The following is an example of a FakeRepository and associated interfaces. The FakeRepository is a fake implementation of a generic repository.

public interface IRepository<T>
{
  T Get(Func<T, bool> predicate);
  IQueryable<T> GetAll();
  T Save(T item);
  IRepository<T> Include(Expression<Func<T, object>> path);
}

public interface IIdentity
{
  int Id {get;set;}
}

public class FakeRepository<T> : IRepository<T> where T : IIdentity
{
  private int _identityCounter = 0;
  public IList<T> DataSet { get; set; } = new List<T>();

  public T Get(Func<T, bool> predicate)
  {
    return GetAll().Where(predicate).FirstOrDefault();
  }

  public IQueryable<T> GetAll()
  {
    return DataSet.AsQueryable();
  }

  public T Save(T item)
  {
    return item.Id == default(int) ? Create(item) : Update(item);
  }           

  public IRepository<T> Include(Expression<Func<T, object>> path)
  {
    // Nothing to do here since this function is for EntityFramework
    // We are using Linq to Objects so there is not need for Include
    return this;
  }

  private T Create(T item)
  { 
    item.Id = ++_identityCounter;
    DataSet.Add(item);
    return item;
  }

  private T Update(T item)
  {
    var found = Get(x => x.Id == item.Id);

    if(found == null)
    {
      throw new KeyNotFoundException($"Item with Id {item.Id} not    
        found!");
    }

    DataSet.Remove(found);
    DataSet.Add(item);

    return item;
  }
}