我有一个IMstTuver类实现的接口MstTuver。 IMstTuver包含MaxVersion和Agenttype个参数。
public class GetTable<T> where T : IMstTuver
{
public IMstTuver GetEntities(DbContext context, string agenttype)
{
long maxVersion = context.Set<T>().Max(x => x.MaxVersion);
IMstTuver mstTuver = context.Set<T>()
.Where(x => x.MaxVersion == maxVersion &&
x.AgentType == agenttype)
.FirstOrDefault();
return mstTuver;
}
}
在我班上：
table.GetEntities(MyDbContext, "MSMA") as MstTuver;
我收到错误
类型＆＃39; T＆＃39;必须是参考类型才能将其用作参数＆＃39; TEntity＆＃39;在泛型类型或方法中，System.Data.Entity.DbSet＆＃39;
请帮忙。
答案 0 :(得分：3)
Set<T>有一个通用约束，T必须是引用类型，这意味着你的类也需要有一个。修复方法是对调用链上的所有泛型类和/或方法应用以下修改：
public class GetTable<T> where T : class, IMstTuver
答案 1 :(得分：1)
您可以通过在方法中添加angular.module('starter', ['ionic', 'ngCordova'])
.run(function($ionicPlatform, GoogleMaps) {
$ionicPlatform.ready(function() {
// Hide the accessory bar by default (remove this to show the accessory bar above the keyboard
// for form inputs)
if(window.cordova && window.cordova.plugins.Keyboard) {
cordova.plugins.Keyboard.hideKeyboardAccessoryBar(true);
}
if(window.StatusBar) {
StatusBar.styleDefault();
}
GoogleMaps.init();
})
})
.config(function($stateProvider, $urlRouterProvider) {
$stateProvider
.state('map', {
url: '/',
templateUrl: 'templates/map.html',
controller: 'MapCtrl'
});
$urlRouterProvider.otherwise("/");
})
.factory('Markers', function($http) {
var markers = [];
return {
getMarkers: function(){
return $http.get("http://localhost:8080/markers.php").then(function(response){
markers = response;
return markers;
});
}
}
})
.factory('GoogleMaps', function($cordovaGeolocation, Markers){
var apiKey = false;
var map = null;
function initMap(){
var options = {timeout: 10000, enableHighAccuracy: true};
$cordovaGeolocation.getCurrentPosition(options).then(function(position){
var latLng = new google.maps.LatLng(position.coords.latitude, position.coords.longitude);
var mapOptions = {
center: latLng,
zoom: 15,
mapTypeId: google.maps.MapTypeId.ROADMAP
};
map = new google.maps.Map(document.getElementById("map"), mapOptions);
//Wait until the map is loaded
google.maps.event.addListenerOnce(map, 'idle', function(){
//Load the markers
loadMarkers();
});
}, function(error){
console.log("Could not get location");
//Load the markers
loadMarkers();
});
}
function loadMarkers(){
//Get all of the markers from our Markers factory
Markers.getMarkers().then(function(markers){
console.log("Markers: ", markers);
var records = markers.data.markers;
for (var i = 0; i < records.length; i++) {
var record = records[i];
var markerPos = new google.maps.LatLng(record.lat, record.lng);
// Add the markerto the map
var marker = new google.maps.Marker({
map: map,
animation: google.maps.Animation.DROP,
position: markerPos
});
var infoWindowContent = "<h4>" + record.name + "</h4>";
addInfoWindow(marker, infoWindowContent, record);
}
});
}
function addInfoWindow(marker, message, record) {
var infoWindow = new google.maps.InfoWindow({
content: message
});
google.maps.event.addListener(marker, 'click', function () {
infoWindow.open(map, marker);
});
}
return {
init: function(){
initMap();
}
}
})
.controller('MapCtrl', function($scope, $state, $cordovaGeolocation) {
});
类型约束来将T声明为引用类型：
class答案 2 :(得分：0)
我过去做过类似的事情，发现在方法上放置泛型和约束比在类型上更容易。这将允许相同的存储库类型为多个实体类型提供服务（只要它们满足约束条件）。
示例：
public class GetTable
{
public T GetEntities<T>(DbContext context, string agenttype)where T : class, IMstTuver
{
long maxVersion = context.Set<T>().Max(x => x.MaxVersion);
IMstTuver mstTuver = context.Set<T>().Where(x => x.MaxVersion == maxVersion && x.AgentType == agenttype).FirstOrDefault();
return mstTuver;
}
}
将来当您从现有库返回泛型作为泛型时，可以检查要调用的方法的约束并实现相同的约束。在这种情况下，Set<T>方法被限制为类。然后，您可以根据需要添加其他非冲突约束。
public virtual DbSet<TEntity> Set<TEntity>() where TEntity : class