一旦成功的发布API后，使用者将依赖于你所提供的服务。但是变更总是无法避免的，因此谨慎的制定ASP.NET Web API的版本策略就变得非常重要。一般来说，新的功能需要无缝的接入，有时新老版本需要并行，以便给使用者足够的时间来进行迁移和配套的变更。设置，老的版本会一直持续被使用。
简单版本管理
假设我们现在对StudentsController进行修改，将GET方法中返回“FirstName” 和 “LastName”修改为返回“FullName”和 “CoursesDuration”。
最简单的方法就是新建一个类似于StudentsV2Controller的新服务。请留意V2，接下来将依赖这个引入管理策略。这种方法来自于Shawn Wildermuth的plural sight course文章。旧版本的服务返回的json应该是这样的：
1: [{
2: "id": 2,
3: "url": "http://localhost:8323/api/students/HasanAhmad",
4: "firstName": "Hasan",
5: "lastName": "Ahmad",
6: "gender": 0,
7: "enrollmentsCount": 4
8: },
9: {
10: "id": 3,
11: "url": "http://localhost:8323/api/students/MoatasemAhmad",
12: "firstName": "Moatasem",
13: "lastName": "Ahmad",
14: "gender": 0,
15: "enrollmentsCount": 4
16: }]
而新版本的返回如下：
1: [{
2: "id": 2,
3: "url": "http://localhost:8323/api/students/HasanAhmad",
4: "fullName": "Hasan Ahmad",
5: "gender": 0,
6: "enrollmentsCount": 4,
7: "coursesDuration": 13
8: },
9: {
10: "id": 3,
11: "url": "http://localhost:8323/api/students/MoatasemAhmad",
12: "fullName": "Moatasem Ahmad",
13: "gender": 0,
14: "enrollmentsCount": 4,
15: "coursesDuration": 16
16: }]
新版本的GET函数的实现为：
1: public IEnumerable<StudentV2BaseModel> Get(int page = 0, int pageSize = 10)
2: {
3: IQueryable<Student> query;
4:
5: query = TheRepository.GetAllStudentsWithEnrollments().OrderBy(c => c.LastName);
6:
7: var totalCount = query.Count();
8: var totalPages = Math.Ceiling((double)totalCount / pageSize);
9:
10: var urlHelper = new UrlHelper(Request);
11: var prevLink = page > 0 ? urlHelper.Link("Students", new { page = page - 1, pageSize = pageSize }) : "";
12: var nextLink = page < totalPages - 1 ? urlHelper.Link("Students", new { page = page + 1, pageSize = pageSize }) : "";
13:
14: var paginationHeader = new
15: {
16: TotalCount = totalCount,
17: TotalPages = totalPages,
18: PrevPageLink = prevLink,
19: NextPageLink = nextLink
20: };
21:
22: System.Web.HttpContext.Current.Response.Headers.Add("X-Pagination",
23: Newtonsoft.Json.JsonConvert.SerializeObject(paginationHeader));
24:
25: var results = query
26: .Skip(pageSize * page)
27: .Take(pageSize)
28: .ToList()
29: .Select(s => TheModelFactory.CreateV2Summary(s));
30:
31: return results;
32: }
StudentV2BaseModel和CreateV2Summary只是为了举例方便，实际可以根据需要是否要进行这样的修改或者无须修改。
具体的代码如下：
1: public class StudentV2BaseModel
2: {
3: public int Id { get; set; }
4: public string Url { get; set; }
5: public string FullName { get; set; }
6: public Data.Enums.Gender Gender { get; set; }
7: public int EnrollmentsCount { get; set; }
8: public double CoursesDuration { get; set; }
9: }
10:
11: public class ModelFactory
12: {
13: public StudentV2BaseModel CreateV2Summary(Student student)
14: {
15: return new StudentV2BaseModel()
16: {
17: Url = _UrlHelper.Link("Students", new { userName = student.UserName }),
18: Id = student.Id,
19: FullName = string.Format("{0} {1}", student.FirstName, student.LastName),
20: Gender = student.Gender,
21: EnrollmentsCount = student.Enrollments.Count(),
22: CoursesDuration = Math.Round(student.Enrollments.Sum(c => c.Course.Duration))
23: };
24: }
25: }