<div>Contributed By: <a href="http://carloscds.net/" target="_blank">Carlos dos Santos</a></div>


<div>Published on: 2010-07-19</div>

<hr/>
<div class="font-italic">That article was published more than three years ago. Some content such as older Visual Studio version, "incoming" features, etc. might be outdated. However, the article may still be pertinent to read.</div>

Entity Framework Generic queries with Linq

In the concept of object orientation, we always have in mind to create codes that can be reused within the application or even in different applications. In this context, I have been asked how it is possible to use LINQ, which is totally object-oriented and strongly typed, and still create reusable codes.

Taking advantage of this situation, I will show how it is possible to create a query method using LINQ, which is used to query practically any entity (table) of your model. We will create a completely generic query, and one of the possible places that you can use this type of method, it is in a consultation routine that you call throughout your system, for example.

For our example, let's create a very simple console project, remembering that we will use Visual Studio 2010 with Entity Framework 4:

Now let's add the Entity Framework model, for this example we will use the Northwind database, if you don't have it, download it here. To add the model, right-click on your project and go to Add / New Item, or CTRL + SHIFT + A and select Data and then ADO.Net Entity Data Model. Choose Generate from a database and create a connection to your Northwind database. Select some tables, for example: Categories, Products, Customers:

After creating the EDMX file, let's go to the Query method:

static ObjectQuery<DbDataRecord> Query(string query, ObjectContext ctx)
{
    return new ObjectQuery<DbDataRecord>(query, ctx);
}

Note: add the following namespaces:

using System.Data.Objects;
using System.Data.Common;

It looks like it got complicated, but it is very simple. We are returning a generic object of type DbDataRecord, and within the method, we are returning a new ObjectQuery, which allows us to create queries dynamically. Let's see an example of a method call:

static void Main(string[] args)
{
    NorthwindEntities ctx = new NorthwindEntities();

    var datas = Query("SELECT c.ProductName, c.UnitPrice FROM Products AS c", ctx);

    foreach (DbDataRecord data in datas)
    {
        Console.WriteLine(data["ProductName"] + " - " + data["UnitPrice"]);
    }
}

First, we create the context for the Northwind database, and then we call the Query method passing in our query and the context. See that the query looks a lot like an SQL query, and in this case, we are bringing the ProductName and UnitPrice from the Products table.

Then we run a foreach() to show the data, using DbDataRecord to access the query fields. Let's look at another example, now also bringing data from the Categories table:

var datas = Query("SELECT c.ProductName, c.UnitPrice, c.Categories.CategoryName FROM Products AS c" , ctx);

foreach (DbDataRecord data in datas)
{
    Console.WriteLine(data["ProductName"] + " - " + data["UnitPrice"]+" - "+data["CategoryName"]);
}

And to end an example with conditional query:

var datas = Query("SELECT c.ProductName, c.UnitPrice, c.Categories.CategoryName FROM Products AS c WHERE c.UnitPrice < 10" , ctx);

foreach (DbDataRecord data in datas)
{
    Console.WriteLine(data["ProductName"] + " - " + data["UnitPrice"]+" - "+data["CategoryName"]);
}

I believe that this simple example can help in the development of your application and also show the amount of existing features in Entity Framework 4.