WPF MVVM - Repository pattern query

Question

I have a web service which provides an interface to query data. I am writing a WPF application using MVVM. I am working on creating a repository that my View Models can use to retrieve models. The repository will call the Web service to fetch data, when required.

I would require various Find methods in my repository that finds the data based on various criteria and not just one criteria like 'Id'.

In my Repository, I have created a Find method that takes a Specification as input

void IList<MyData> Find (ISpecification spec) 

where a basic ISpecification interface is

public interface ISpecification<T>
{
    bool IsSatisfiedBy(T candidate);
}

A high level implemenation of the Find method would be as follows

• first searches the cache for candidates that satisfy specification
• If found return the list of candidates
• Else use the specification / criteria to call web service to fetch the candidates and return the list

I am confused about the Else scenario above - What is the correct way of designing the Specification so that if I have no data in repository cache that satisfies the specification, I should be able to retrieve the criteria from specification and call the web service passing the web method this criteria?

A few things on my mind-

• If I specialize Find methods to take specialized specifications which have properties / criterias then Repository - Specification has tight coupling
• If I would have to connect directly to DB (which I am not) then I could have supported a method that returns an SQL for example. LINQ could have been an option etc

Answer 1

Why don't you use a linq expression as the parameter input?

e.g.

public class MyModel
{
  public int Prop1 {get;set;}
  public string Prop2 {get;set;}
}

public interface IRepository
{
  T Find<T>(Expression<Func<T,object>> expression);
}

public class MyRepository : IRepository
{
  public  T Find<T>(Expression<Func<T,object>> expression) where T : class
  {
    //Implement your caching/ calling your web service here
  }
}

So you could then call your repository like so:

MyRepository repository = new MyRepository();
var model = repository.Find<MyModel>(a=> a.Prop1 == 5);

If you want to not allow the user to put any kind of type int the generic argument you could have your models all inherit from a base class or impelement an interface and then change the find method to:

public  T Find<T>(Expression<Func<T,object>> expression) where T : IMyModelInterface //or whatever base class you want