ABP框架源码中的Linq扩展方法

文件目录：aspnetboilerplate-dev\aspnetboilerplate-dev\src\Abp\Collections\Extensions\EnumerableExtensions.cs

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;
}
}
}

WhereIf很好用：

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)
};
}

以前都需要这样写：