Living in the Tech Avalanche Generation

LINQ To XML and the Specification Pattern - [In a static world]

April 04th, 2009 | Category: C#, Design Patterns, LINQ, LINQ To XML

I have documented quite extensively on how to use the Specification Pattern and in particular how that can work with a variety of LINQ flavours. The pattern has proven useful in matching objects based on a computation that seeks to test equality and when turned slightly on it’s head a little, it can provide dynamic querying capability to the current Microsoft group of ORM products, namely LINQ To SQL and the Entity Framework. You can check out most of the series on this topic from here. In more recent times I looked at how to use the same pattern with IronRuby which whilst very satisfying for the author, it currently lacks the ability to produce expressions that either ORM products are capable of utilising.

So, for the moment I want to go back again to the Specification pattern and explore it’s use specifically in the world of C# and it’s statically typed sibling languages. The Specification that I currently use is based primarily around our ability to push around Expression<Func<T>> and Func<T> as delegates that can be used as arguments to the extension methods in a given LINQ implementation. Therefore, LINQ to XML should be no exception. Given that the class at the centre of all this ( Specification<T> ) matches objects that satisfy a boolean condition that is asserted using the generic T argument, we are able to specify a function to pass to any extension method implemented by the LINQ To XML provider, that takes a generic as it’s input parameter and returns a boolean. A quick reminder of the Specification classes interface:

public interface ISpecification<T>
{
    Expression<Func<T, bool>> EvalPredicate { get; }
    Func<T, bool> EvalFunc { get; }
    bool Matches(T entity);
}

Here is a simple example of filtering a query over XML data by supplying a predicate in the form of a specification.

public void employees_from_uk()
{
    var all_employees_on_file =
        XDocument.Load(@"..\..\XML\Employees.xml").Descendants();

    Specification<XElement> aus_emp_spec =
        new Specification<XElement>
            (x => x.Attribute("Country").Value == "Australia");

    var aus_employees = from e in all_employees_on_file.Descendants()
                        .Where(aus_emp_spec.EvalFunc)
                        select e;

    aus_employees.ToList().ForEach(x =>
        { Console.WriteLine(x.ToString()); });
}

And now a more complex specification that uses the AND operator to create a combined set of conditions to be satisfied in a .Where extension method.

public void employees_from_uk_that_are_ceo()
{
    var all_employees_on_file =
        XDocument.Load(@"..\..\XML\Employees.xml").Descendants();

    var aus_emp_spec =
        new Specification<XElement>
            (x => x.Attribute("Country").Value == "UK");

    var ceo_emp_spec =
        new Specification<XElement>
            (x => x.Attribute("Title").Value == "CEO");

    var compound_spec = aus_emp_spec & ceo_emp_spec;

    var aus_employees = from e in all_employees_on_file.Descendants()
                        .Where(compound_spec.EvalFunc)
                        select e;

    aus_employees.ToList().ForEach(x =>
        { Console.WriteLine(x.ToString()); });
}

More than just a ‘Where’ predicate?

Your probably noticing that we are always applying our specifications to the “where” method. As I mentioned, a Specification in it’s current incarnation can only be applied to extension methods that take a function that accepts a generic argument as an input parameter and returns a boolean, therefore there are some other methods that can be specified. Here is example of using the .SkipWhile() extension method.

public void skip_while_employees_from_usa_then_return_the_rest_regardless()
{
    var all_employees_on_file =
        XDocument.Load(@"..\..\XML\Employees.xml").Descendants();

    var usa_empoyee_spec =
        new Specification<XElement>
            (x => x.Attribute("Country").Value == "USA");

    var usa_emps = from e in all_employees_on_file.Descendants()
                       .SkipWhile(usa_empoyee_spec.EvalFunc)
                       select e;

    usa_emps.ToList().ForEach(x =>
        { Console.WriteLine("Skipping from the USA : " +
            x.ToString()); });
}

The final example combines data from two XML sources, each one requiring it’s own specification, where the LINQ query will bring the disparate data together.

public void customer_joined_with_customer_representative()
{
    var cust_rep_spec =
        new Specification<XElement>(x => x.Name == "CustomerRep");
    var emp_id_spec =
        new Specification<XAttribute>(x => x.Name == "EmployeeID");

    var all_employees_on_file =
        XDocument.Load(@"..\..\XML\Employees.xml").Descendants().Attributes();

    var all_customers_on_file =
        XDocument.Load(@"..\..\XML\Customers.xml").Root.Descendants().Elements();

    var customer_with_rep =
        from c in all_customers_on_file.Where(cust_rep_spec.EvalFunc)
        from e in all_employees_on_file.Where(emp_id_spec.EvalFunc)
        where c.Value == e.Value
        select new
        {
            c = c.Parent.Element("FirstName").Value + " " +
                c.Parent.Element("LastName").Value,
            e = e.Value
        };

    customer_with_rep.ToList().ForEach(x =>
    {
        Console.WriteLine(
            string.Format("Customer : {0} is represented by Rep Number {1}",
            x.c, x.e));
    });
}

For ease of reading this code without the necessity of downloading the code, I have included the XML directly in the post here.

