属性类型作为通用参数 [英] Property Type as Generic parameter
问题描述
我试图弄清楚如何才能使泛型调用为类型创建一个变量。在下面的调用中，它采用类型DAL.Account并且工作正常。
var tst = ctx.GetTable< DAL。帐户>（）。其中（t => t.Sbank ==000134）;
我想改变它以便我可以传递一个变量来代替DAL.Account 。像这样的东西，但我知道这是行不通的，因为你不能将属性作为Type来传递。
ctx.GetTable< Criteria.EntityType>（）。其中（LinqToSQLHelper.BuildWhereStatement（Criteria.StateBag），Criteria.StateBag.Values.ToArray（））
下面是我认为解释我想要做的事情的shell代码段。泛型不是我的强项，所以我正在寻求一些帮助。无论如何，我可以做到这一点？
//存储一个Type，表示Object是一个Criteria标准。
public class AccountCriteria：IGeneratedCriteria
{
...
public Type EntityType
{
get {return typeof（DAL.Account） ;}
}
}
//我在DataContext中添加了一个名为GetTable的函数
//然后在Console中用它作为示例应用程序来测试其功能。
public class ADRPDataContext：NHibernateDataContext
{
...
public CodeSmith.Data.NHibernate.ITable< T> GetTable< T>（）其中T：EntityBase
{
var tb = new CodeSmith.Data.NHibernate.Table< T>（this）;
返回tb;
//使用DataContext.GetTable的控制台应用程序
class程序
{
static void Main（string [] args ）
{
using（var ctx = new ADRPDataContext（））
{
var tst = ctx.GetTable< DAL.Account>（）其中（t => t .Sbank ==000134）;
}
}
}
//使用Critera的EntityType属性生成数据的ExistsCommand类。
public class ExistsCommand
{
private IGeneratedCriteria Criteria {get;组; }
$ b保护覆盖无效DataPortal_Execute（）
{
使用（var ctx = new DC.ADRPDataContext（））
{
//这是我的第一次尝试但不起作用，因为您无法为类型传递属性。
//但是我可以弄清楚如何编写它以便它能够工作。
Result = ctx.GetTable< Criteria.EntityType>（）。其中（LinqToSQLHelper.BuildWhereStatement（Criteria.StateBag），Criteria.StateBag.Values.ToArray（））。Count（）> 0;
您正在寻找实例化一个泛型类型。一些信息可以在这里找到
这是一个简单的例子，演示如何实例化一个容量为3的List。下面是一个方法，您可以在不知道类型时调用它来创建泛型：
public static Object CreateGenericListOfType（Type typeGenericWillBe）
{
//替代以下内容：
// List<串GT; myList = new List< String>（3）;
//为泛型的构造函数构建参数（显然，如果每种潜在类型都有不同的构造函数，此代码将不起作用）
object [] constructorArgs = new Object [ 1];
constructorArgs [0] = 3;
//实例化泛型。与调用上面的一行示例（注释掉）相同。结果在列表< String>带有3个列表项
类型genericListType = typeof（List<>）;
类型[] typeArgs = {typeGenericWillBe};
类型myNewGeneric = genericListType.MakeGenericType（typeArgs）;
对象GenericOfType = Activator.CreateInstance（myNewGeneric，constructorArgs）;
返回GenericOfType;
$ / code> 下面是一些示例代码，它们将向您展示示例方法的作用：
列表< String> Strings =（List< String>）InstantiateGenericTypeWithReflection.CreateGenericListOfType（typeof（String））;
//演示该对象实际上是一个List< String>我们可以像使用linq扩展一样做东西（对linq没有太大的用处，但是可以作为例子）
Strings.Add（frist）;
Strings.Add（2nd）;
Strings.Add（tird）;
Console.WriteLine（item index 2 value：+ Strings.Where（strings => strings ==2）。First（））;
在你的例子中，替换你的 I'm trying to figure out how I can make a Generics call take a variable for the Type. In the call below it take a type "DAL.Account" and works fine. I want to change that so that I can pass a variable in place of the "DAL.Account". Something like this but I know that won't work as you can't pass property as a Type. Below is the shell pieces of code I think explains what I'm trying to do. Generics is not my strong suit so I'm looking for some help. Is there anyway that I can make this happen?
