Hacking Linq to Entities with Expressions Part 1: Clean Generic Repository

The repository pattern is intended to create an abstraction layer between the data access layer and the business logic layer of an application and is often used with Entity Framework. To avoid creating repository classes specific to each entity type it is a common practice to create a generic repository class that can be used for any entity. However, most examples I have seen could not really be used easily for any entity. For instance the repository type requires to provide a generic type for the key (e.g. class Repository<TEntity, TKey>) which should not really be required as the type of the entity is provided. Another thing to look at it is the GetById() method. It’s interesting at least for a couple of reasons:

• key properties of different entity types may have different types (e.g. string key properties vs. int key properties)
• key properties of different entity types may have different names

I have seen several ways of solving the above problems, for instance: enforcing all entities to be derived from a base (possibly generic) entity type containing the key property (it does not solve the problem of entity types with different key property names since the names of all key properties in the whole model will be the same) or passing a lambda expression/query to the GetById() method (feels wrong to me since the GetById() method should just take the value of the key for which to return the entity and not query, property name and whatnot). I thought a little bit on this I concluded that it should be possible to create a generic repository type without any additional overhead since we already have all the information that is needed. We know the entity type – it is the generic parameter to the repository type. We are able to reason about the entity type (i.e. figure out what the key property are) because we do have the context and – as a result – we can access all the metadata. Finally – for the GetById() we have the value of the key since it is provided by the user. The only obstacle here is to create the right query to send to the database but this can be easily solved by creating the query dynamically with Expression Trees.
** EDIT **
As pointed out by Florim below in the comments there is a better option than building a dynamic query – namely DbSet.Find() method. Not it is simpler (it does not require building the dynamic query) but also it may save a trip to the database if the entity is available locally. I am leaving the rest of the post as is to justify the “Hacking Linq to Entities with Expressions” title.
** EDIT END **
Let’s start from finding the key property – given the entity type TEntity (the generic entity type of the repository) and a valid DbContext (derived) instance (passed as a parameter to the constructor of the repository type) we can find the key property as follows:

private PropertyInfo GetKeyProperty(DbContext context)
{
    if (_keyProperty == null)
    {
        var edmEntityType = 
            ((IObjectContextAdapter)context)
                .ObjectContext
                .MetadataWorkspace
                .GetItems<EntityType>(DataSpace.CSpace)
                .Single(e => e.Name == typeof(TEntity).Name);

        _keyProperty = 
            typeof(TEntity)
                .GetProperty(
                    edmEntityType.KeyMembers.Single().Name, 
                    BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

        if (_keyProperty == null)
        {
            throw new InvalidOperationException("Key property not found.");
        }
    }

    return _keyProperty;
}

Building the filter (i.e. the e => e.{keyProperty} == value) using Expression Trees is just a few lines of code:

private IQueryable<TEntity> Filter<TKey>(
    IQueryable<TEntity> dbSet,
    PropertyInfo keyProperty,
    TKey value)
{
    var entityParameter = Expression.Parameter(typeof(TEntity), "e");

    var lambda =
        Expression.Lambda<Func<TEntity, bool>>(
            Expression.Equal(
                Expression.Property(entityParameter, keyProperty),
                // no cast required if the passed value is of the 
                // same type as the key property
                typeof(TKey) == keyProperty.PropertyType ?
                    (Expression)Expression.Constant(value) :
                    (Expression)Expression.Convert(
                        Expression.Constant(value), keyProperty.PropertyType)),
                entityParameter);

    return dbSet.Where(lambda);
}

And finally we will connect the dots and create the GetById() method:

public TEntity GetById<TKey>(TKey value)
{
    return Filter(
        _context.Set<TEntity>(),
        GetKeyProperty(_context), value).SingleOrDefault();
}

Yes, the GetById() is generic. This is to avoid the value to be of the object type. Note that this does not add any overhead since the generic type does not have to be provided when invoking this method – the compiler is able to infer the type from the value of the parameter. In addition the Filter method will add a cast if the type of the passed value is different from the type of the key property (which will result in an exception at runtime if the provided value cannot be cast to the type of the key property).
For completeness here is the generic repository class (it does not include the Add, Delete etc. methods as they are not as interesting to me as the GetById() method):

public class Repository<TEntity> where TEntity : class
{
    private readonly DbContext _context;

    // for brevity composite keys are not supported
    private PropertyInfo _keyProperty;

    public Repository(DbContext context)
    {
        _context = context;
    }

    public TEntity GetById<TKey>(TKey value)
    {
        return Filter(
            _context.Set<TEntity>(),
            GetKeyProperty(_context), value).SingleOrDefault();
    }

    private IQueryable<TEntity> Filter<TKey>(
        IQueryable<TEntity> dbSet,
        PropertyInfo keyProperty,
        TKey value)
    {
        var entityParameter = Expression.Parameter(typeof(TEntity), "e");

        var lambda =
            Expression.Lambda<Func<TEntity, bool>>(
                Expression.Equal(
                    Expression.Property(entityParameter, keyProperty),
                    // no cast required if the passed value is of the
                    // same type as the key property
                    typeof(TKey) == keyProperty.PropertyType ?
                        (Expression)Expression.Constant(value) :
                        (Expression)Expression.Convert(
                            Expression.Constant(value), keyProperty.PropertyType)),
                    entityParameter);

        return dbSet.Where(lambda);
    }

    private PropertyInfo GetKeyProperty(DbContext context)
    {
        if (_keyProperty == null)
        {
            var edmEntityType =
                ((IObjectContextAdapter)context)
                    .ObjectContext
                    .MetadataWorkspace
                    .GetItems<EntityType>(DataSpace.CSpace)
                    .Single(e => e.Name == typeof(TEntity).Name);

            _keyProperty =
                typeof(TEntity)
                    .GetProperty(
                        edmEntityType.KeyMembers.Single().Name,
                        BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

            if (_keyProperty == null)
            {
                throw new InvalidOperationException("Key property not found.");
            }
        }

        return _keyProperty;
    }

    // other "less interesting" methods
}

… and an example of how to use it. For the following, simple model with entities having keys with different names and of different types:

public class Customer
{
    public string CustomerId { get; set; }
    
    // ...
}

public class Order
{
    public Guid OrderId { get; set; }

    // ...
}

public class Item
{
    public int ItemId { get; set; }

    // ...
}

public class Context : DbContext
{
    public DbSet<Customer> Customers { get; set; }
    public DbSet<Order> Orders { get; set; }
    public DbSet<Item> Items { get; set; }
}

The entities can be retrieved by id as simple as:

using (var ctx = new Context())
{
    Console.WriteLine(
        new Repository<Customer>(ctx)
            .GetById("ALFKI").CustomerId);

    Console.WriteLine(
        new Repository<Order>(ctx)
            .GetById(new Guid("00000000-0000-0000-C000-000000000046")).OrderId);

    Console.WriteLine(
        new Repository<Item>(ctx)
            .GetById((byte)1).ItemId);
}

As you can see the code is clean – no extraneous information is provided – just the type of the entity and the value of the key (Yeah, the cast to byte is not needed – it is just to test that the logic in the dynamically built filter works)