring

April 9, 2013

A Brief Recap

First of all I’d like to apologise for the delay in the coming of the second part of this series. It was originally intended to be a guide to building a simple oData query parser with ANTLR, but as I worked on the samples, it quickly turned into a full scale project.

I few months back I came across a need to use OData with a loosely typed data structure. I quickly found that OData support in Asp.NET Web API was readily available… but only when coding against a known entity model. Also not all features of OData are available out of the box, and even fewer work without having to jump through significant hoops.  I started playing around with a solution, and this is the result.

Presenting: Linq to Querystring

The aim of the Linq to Querystring project is to provide a fast, lightweight subset of the OData URI Specification, with additional flexibility to allow use of loosely typed\dynamic data structures. It also supports the $inlinecount and $select operators, and at the time of writing support for $expand is in development.

Linq to Querystring works by parsing an OData query using ANTLR, and then mapping the resulting syntax tree on to a .NET IQueryable. This means that in theory it can work with any Queryable Provider, at present it has been tested with Linq to Objects, Entity Framework & MongoDB

To get started, first add it to your project using NuGet. Once you’ve done that, simply add the following attribute to a Web API controller action that returns an IQueryable or Task<IQueryable>:

public IQueryable<Movie> Get()

And that’s all there is to it. You can now append OData query parameters to your API calls and see the results. You can also use the built in IQueryable extension methods manually if you need to.

Addressing issues with OData

One thing I should stress is that Linq to Querystring is that the OData specification itself is very extensive, and Linq to Querystring does not claim (or intend) to support all of it. In fact, OData itself seems to split opinion – see here for example:

In to the answer above, Mythz states some concerns that proponents of REST often have about OData, which Linq to Querystring goes some way towards addressing:

  • Poor development practices – Linq to Querystring is simple, flexible and open source, so it can respond to new technologies and paradigms.
  • Promotes bad web service practices – No longer tied to your DBMS as it works with any IQueryable, so you don’t have to expose your data model through your services.
  • Only used in Microsoft technologies – The main expression parsing engine of Linq to Querystring is written in ANTLR so can be easily ported to other languages that support construction of expression trees.
  • OData is slow – Leaving out certain elements of the protocol helps to keep things fast compared to full blown OData implementations. All Linq to Querystring does is map the AST produced by ANTLR onto an IQueryable expression tree.

Of course this is probably not going to convince true REST zealots, but I definately see a need and use for in-query filtering regardless if you prefer HATEOAS or CSDS.

Flexibility and extra functionality

Additionally – due to it’s flexbility – the project may also include features that are not present in the standard OData query specification. Such features are carefully designed not to detract from the power of OData, always augmenting the existing functionality.

For example, in Linq to Querystring you can use the ‘[‘ and ‘]’ brackets to designate that a property should be interpreted dynamically. For example the following filter query

$filter=[Age] gt 18

Is equivilent to:

looselyTypedList.Where(o => o["Age"] > 18);

Current features & roadmap

Please consult the Github site for currently supported features and documentation, as these are changing all the time. Some highlights include:

  • Support for other API frameworks, NancyFX\ServiceStack
  • UI plugin for constructing Linq to Querystring OData queries
  • Support for the $expand operator
  • Testing for other NoSQL Linq Providers

The project is still in development, so some things might not work exactly correctly so please let me know if they do by registering an issue on github, or submit a pull request. Be sure to check back in on the github page regularly for updates in the next few weeks, and l’ll also be writing a series of articles on how to make the most of OData, so stay tuned!

If you’re looking for better OData functionality in your API, I think Linq to Querystring could be just what you need. Don’t take my word for it though, take a test drive over at the demo site: and see for yourself!



From → OData, Web API

  1. Nav permalink

    great work Pete

  2. Hi,
    This seems like a great initiative. I stumbled upon this link as I have a similar problem where I have an IQueryable object created out of a sql dataset and need to expose filtering capabilities to end users. To make matters worse this has to be done on top of a ASP.Net 4 web forms application. So Hopefully linq-to-querystring will give me an idea 🙂

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