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。

好了，以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对的支持。