使用LINQ的.Cast()运算符时，显式/隐式强制转换运算符失败 [英] Explicit/implicit cast operator fails when using LINQ's .Cast() operator

问题描述

我正在尝试使用一个具有显式(但也会因隐式而失败)强制转换运算符的类，该运算符在使用LINQ的Cast<T>()函数时会失败.这是两个类的定义

I am trying to use a class that has a explicit (but also fails with implicit) cast operator that fails when using LINQ's Cast<T>() function. Here is the definitions of the two classes

public class DatabaseInfoElement : ConfigurationElement
{
    [ConfigurationProperty("AllowedServer", IsRequired = true)]
    public string AllowedServer { get { return (string)base["AllowedServer"]; } }

    [ConfigurationProperty("DatabaseName", IsRequired = true)]
    public string DatabaseName { get { return (string)base["DatabaseName"]; } }

    [ConfigurationProperty("SqlInstance", IsRequired = true)]
    public string SqlInstance { get { return (string)base["SqlInstance"]; } }

    public static explicit operator DatabaseInfo(DatabaseInfoElement element)
    {
        return new DatabaseInfo(element.AllowedServer, element.DatabaseName, element.SqlInstance);
    }

}

public class DatabaseInfo
{
    public DatabaseInfo(string allowedServer, string sqlInstance, string databaseName)
    {
        AllowedServer = allowedServer;
        SqlInstance = sqlInstance;
        DatabaseName = databaseName;
    }

    public string AllowedServer { get; set; }
    public string SqlInstance { get; set; }
    public string DatabaseName { get; set; }
}

这是我用来测试的代码.

Here is the code I am using to test it.

//Gets the ConfigurationSection that contains the collection "Databases"
var section = DatabaseInfoConfig.GetSection();

//This line works perfectly.
DatabaseInfo test = (DatabaseInfo)section.Databases[0];

//This line throws a execption
var test2 = new List<DatabaseInfo>(section.Databases.Cast<DatabaseInfo>());

这是我得到的例外情况

System.InvalidCastException was unhandled by user code
  HResult=-2147467262
  Message=Unable to cast object of type 'Server.Config.DatabaseInfoElement' to type 'Server.DatabaseInfo'.
  Source=System.Core
  StackTrace:
       at System.Linq.Enumerable.d__b1`1.MoveNext()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at Sandbox.Main() in E:\Code\Sandbox\Program.cs:line 82
  InnerException: 

为了让它按我想要的方式工作，我在演员表中做错什么了?

What am I doing wrong in my casting to get this to work the way I want?

推荐答案

定义显式/隐式强制转换运算符时，它们将在编译时绑定在调用位置.这就是第一行起作用的原因:编译器可以计算出所需的所有类型信息，因此可以将您的自定义显式强制转换运算符替换为默认的.

When you define explicit/implicit cast operators, they are bound at call-sites at compile-time. That's why the first line works: the compiler can work out all the type information needed, and so it can substitute your custom explicit cast operator for the default one.

但是，由于Cast<T>仅执行通用类型转换，因此编译器不了解您的运算符，因此将其忽略.结果:无效的强制转换异常.

However, since the Cast<T> just performs a generic cast, the compiler doesn't know about your operator, and thus it is ignored. Result: invalid cast exception.

您可以通过执行.Select(x => (DatabaseInfo)x)来解决此问题.另外，您可以添加一个名为ToDatabaseInfo()的方法，这样就不会隐藏实际发生的情况.

You can get around this by instead performing a .Select(x => (DatabaseInfo)x). Alternatively, you could add on a method called ToDatabaseInfo(), so that you're not hiding what's actually going on.

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