ABP框架源码中的Linq扩展方法

using System;
using System.Collections.Generic;
using System.Linq;

namespace Abp.Collections.Extensions
{    
    /// <summary> 
    /// Extension methods for <see cref="IEnumerable{T}"/>.
    /// </summary>
    public static class EnumerableExtensions
    {
        /// <summary>
        /// Concatenates the members of a constructed <see cref="IEnumerable{T}"/> collection of type System.String, using the specified separator between each member.
        /// This is a shortcut for string.Join(...)
        /// </summary>
        /// <param name="source">A collection that contains the strings to concatenate.</param>
        /// <param name="separator">The string to use as a separator. separator is included in the returned string only if values has more than one element.</param>
        /// <returns>A string that consists of the members of values delimited by the separator string. If values has no members, the method returns System.String.Empty.</returns>
        public static string JoinAsString(this IEnumerable<string> source, string separator)
        {
            return string.Join(separator, source);
        }

        /// <summary>
        /// Concatenates the members of a collection, using the specified separator between each member.
        /// This is a shortcut for string.Join(...)
        /// </summary>
        /// <param name="source">A collection that contains the objects to concatenate.</param>
        /// <param name="separator">The string to use as a separator. separator is included in the returned string only if values has more than one element.</param>
        /// <typeparam name="T">The type of the members of values.</typeparam>
        /// <returns>A string that consists of the members of values delimited by the separator string. If values has no members, the method returns System.String.Empty.</returns>
        public static string JoinAsString<T>(this IEnumerable<T> source, string separator)
        {
            return string.Join(separator, source);
        }

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

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

public GetTasksOutput GetTasks(GetTasksInput input)
        {
            var query = _taskRepository.GetAll().Include(t => t.AssignedPerson)
                .WhereIf(input.State.HasValue, t => t.State == input.State.Value)
                .WhereIf(!input.Filter.IsNullOrEmpty(), t => t.Title.Contains(input.Filter))
                .WhereIf(input.AssignedPersonId.HasValue, t => t.AssignedPersonId == input.AssignedPersonId.Value);
            //WhereIf为Linq扩展方法，表示如果第一个表达式为true，则加第二个过滤条件
            //这样就不需要用那么多个if()了

            //排序
            if (!string.IsNullOrEmpty(input.Sorting))
                query = query.OrderBy(input.Sorting);
            else
                query = query.OrderByDescending(t => t.CreationTime);

            var taskList = query.ToList();

            //Used AutoMapper to automatically convert List<Task> to List<TaskDto>.
            return new GetTasksOutput
            {
                Tasks = Mapper.Map<List<TaskDto>>(taskList)
            };
        }