FakeRepository.cs

/// <summary>
/// An in-memory repository of T.
/// </summary>
/// <typeparam name="T">Type of entities to emulate.</typeparam>
public class FakeRepository<T> : IRepository<T>
	where T : class
{
	private readonly PropertyInfo _key;
	private readonly Dictionary<int, T> _entities;

	public FakeRepository()
	{
		_key = GetIdProperty();

		if (_key.PropertyType != typeof(int))
		{
			throw new NotSupportedException("Cannot fake entities where the key is not an int.");
		}

		_entities = new Dictionary<int, T>();
	}

	public void Create(T entity) => _entities.Add((int)_key.GetValue(entity), entity);

	public void Delete(int? id) => _entities.Remove(id.Value);

	public void Delete(T entity) => _entities.Remove((int)_key.GetValue(entity));

	public void Dispose() => _entities.Clear();

	public T Find(int? id)
	{
		T obj = default(T);
		_entities.TryGetValue(id.Value, out obj);
		return obj;
	}

	public IQueryable<T> GetAll() => _entities.Values.AsQueryable();

	public void Update(T entity) => _entities[(int)_key.GetValue(entity)] = entity;

	/// <summary>
	/// Get the Id property of an entity.  This is roughly the same thing that Entity Framework does when not provided with a [Key] override.
	/// </summary>
	/// <see cref="https://github.com/mono/entityframework/blob/70998a4be0ed566ad3f21d6c5a51016b7c16f9bd/src/EntityFramework/ModelConfiguration/Conventions/Edm/IdKeyDiscoveryConvention.cs#L22"/>
	/// <param name="obj">Object from which to get the key.</param>
	/// <returns>Property info for the key, or null if a key was not found.</returns>
	private PropertyInfo GetIdProperty()
	{
		var name = typeof(T).Name;
		var primitiveProperties = typeof(T).GetProperties().Where(p => p.PropertyType.IsPrimitive);

		return (from p in primitiveProperties
				where p.Name.Equals("Id", StringComparison.OrdinalIgnoreCase)
				|| p.Name.Equals(name + "Id", StringComparison.OrdinalIgnoreCase)
				select p).Single();
	}
}