简述
OData，即Open Data Protocol，是由微软在2007年推出的一款开放协议，旨在通过简单、标准的方式创建和使用查询式及交互式RESTful API。
类库
在.NET Core中想要使用OData功能的话需要添加Microsoft.AspNetCore.OData包。
dotnet add package Microsoft.AspNetCore.OData准备模型类
public class Address{ ???public string City { get; set; } ???public string Street { get; set; }}public enum Category{ ???Book, ???Magazine, ???EBook}public class Press{ ???public int Id { get; set; } ???public string Name { get; set; } ???public string Email { get; set; } ???public Category Category { get; set; }}public class Book{ ???public int Id { get; set; } ???public string ISBN { get; set; } ???public string Title { get; set; } ???public string Author { get; set; } ???public decimal Price { get; set; } ???public Address Address { get; set; } ???public Press Press { get; set; }}创建Edm模型
OData使用EDM，即Entity Data Model来描述数据的结构。在Startup文件中添加创建方法。
private static IEdmModel GetEdmModel(){ ???var builder = new ODataConventionModelBuilder(); ???builder.EntitySet<Book>("Books"); ???builder.EntitySet<Press>("Presses"); ???return builder.GetEdmModel();}注册OData服务
在Startup文件的ConfigureServices方法里注册OData服务。
services.AddOData();services.AddMvc(options => ???{ ???????options.EnableEndpointRouting = false; ???}).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);这里要注意的是在.NET Core 2.2里，默认已经有终结点，所以要使用OData的终结点的话需要将默认选项禁用掉。
注册OData终结点
同样在Startup文件里，在其Configure方法内将原来的注册路由内容改为注册OData的终结点。
app.UseMvc(b =>{ ???b.MapODataServiceRoute("odata", "odata", GetEdmModel());});显示元数据
运行程序后访问https://localhost:5001/odata/$metadata地址，可以看到所有可用模型的元数据。
<edmx:Edmx xmlns:edmx="http://docs.oasis-open.org/odata/ns/edmx" Version="4.0"> ???<edmx:DataServices> ???????<Schema xmlns="http://docs.oasis-open.org/odata/ns/edm" Namespace="Default"> ???????????<EntityType Name="Book"> ???????????????<Key> ???????????????????<PropertyRef Name="Id"/> ???????????????</Key> ???????????????<Property Name="Id" Type="Edm.Int32" Nullable="false"/> ???????????????<Property Name="ISBN" Type="Edm.String"/> ???????????????<Property Name="Title" Type="Edm.String"/> ???????????????<Property Name="Author" Type="Edm.String"/> ???????????????<Property Name="Price" Type="Edm.Decimal" Nullable="false"/> ???????????????<Property Name="Address" Type="Default.Address"/> ???????????????<NavigationProperty Name="Press" Type="Default.Press"/> ???????????</EntityType> ???????????<EntityType Name="Press"> ???????????????<Key> ???????????????????<PropertyRef Name="Id"/> ???????????????</Key> ???????????????<Property Name="Id" Type="Edm.Int32" Nullable="false"/> ???????????????<Property Name="Name" Type="Edm.String"/> ???????????????<Property Name="Email" Type="Edm.String"/> ???????????????<Property Name="Category" Type="Default.Category" Nullable="false"/> ???????????</EntityType> ???????????<ComplexType Name="Address"> ???????????????<Property Name="City" Type="Edm.String"/> ???????????????<Property Name="Street" Type="Edm.String"/> ???????????</ComplexType> ???????????<EnumType Name="Category"> ???????????????<Member Name="Book" Value="0"/> ???????????????<Member Name="Magazine" Value="1"/> ???????????????<Member Name="EBook" Value="2"/> ???????????</EnumType> ???????????<EntityContainer Name="Container"> ???????????????<EntitySet Name="Books" EntityType="Default.Book"> ???????????????????<NavigationPropertyBinding Path="Press" Target="Presses"/> ???????????????</EntitySet> ???????????????<EntitySet Name="Presses" EntityType="Default.Press"/> ???????????</EntityContainer> ???????</Schema> ???</edmx:DataServices></edmx:Edmx>创建Controller
本文实例中不考虑数据库的操作，故而使用hard code方式构建必要的模型对象。
