public interface ITaskGenerator<T> where T : ITaskRequest
{
    int CreateTask(T request);
}

public class OrderTaskGenerator : ITaskGenerator<OrderTaskRequest>
{
        public int CreateTask(OrderTaskRequest request)
        {
          ..
        }
}

[TestFixture(typeof(OrderTaskGenerator))]
    public class ITaskGeneratorContract<T> where T : ITaskGenerator<ITaskRequest>, new()
    {

        [Test]
        public void CreateTask_Returns_InvalidException_If_Task_ID_Is_Empty()
        {
          // Test goes here
        }
    }

[TestFixture(typeof(OrderTaskGenerator), typeof(OrderTaskRequest))]
public class TaskGeneratorContractTest<T, TRequest> where T : ITaskGenerator<TRequest>, new() where TRequest : ITaskRequest
{
    private T _sut;

    public TaskGeneratorContractTest()
    {
        _sut = new T();
    }
    [Test]
    public void CreateTask_Returns_InvalidException_If_Task_ID_Is_Empty()
    {
        Assert.IsTrue(true);
    }
}