​
using System;
using System.Linq;
using System.Linq.Expressions;
using Abp.Application.Services.Dto;

namespace Abp.Linq.Extensions
{
    /// <summary>
    /// Some useful extension methods for <see cref="IQueryable{T}"/>.
    /// </summary>
    public static class QueryableExtensions
    {
        /// <summary>
        /// Used for paging. Can be used as an alternative to Skip(...).Take(...) chaining.
        /// </summary>
        public static IQueryable<T> PageBy<T>(this IQueryable<T> query, int skipCount, int maxResultCount)
        {
            if (query == null)
            {
                throw new ArgumentNullException("query");
            }

            return query.Skip(skipCount).Take(maxResultCount);
        }

        /// <summary>
        /// Used for paging with an <see cref="IPagedResultRequest"/> object.
        /// </summary>
        /// <param name="query">Queryable to apply paging</param>
        /// <param name="pagedResultRequest">An object implements <see cref="IPagedResultRequest"/> interface</param>
        public static IQueryable<T> PageBy<T>(this IQueryable<T> query, IPagedResultRequest pagedResultRequest)
        {
            return query.PageBy(pagedResultRequest.SkipCount, pagedResultRequest.MaxResultCount);
        }

        /// <summary>
        /// Filters a <see cref="IQueryable{T}"/> by given predicate if given condition is true.
        /// </summary>
        /// <param name="query">Queryable to apply filtering</param>
        /// <param name="condition">A boolean value</param>
        /// <param name="predicate">Predicate to filter the query</param>
        /// <returns>Filtered or not filtered query based on <paramref name="condition"/></returns>
        public static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition, Expression<Func<T, bool>> predicate)
        {
            return condition
                ? query.Where(predicate)
                : query;
        }

        /// <summary>
        /// Filters a <see cref="IQueryable{T}"/> by given predicate if given condition is true.
        /// </summary>
        /// <param name="query">Queryable to apply filtering</param>
        /// <param name="condition">A boolean value</param>
        /// <param name="predicate">Predicate to filter the query</param>
        /// <returns>Filtered or not filtered query based on <paramref name="condition"/></returns>
        public static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition, Expression<Func<T, int, bool>> predicate)
        {
            return condition
                ? query.Where(predicate)
                : query;
        }
    }
}

​

 public async Task<PagedResultDto<ProjectListDto>> GetProjects(GetProjectsInput input)
        {
            var query = _projectRepository.GetAll();
            query = query
                .WhereIf(!input.Name.IsNullOrWhiteSpace(), item => item.Name.Contains(input.Name))
                .WhereIf(!input.Address.IsNullOrWhiteSpace(), item => item.Address.Contains(input.Address))
                .WhereIf(!input.ResponbleDepart.IsNullOrWhiteSpace(), item => item.ResponbleDepart.Contains(input.ResponbleDepart))
                .WhereIf(!input.Type.IsNullOrWhiteSpace(), item => item.Type.Contains(input.Type))
                .WhereIf(input.ProjectDateStart.HasValue, item => item.StartTime >= input.ProjectDateStart)
                .WhereIf(input.ProjectDateEnd.HasValue, item => item.EndTime <= input.ProjectDateEnd)
                .WhereIf(input.ReportDateStart.HasValue, item => item.ReportTime >= input.ReportDateStart)
                .WhereIf(input.ReportDateEnd.HasValue, item => item.ReportTime <= input.ReportDateEnd);


            var projectCount = await query.CountAsync();

            var projects = await query.OrderBy(input.Sorting).PageBy(input).ToListAsync();

            var projectListDtos = ObjectMapper.Map<List<ProjectListDto>>(projects);

            return new PagedResultDto<ProjectListDto>(
                projectCount,
                projectListDtos
                );
        }