Employee.xml File

<?xml version="1.0" encoding="utf-8" ?>
<Employees>
  <Employee EmployeeID="1" FirstName="Nancy"
            LastName="Davolio" Title="Sales Representative"
            HireDate="1992-05-01T00:00:00" Country="USA"
            Extension="5467" />
  <Employee EmployeeID="2" FirstName="Andrew"
            LastName="Fuller" Title="Vice President, Sales"
            HireDate="1992-08-14T00:00:00" Country="AUSTRALIA"
            Extension="3457" />
  <Employee EmployeeID="3" FirstName="Janet"
            LastName="Leverling" Title="Sales Representative"
            HireDate="1992-04-01T00:00:00" Country="USA"
            Extension="3355" />
  <Employee EmployeeID="4" FirstName="Margaret"
            LastName="Peacock" Title="Sales Representative"
            HireDate="1993-05-03T00:00:00" Country="AUSTRALIA"
            Extension="5176" />
  <Employee EmployeeID="5" FirstName="Steven" LastName="Buchanan"
            Title="Sales Manager" HireDate="1993-10-17T00:00:00"
            Country="UK" Extension="3453" />
  <Employee EmployeeID="6" FirstName="Michael"
            LastName="Suyama" Title="Sales Representative"
            HireDate="1993-10-17T00:00:00" Country="UK"
            Extension="428" />
  <Employee EmployeeID="7" FirstName="Robert" LastName="King"
            Title="CEO" HireDate="1994-01-02T00:00:00"
            Country="UK" Extension="465" />
  <Employee EmployeeID="8" FirstName="Laura" LastName="Callahan"
            Title="Inside Sales Coordinator"
            HireDate="1994-03-05T00:00:00" Country="USA"
            Extension="2344" />
  <Employee EmployeeID="9" FirstName="Anne" LastName="Dodsworth"
            Title="Sales Representative" HireDate="1994-11-15T00:00:00"
            Country="UK" Extension="452" />
</Employees>

Customers.xml File

<?xml version="1.0" encoding="utf-8" ?>
<Customers>
  <Customer>
    <FirstName>Simon</FirstName>
    <LastName>Segal</LastName>
    <JoinDate>19-10-2008</JoinDate>
    <Country>Australia</Country>
    <Email>simon.segal@techavalanche.com</Email>
    <MobilePhone>0405987367</MobilePhone>
    <CustomerRep>2</CustomerRep>
  </Customer>
  <Customer>
    <FirstName>Bob</FirstName>
    <LastName>Jones</LastName>
    <JoinDate>23-10-2008</JoinDate>
    <Country>Germany</Country>
    <Email>bob.j@bundes.com</Email>
    <MobilePhone>8983478347</MobilePhone>
    <CustomerRep>1</CustomerRep>
  </Customer>
  <Customer>
    <FirstName>Clair</FirstName>
    <LastName>Humgola</LastName>
    <JoinDate>10-10-2008</JoinDate>
    <Country>Australia</Country>
    <Email>chumg@telstra.com.au</Email>
    <MobilePhone>0458193487</MobilePhone>
    <CustomerRep>4</CustomerRep>
  </Customer>
  <Customer>
    <FirstName>Milton</FirstName>
    <LastName>Nomik</LastName>
    <JoinDate>20-12-2008</JoinDate>
    <Country>Australia</Country>
    <Email>milt.n@techavalanche.com</Email>
    <MobilePhone>09072625717</MobilePhone>
    <CustomerRep>6</CustomerRep>
  </Customer>
</Customers>

On a final note: I am going to have a go shortly at putting together some variations of the Specification<T> class to allow for a more complete use with LINQ in General, remembering that the initial requirement was based around testing objects for equality and then creating a more manageable and malleable system of creating dynamic queries in LINQ To SQL and the Entity Framework. The “new” Specification class will need to support more than the just Func<T, bool> and should prove to be in interesting exercise.

Full code download for this post can be found here.