Connected.Common.Types/Common.Types.Tests/InstanceFaker.cs

using Moq;

namespace Common.Types;

internal class InstanceFaker<T>
{
	private readonly Dictionary<Type, Mock> _mocks = new();
	private bool _mocksGenerated = false;

	public InstanceFaker()
	{
		if (typeof(T).GetConstructors().Length > 1)
			throw new Exception($"Multiple constructors for type {typeof(T).FullName} found. Auto resolve not possible.");

		GenerateMocks();
	}

	private void GenerateMocks()
	{
		if (_mocksGenerated)
			return;

		_mocksGenerated = true;

		var constructor = typeof(T).GetConstructors()[0];

		var parameters = constructor.GetParameters();

		foreach (var parameter in parameters)
		{
			if (!parameter.ParameterType.IsInterface)
				throw new Exception($"Constructor parameter {parameter.Name} of type {parameter.ParameterType.FullName} is not an interface, cannot mock.");

			_mocks.Add(parameter.ParameterType, CreateMock(parameter.ParameterType));
		}
	}

	private T ProcessType()
	{
		GenerateMocks();

		var constructor = typeof(T).GetConstructors()[0];

		var mocks = _mocks.Select(e => e.Value.Object);

		return (T)constructor.Invoke(mocks.Cast<object>().ToArray());
	}

	private Mock? CreateMock(Type type)
	{
		var creator = typeof(Mock<>).MakeGenericType(type);
		return Activator.CreateInstance(creator) as Mock;
	}

	public Mock<T>? GetMock<T>()
		where T : class => _mocks.GetValueOrDefault(typeof(T)) as Mock<T>;

	private T _instance;

	public T Instance => _instance ??= ProcessType();
}