public class BooksController : ODataController{ ???private static IList<Book> Books {get; set;} ???public BooksController() ???{ ???????Books = new List<Book> ???????{ ???????????new Book ???????????{ ???????????????Id = 1, ???????????????ISBN = "111-0-321-56789-1", ???????????????Title = "Calculus", ???????????????Price = 66.6m, ???????????????Address = new Address ???????????????{ ???????????????????City = "Shanghai", ???????????????????Street = "Beijin Xi Road" ???????????????}, ???????????????Press = new Press ???????????????{ ???????????????????Id = 1, ???????????????????Name = "Shanghai Tongji", ???????????????????Category = Category.Book ???????????????} ???????????}, ???????????new Book ???????????{ ???????????????Id = 2, ???????????????ISBN = "222-2-654-00000-2", ???????????????Title = "Linear Algebra", ???????????????Price = 53.2m, ???????????????Address = new Address ???????????????{ ???????????????????City = "Shanghai", ???????????????????Street = "Beijin Dong Road" ???????????????}, ???????????????Press = new Press ???????????????{ ???????????????????Id = 2, ???????????????????Name = "Shanghai Fudan", ???????????????????Category = Category.EBook ???????????????} ???????????} ???????????????????}; ??????} ???[EnableQuery] ???public IActionResult Get() ???{ ???????return Ok(Books); ???} ???[EnableQuery] ???public IActionResult Get(int key) ???{ ???????return Ok(Books.FirstOrDefault(b => b.Id == key)); ???}}EnableQuery特性在需要高级查询的场景时必须添加。
查询
加入Controller之后，访问https://localhost:5001/odata/Books地址，可得到所有Book数据。
{ ???"@odata.context": "https://localhost:5001/odata/$metadata#Books", ???"value": [ ???????{ ???????????"Id": 1, ???????????"ISBN": "111-0-321-56789-1", ???????????"Title": "Calculus", ???????????"Author": null, ???????????"Price": 66.6, ???????????"Address": { ???????????????"City": "Shanghai", ???????????????"Street": "Beijin Xi Road" ???????????} ???????}, ???????{ ???????????"Id": 2, ???????????"ISBN": "222-2-654-00000-2", ???????????"Title": "Linear Algebra", ???????????"Author": null, ???????????"Price": 53.2, ???????????"Address": { ???????????????"City": "Shanghai", ???????????????"Street": "Beijin Dong Road" ???????????} ???????} ???]}访问https://localhost:5001/odata/Books(1)地址，可得到key值为1的Book数据。
{ ???"@odata.context": "https://localhost:5001/odata/$metadata#Books/$entity", ???"Id": 1, ???"ISBN": "111-0-321-56789-1", ???"Title": "Calculus", ???"Author": null, ???"Price": 66.6, ???"Address": { ???????"City": "Shanghai", ???????"Street": "Beijin Xi Road" ???}}高级查询
如果想要使用OData查询的高级功能，可以在注册终结点时额外加上相应的配置。
app.UseMvc(b =>{ ???b.Select().Expand().Filter().OrderBy().MaxTop(100).Count(); ???b.MapODataServiceRoute("odata", "odata", GetEdmModel());});访问网址时加上所需的查询内容：https://localhost:5001/odata/Books?$select=Id,Title
{ ???"@odata.context": "https://localhost:5001/odata/$metadata#Books(Id,Title)", ???"value": [ ???????{ ???????????"Id": 1, ???????????"Title": "Calculus" ???????}, ???????{ ???????????"Id": 2, ???????????"Title": "Linear Algebra" ???????} ???]}如果想要按特定条件过滤数据内容的话也很容易：https://localhost:5001/odata/Books?$filter=Price%20le%2060
{ ???"@odata.context": "https://localhost:5001/odata/$metadata#Books", ???"value": [ ???????{ ???????????"Id": 2, ???????????"ISBN": "222-2-654-00000-2", ???????????"Title": "Linear Algebra", ???????????"Author": null, ???????????"Price": 53.2, ???????????"Address": { ???????????????"City": "Shanghai", ???????????????"Street": "Beijin Dong Road" ???????????} ???????} ???]}总结
不难看出，OData的真正魅力在于其对那些高级查询功能的支持，所以在创建RESTful API时，不妨考虑使用OData，这样应该能减少许多不必要的代码工作。
.NET Core开发日志——OData
原文地址：https://www.cnblogs.com/kenwoo/p/10360260.html