如何使用接口，基础和具体实现存储库模式

Question

I have almost completed implementing my repository pattern by having a IRepository<T> interface, a NewsRepository class and a News entity. The problem I ran into was trying to abstract out common methods to a base Repository class.

I could not find a way to abstract the Get method in the NewsRepository as it contains a specific Linq expression.

My questions are :

1) How do I abstract to a base class the public T Get(int id) method please? The only way I have done it so far is by passing in Expression<Func<T,bool>> instead of an int, but then that deosn't really abstract out common behaviour as each sub-class will still need to pass in an expression that is almost identical in each case ie n => n.id == id .

2) How do I pass into the base class on the Update method the sub-class GetViolations and map methods please? I imagine the solution is possibly by using delegates, but I couldn't get the syntax to compile

This is a simplified set of the code - in practice I have a Save method which does Update and Insert rather than just the Update shown here.

public interface IRepository<T>
{
    T Get(int id);
    void Update(T item);
}

public class NewsRepository : IRepository<News>
{
    private Table<News> _newsTable;
    public NewsRepository(string connectionString)
    {
        _newsTable = new DataContext(connectionString).GetTable<News>();
    }

    public News Get(int id)
    {
        return _newsTable.SingleOrDefault(n => n.NewsId == id);
    }

    public void Update(News item)
    {
        var errors = item.GetRuleViolations();
        if (errors.Count > 0)
            throw new RuleException(errors);

        News dbNews = _newsTable.SingleOrDefault(n => n.NewsId == item.NewsId);
        map(dbNews, item);

        _newsTable.Context.SubmitChanges();
    }

    private void map(News dbNews, News news)
    {
        dbNews.Title = news.Title;
        dbNews.Article = news.Article;
    }
}

public class Repository<T> where T : class
{
    protected Table<T> _table;

    public Repository(Table<T> t)
    {
        _table = t;
    }

    //How do i do this??! - This doesn't compile due to T no having a NewsId
    public T Get(int id)
    {
    return _table.SingleOrDefault(n => n.NewsId == id);
    }

    //This seems to be a solution, but it's not really abstracting common behaviour as each
    //sub-class will still need to pass in the same linq expression...
    public T Get(Expression<Func<T,bool>> ex)
    {
        return _table.SingleOrDefault(ex);
    }

    public void Update(T item)
    {
        //How is it possible to pass in the GetRuleViolations and map functions to this method?
        var errors = item.GetRuleViolations();
        if (errors.Count > 0)
            throw new RuleException(errors);

        T dbNews = _table.SingleOrDefault(n => n.NewsId == item.NewsId);
        map(dbNews, item);

        _table.Context.SubmitChanges();
    }
}

Answer 1

1. L2S supports neither layer supertypes nor using interface members in queries, which makes reuse quite difficult. One option is to dynamically build an expression tree. It's a bit messy, but if you isolate it to your base class repository it's not that bad.

Here is an example:

public interface IEntity
{
    int Id { get; }
}

public partial class News : IEntity
{
}

public class Repository<T> where T : class, IEntity
{

    private readonly DataContext _db;

    public Repository(DataContext db)
    {
        _db = db;
    }

    public T Get(int id)
    {
        Expression<Func<T, bool>> hasId = HasId(id);
        return _db.GetTable<T>().Single(hasId);
    }

    // entity => entity.Id == id;   
    private Expression<Func<T, bool>> HasId(int id)
    {
        ParameterExpression entityParameter = Expression.Parameter(typeof (T), "entity");
        return Expression.Lambda<Func<T, bool>>(
            Expression.Equal(
                Expression.Property(entityParameter, "Id"),
                Expression.Constant(id)
                ),
            new[] {entityParameter}
            );
    }
}

See also http://msdn.microsoft.com/en-us/library/bb397951.aspx

Answer 2

1. It really helps to have a layer supertype for entities . They will share an Id property. You won't have to deal with an expression to represent the id proeprty, you'll just know what it is.

2. The template method pattern . In this pattern your base Update does all the work calling helper methods in order, and your derived classes implement those protected abstract helper methods.