【ABP杂烩】Extensions后缀扩展方法

1.Extensions介绍

扩展方法使你能够向现有类型“添加”方法，而无需创建新的派生类型、重新编译或以其他方式修改原始类型。 扩展方法是一种特殊的静态方法，但可以像扩展类型上的实例方法一样进行调用。 对于用 C#、F# 和 Visual Basic 编写的客户端代码，调用扩展方法与调用在类型中实际定义的方法没有明显区别。

详细见官方文档：扩展方法（C# 编程指南）

2.语法介绍

* 需要写在一个静态类中 
* 必须是一个静态方法 
* 通过第一个参数和this关键字指定扩展的目标类型 
* 不同类型的扩展方法不一定要写在同一个类中

3.事例介绍

 在ABP框架原中会发现在很多地方带有Extensions的类，其主要作用是在不改变原有接口或类的基础上扩展自定义的方法，从而方便使用，如QueryableExtensions类是对IQueryable进行扩展，里面添加了分页PageBy、条件判断WhereIf方法，代码如下：

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

​ 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;         }     } }   ​

 

如上所述，对IQueryable扩展后，可以像调用原生方法一样，使上层的调用感受不到区别和不用做过多的操作，方便对第三方的库进行扩展，从而增加自定义需求，有效提高项目的开发效率。调用代码如下：

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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                );        }