让我描述一下情况。
我使用 Web API,在定义 http 动词的控制器类中,我注意到我无法创建构造函数。事实上,如果我添加一个空的构造函数,那就可以了,但是当我修改这个构造函数时,就不行了。当我尝试使用邮递员发布到控制器类内的方法时,它给了我以下错误:
{
"Message": "An error has occurred.",
"ExceptionMessage": "An error occurred when trying to create a controller of type 'DataController'. Make sure that the controller has a parameterless public constructor.",
"ExceptionType": "System.InvalidOperationException",
"StackTrace": " bij System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)\r\n bij System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request)\r\n bij System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()",
"InnerException": {
"Message": "An error has occurred.",
"ExceptionMessage": "Type Chatbot.Controller.DataController heeft geen standaardconstructor",
"ExceptionType": "System.ArgumentException",
"StackTrace": " bij System.Linq.Expressions.Expression.New(Type type)\r\n bij System.Web.Http.Internal.TypeActivator.Create[TBase](Type instanceType)\r\n bij System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator)\r\n bij System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)"
}
}
控制器类
public class DataController : ApiController {
public string data { get; set; }
public DataController(string analyticsData) {
data = analyticsData;
}
public void Post([FromBody] dynamic data) {
_analyticsData.InsertCredentials("fdf", data, "dfdf");
如何实现向控制器类添加非空构造函数而不导致此错误的目标?
[编辑]我想要这个是因为以下原因:
private AnalyticsData _analyticsData;
public CredentialsController(AnalyticsData analyticsData) {
_analyticsData = analyticsData;
}
//acces the following method send data to database
_analyticsData.InsertCredentials("fdf", "fdd", "dfdf");
我想从构造函数访问此方法,但是如果我的构造函数不为空,我已经收到此错误。就像我说的,当我在它旁边添加一个空构造函数时,它不会再收到错误,但无法找到 _analyticsData,因为它仅在第二个构造函数中可用
[编辑] 我现在使用Unity进行DI 这是我在 WebApiConfig 中添加的行
container.RegisterType<IAnalyticsData, AnalyticsData>(new HierarchicalLifetimeManager());
我用其他类和接口测试了它,它有效。但它不起作用 当 AnalyticsData 等类内部有一个构造函数时。所以 仅当我从 AnalyticsData 类中删除构造函数时,这才有效
AnalyticsData 类的构造函数
public AnalyticsData(ResourceFile file, ResourceFile creationFile) {
var exists = file.Exists();
_connection = SQLite.Open(file);
if (!exists && creationFile.Exists()) {
SQLite.Execute(_connection, creationFile.GetText());
}
}
需要构造函数来加载当前的sqlite数据库
AnalyticsData = new AnalyticsData(new ResourceFile(Settings.String("DatabaseLocation")), Resources.GetFolder("data").GetFile("create.sql"));
我现在收到的唯一问题是我需要使用没有它就无法使用的构造函数,因为那样我就无法再与数据库建立连接。
除了添加这一行之外,我还需要做更多的事情吗?
container.RegisterType<IAnalyticsData, AnalyticsData>(new HierarchicalLifetimeManager());
当您创建一个没有构造函数的类时,编译器会自动为其创建一个无参数构造函数。 当您创建带有构造函数(带或不带参数)的类时,编译器不会再自动为该类创建无参数构造函数,因为已经存在一个。
对于您的解决方案,您只需在带参数的构造函数旁边添加无参数构造函数即可:
public class DataController : ApiController {
public string data { get; set; }
public DataController() : this("somedefaultvalue") { } //calls the parameter constructor with "somedefaultvalue"
public DataController(string analyticsData) {
data = analyticsData;
}
//other code
}
查看差异的示例:
static void Main()
{
var decaultConstr = new IOwnNothing(); //valid
var customConst = new IOwnAParamConstructor(); // invalid
var customConst2 = new IOwnAParamConstructor(6); //valid
var privateConstr = new IOwnAPrivateParamConstructor(); //no constructor available
}
private class IOwnNothing
{
}
private class IOwnAParamConstructor
{
public IOwnAParamConstructor(int a) { }
}
private class IOwnAPrivateParamConstructor
{
private IOwnAPrivateParamConstructor(int a) { }
}