这是最终目标：通过名称从工厂获取新连接 factory.CreateConnection<SqlConnection>("my db")
到目前为止，这是我的代码：
type IConnectionFactory =
abstract member CreateConnection : unit -> IDbConnection
type DefaultConnectionFactory<'t when 't : (new : unit -> 't) and 't :> IDbConnection> () =
member this.CreateConnection () = new 't()
interface IConnectionFactory with
member this.CreateConnection () = this.CreateConnection() :> IDbConnection
type DefaultConnectionFactoryFactory() =
member this.CreateConnectionFactory connectionType =
if not (typeof<IDbConnection>.IsAssignableFrom connectionType)
then invalidArg "connectionType" (sprintf "type %O is not assignable to IDbConnection" connectionType)
let genericFactoryType = typedefof<DefaultConnectionFactory<_>>
let factorytype = genericFactoryType.MakeGenericType([| connectionType |])
Activator.CreateInstance(factorytype) :?> IConnectionFactory
type ConnectionFactory (connectionStrings : Dictionary<string, (IConnectionFactory * string)>) =
let ConnectionStrings = connectionStrings
member this.CreateConnection connectionStringName : 't :> IDbConnection =
if ConnectionStrings.ContainsKey connectionStringName
then
let connectionFactory, connectionString = ConnectionStrings.Item connectionStringName
let connection = connectionFactory.CreateConnection ()
connection.ConnectionString <- connectionString
connection :?> 't
else invalidArg "dataSourceName" (sprintf "no data source found named %s" connectionStringName)
在这条线上：
let genericFactoryType = typedefof<DefaultConnectionFactory<_>>
我收到此错误：为类型推断变量应用默认类型“IDbConnection”时，键入约束不匹配 . 泛型构造要求类型“IDbConnection”具有公共默认构造函数 . 考虑添加其他类型约束
我不确定如何解决它...欢迎任何其他建议
1 回答
这与this question有关 . 问题是
typedefof<DefaultConnectionFactory<_>>.DefaultConnectionFactory上的约束需要一个实现IDbConnection的类型，并且具有默认构造函数 . 编译器知道没有这种类型 . 所以，你必须提供一个 . 你可以使用SqlConnection，例如：效果是一样的 .
typedefof将返回无界类型DefaultConnectionFactory<>（以C＃术语表示），这在F＃中无法表达 .