public class GetAllTasksInput : PagedAndSortedResultRequestDto
{
    public TaskState? State { get; set; }
}

public class TaskAppService : AsyncCrudAppService<Task, TaskDto, int, GetAllTasksInput>
{
    public TaskAppService(IRepository<Task> repository)
        : base(repository)
    {

    }

    protected override IQueryable<Task> CreateFilteredQuery(GetAllTasksInput input)
    {
        return base.CreateFilteredQuery(input)
            .WhereIf(input.State.HasValue, t => t.State == input.State.Value);
    }
}

参考：
https://aspnetboilerplate.com/Pages/Documents/Application-Services
https://docs.microsoft.com/en-us/aspnet/core/data/ef-rp/sort-filter-page?view=aspnetcore-2.2

abp CrudAppService 自定义分页、排序