显式转换在默认模型绑定中不起作用 [英] Explicit casting doesn't work in default model binding

问题描述

我使用的是ASP.NET MVC2和Entity Framework。我将简化情况一点;希望它会使它更清晰，不会更混乱！

I am using ASP.NET MVC2 and Entity Framework. I am going to simplify the situation a little; hopefully it will make it clearer, not more confusing!

我有一个控制器动作来创建地址，国家是一个查找表（换句话说，国家和地址类之间的一对多关系）。让我们说清楚地址类中的字段称为Address.Land。并且，为了下拉列表的目的，我得到Country.CountryID和Country.Name。

I have a controller action to create address, and the country is a lookup table (in other words, there is a one-to-many relationship between Country and Address classes). Let's say for clarity that the field in the Address class is called Address.Land. And, for the purposes of the dropdown list, I am getting Country.CountryID and Country.Name.

我知道模型与输入验证。所以，如果我调用下拉字段 formLand - 我可以使它工作。但是如果我调用字段 Land （即匹配Address类中的变量） - 我得到以下错误：

I am aware of Model vs. Input validation. So, if I call the dropdown field formLand - I can make it work. But if I call the field Land (that is, matching the variable in Address class) - I am getting the following error:

从类型
'System.String'到类型'App.Country'
的参数转换失败，因为没有类型转换器可以
在这些类型之间转换。

"The parameter conversion from type 'System.String' to type 'App.Country' failed because no type converter can convert between these types."

好的，这很有意义。字符串（CountryID）来自表单，并且binder不知道如何将它转换为Country类型。所以，我写了转换器：

OK, this makes sense. A string (CountryID) comes from the form and the binder doesn't know how to convert it to Country type. So, I wrote the converter:

namespace App {
    public partial class Country {
        public static explicit operator Country(string countryID) {
            AppEntities context = new AppEntities();
            Country country = (Country) context.GetObjectByKey(
                new EntityKey("AppEntities.Countries", "CountryID", countryID));
            return country;
        }
    }
}

和隐式。我从控制器测试它 - 国家c =（国家）fr - 它工作正常。然而，它从来没有被调用时发布视图。我在模型中得到相同的无类型转换器错误。

FWIW, I tried both explicit and implicit. I tested it from the controller - Country c = (Country)"fr" - and it works fine. However, it never got invoked when the View is posted. I am getting the same "no type converter" error in the model.

任何想法如何提示模型绑定器，是类型转换器？
感谢

Any ideas how to hint to the model binder that there is a type converter? Thanks

推荐答案

类型转换器与显式或隐式转换不同，

A type converter is not the same as an explicit or implicit conversion, it's an object that converts values between various types.

我认为你需要创建一个继承自 TypeConverter 的类，可以在 Country 和其他类型，并将 TypeConverterAttribute 应用到您的类以指定要使用的转换器：

I think you need to create a class inherited from TypeConverter that converts between Country and other types, and apply the TypeConverterAttribute to your class to specify the converter to use :

using System.ComponentModel;

public class CountryConverter : TypeConverter
{
    // override CanConvertTo, CanConvertFrom, ConvertTo and ConvertFrom
    // (not sure about other methods...)
}

[TypeConverter(typeof(CountryConverter))]
public partial class Country
{

...

}

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