How to access OData service with LINQ?

Open Data Protocol (OData) relates to creation and use of RESTful APIs. OData uses URIs to identify resources on the Internet. The generic syntax for accessing the root of such service is http://host/Service. OData is built upon HTTP, ATOMPub and JSON.

An example of such a resource is the Northwind Service:

http://services.odata.org/northwind/northwind.svc/

LINQ, short for Language Integrated Query, provides an object oriented approach to not only querying relational databases but also any kind of source such as XML, Collection of objects, etc.

Want to know more about LINQ, go here.
Accessing OData with LinqPad.

Launch LinqPad (version used here is v4.55.03) and click on Add Connection link shown here:

OData_02.png

Choose Data Context window opens.

OData_03.png

Click WCF Data Services 5.5 (OData 3) and Click Next.
 In the WCF Data Conneciton 5.5 window type in the URI as shown (you have seen what this is earlier). Leave username and password blank. You can get to the XML or the JSON formatted resources. Remembering this connection is OK for the next time you come here.

Hit Test with Default(XML) checked. Your connection gets populated as shown.

OData_04.png

In the Query pane, Click on Connection and choose the option shown. As to query language you have a number of options.

Odata_05.png

I have just chosen SQL as the language to query. It looks like the driver does not support SQL.

Odata_06.png

Change the language option to C# Expression. Query for Employees table contents as shown:

Odata_07.png

Here is another select statement choosing two columns from Customers table:

OData_08.png

You can easily query OData using C#, but this interface does not support SQL.

You can easily connect to OData using PowerBI, review this post:
http://hodentek.blogspot.com/2015/09/poweer-bi-using-data-from-odata-web.html

What is LINQ?

We know that a query is a question we pose to the database to retrieve some information that we would like to know. For example, in a database that consists of men and women, you may want to know how many men and how many women. Let us go a step further and say that there are people of all nationalities in the database of men and women. You may want to know how many women who hail from Europe are there. This is only filtering of the information but a query can much more complex. This is of course querying a database, let us say a relational database such as the SQL Server where you use the Structured Query Language(SQL), a special language you have to learn.

All data are not in relational databases and they could be in some kind of an XML document, a web service or something else. For example, in order to find some filtered content in an XML document you would use a language called XQuery which is not the same as SQL. This means there is need to learn another query language.

Beginning with Visual Studio 2008 and .NET Framework 3.5 Microsoft introduced LINQ which stands for Language-Integrated Query an innovation that made it possible to query a SQL Server database, an XML document, Web Services etc with a single lanugage and it is a first class query language.

In Visual Studio you can write LINQ in both VB and C# for any collection that supports IEnumerable or the generic IEnumerable interface (or to put it simply a collection in which one can go through the collection item by item).

For example you may have a list  of names ("John", "Tom", "June", "Diana") etc.  or  the number of rows in a database table , etc. and they can be queried using LINQ.

Need to query SQL Server with LinqPad, review the following posts:

http://hodentekmsss.blogspot.com/2014/12/learn-querying-sql-server-using-linq.html
http://hodentekmsss.blogspot.com/2014/12/learn-querying-sql-server-2012-using.html