You are looking to instantiate a generic type. Some info can be found here This is a simple example demonstrating how to instantiate a List with a capacity of 3. Here is a method that you can call to create a generic when you don't know the type: And here is some sample code that will show you the example method works: In your example, replace your 这篇关于属性类型作为通用参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！ GetTable< Criteria.EntityType> / code>与 CreateGenericTableOfType（Criteria.EntityType）。这将返回一个通用的任何类型的通用表。您当然需要正确实现该方法（处理构造函数参数，将List更改为Table等）。var tst = ctx.GetTable<DAL.Account>().Where(t => t.Sbank == "000134");
ctx.GetTable<Criteria.EntityType>().Where(LinqToSQLHelper.BuildWhereStatement(Criteria.StateBag), Criteria.StateBag.Values.ToArray())
//Stores a "Type" that indicates what Object is a Criteria for.
public class AccountCriteria : IGeneratedCriteria
{
...
public Type EntityType
{
get {return typeof(DAL.Account);}
}
}
//I have added a function to the DataContext called "GetTable"
// And then used it as an example in a Console App to test its functionality.
public class ADRPDataContext : NHibernateDataContext
{
...
public CodeSmith.Data.NHibernate.ITable<T> GetTable<T>() where T : EntityBase
{
var tb = new CodeSmith.Data.NHibernate.Table<T>(this);
return tb;
}
}
// console application that uses DataContext.GetTable
class Program
{
static void Main(string[] args)
{
using (var ctx = new ADRPDataContext())
{
var tst = ctx.GetTable<DAL.Account>().Where(t => t.Sbank == "000134");
}
}
}
//ExistsCommand class that uses the EntityType property of the Critera to generate the data.
public class ExistsCommand
{
private IGeneratedCriteria Criteria { get; set; }
protected override void DataPortal_Execute()
{
using (var ctx = new DC.ADRPDataContext())
{
//This was my first attempt but doesn't work becuase you can't pass a property in for a Type.
//But I can figure out how to write this so that it will work.
Result = ctx.GetTable<Criteria.EntityType>().Where(LinqToSQLHelper.BuildWhereStatement(Criteria.StateBag), Criteria.StateBag.Values.ToArray()).Count() > 0;
}
}
}
public static Object CreateGenericListOfType(Type typeGenericWillBe)
{
//alternative to the followin:
//List<String> myList = new List<String>(3);
//build parameters for the generic's constructor (obviously this code wouldn't work if you had different constructors for each potential type)
object[] constructorArgs = new Object[1];
constructorArgs[0] = 3;
//instantiate the generic. Same as calling the one line example (commented out) above. Results in a List<String> with 3 list items
Type genericListType = typeof(List<>);
Type[] typeArgs = { typeGenericWillBe };
Type myNewGeneric = genericListType.MakeGenericType(typeArgs);
object GenericOfType = Activator.CreateInstance(myNewGeneric, constructorArgs);
return GenericOfType;
}
List<String> Strings = (List<String>)InstantiateGenericTypeWithReflection.CreateGenericListOfType(typeof(String));
//demonstrate the object is actually a List<String> and we can do stuff like use linq extensions (isn't a good use of linq but serves as example)
Strings.Add("frist");
Strings.Add("2nd");
Strings.Add("tird");
Console.WriteLine("item index 2 value: " + Strings.Where(strings => strings == "2").First());
GetTable<Criteria.EntityType>() with CreateGenericTableOfType(Criteria.EntityType). This will return a generic table of whatever type you pass in. You will of course need to implement the method properly (handle constructor args, change List to Table etc).
