Question

I would like to implement a simple (testing purposes only) in memory repository as below. The interfaces it implements are generic. One of the methods “Delete” is used in the sample code below.

The cast of predicate throws an exception. How can I implement the deleted method correctly?

public class InMemoryReportingRepository : IReportingRepository
{
    private readonly List<IDto> m_dtos;

    public InMemoryReportingRepository()
    {
        m_dtos = new List<IDto>();
    }

    // ommitted stuff

    public void Delete<TEntity>(Expression<Func<TEntity, bool>> predicate) where TEntity : class, IDto
    {
        var delete = m_dtos.FirstOrDefault((Func<IDto, bool>) predicate.Compile());

        m_dtos.Remove(delete);
    }
}

Answer 1

Try the following:

public void Delete<TEntity>(Expression<Func<TEntity, bool>> predicate) where TEntity : class, IDto
{
    var compiled = predicate.Compile();
    var toDelete = m_dtos.FirstOrDefault(dto => (dto is TEntity) && compiled((TEntity)dto));
    m_dtos.Remove(delete);
}

Func<TEntity, bool> and Func<IDto, bool> are completely different types, which is why your cast failed.

However, I would recommend using class level generics for your in memory data store:

public class DataStore<TEntity> : IDataStore<TEntity> where TEntity : class, IDto
{
  private readonly List<TEntity> m_dtos = new List<TEntity>();
  ...
  public void Delete(Func<TEntity, bool> predicate)
  {
    var toDelete = m_dtos.FirstOrDefault(predicate);
    m_dtos.Remove(toDelete);
  }
}