Asp.net mvc 如何在MVC6中编写多态模型绑定器和提供程序

在MVC3的上下文中，这个问题以前在SO和其他地方被问过，关于它有一些与ASP.NET核心RC1和RC2相关的细节，但是没有一个例子真正展示了如何在MVC 6中正确地完成它

有以下几类

 public abstract class BankAccountTransactionModel {

    public long Id { get; set; }
    public DateTime Date { get; set; }
    public decimal Amount { get; set; }
    public readonly string ModelType;
    public BankAccountTransactionModel(string modelType) {
        this.ModelType = modelType;
    }
 }

 public class BankAccountTransactionModel1 : BankAccountTransactionModel{

    public bool IsPending { get; set; }

    public BankAccountTransactionModel1():
            base(nameof(BankAccountTransactionModel1)) {}
 }    

 public class BankAccountTransactionModel2 : BankAccountTransactionModel{

    public bool IsPending { get; set; }

    public BankAccountTransactionModel2():
            base(nameof(BankAccountTransactionModel2)) {}
 }

在我的控制器里，我有这样的东西

[Route(".../api/[controller]")]  
public class BankAccountTransactionsController : ApiBaseController 
{

 [HttpPost]
 public IActionResult Post(BankAccountTransactionModel model) {

        try {
            if (model == null || !ModelState.IsValid) {
                // failed to bind the model
                return BadRequest(ModelState);
            }

            this.bankAccountTransactionRepository.SaveTransaction(model);

            return this.CreatedAtRoute(ROUTE_NAME_GET_ITEM, new { id = model.Id }, model);
        } catch (Exception e) {

            this.logger.LogError(LoggingEvents.POST_ITEM, e, string.Empty, null);
            return StatusCode(500);
        }
    }

}        

我的客户可能会发布BankAccountTransactionModel1或BankAccountTransactionModel2，我希望使用自定义模型绑定器根据抽象基类BankAccountTransactionModel上定义的属性ModelType中的值来确定要绑定的具体模型

因此，我做了以下工作

1） 编写了一个简单的模型绑定器提供程序，用于检查类型是否为BankAccountTransactionModel。如果是这种情况，则返回BankAccountTransactionModelBinder的实例

public class BankAccountTransactionModelBinderProvider : IModelBinderProvider {

    public IModelBinder GetBinder(ModelBinderProviderContext context) {

        if (context == null) throw new ArgumentNullException(nameof(context));

        if (context.Metadata.IsComplexType && !context.Metadata.IsCollectionType) {     

            var type1 = context.Metadata.ModelType;
            var type2 = typeof(BankAccountTransactionModel);

            // some other code here?

            // tried this but not sure what to do with it!
            foreach (var property in context.Metadata.Properties) {
                propertyBinders.Add(property, context.CreateBinder(property));
            }

            if (type1 == type2) {
                return new BankAccountTransactionModelBinder(propertyBinders);
            }
        }

        return null;
    }
}

2） 对BankAccountTransactionModel进行编码

 public class BankAccountTransactionModelBinder : IModelBinder {


 private readonly IDictionary<ModelMetadata, IModelBinder> _propertyBinders;

 public BankAccountTransactionModelBinder(IDictionary<ModelMetadata, IModelBinder> propertyBinders){
                this._propertyBinders = propertyBinders;
                  }
 public Task BindModelAsync(ModelBindingContext bindingContext) {

        if (bindingContext == null) throw new ArgumentNullException(nameof(bindingContext));            

        // I would like to be able to read the value of the property
        // ModelType like this or in some way...
        // This does not work and typeValue is...
        var typeValue = bindingContext.ValueProvider.GetValue("ModelType");

        // then once I know whether it is a Model1 or Model2 I would like to
        // instantiate one and get the values from the body of the Http  
        // request into the properties of the instance  

        var model = Activator.CreateInstance(type);

        // read the body of the request in some way and set the 
        // properties of model                                                                                                           

        var key = some key? 
        var result = ModelBindingResult.Success(key, model);

        // Job done  
        return Task.FromResult(result);
    }

 }

整个事情看起来还可以，因为提供者实际上被调用了，模型生成器也是如此。然而，在模型绑定器的bindmodelsync中编写逻辑似乎没有任何进展

正如代码中的注释所述，我想在我的模型绑定器中做的所有事情都是从http请求的主体中读取，特别是从JSON中读取ModelType的值。然后在此基础上，我想实例化BankAccountTransactionModel1或BankAccountTransactionModel，最后通过读取主体中JSON的值来为这个实例的属性赋值

我知道这仅仅是一个粗略的近似方法，但我非常感谢一些帮助，也许是关于如何做到或已经做到这一点的例子

我在ModelBinder中遇到了以下代码行的示例

var typeValue = bindingContext.ValueProvider.GetValue("ModelType");

应该读取值。然而，它在我的模型活页夹中不起作用，typeValue总是如下所示

typeValue
{}
Culture: {}
FirstValue: null
Length: 0
Values: {}
Results View: Expanding the Results View will enumerate the IEnumerable

我也注意到

bindingContext.ValueProvider
Count = 2
[0]: {Microsoft.AspNetCore.Mvc.ModelBinding.RouteValueProvider}
[1]: {Microsoft.AspNetCore.Mvc.ModelBinding.QueryStringValueProvider}

这可能意味着我没有机会从身体上读到任何东西

我是否需要在混合中使用“格式化程序”以获得所需的结果

类似的自定义模型绑定器的参考实现是否已经存在，这样我就可以简单地使用它，或者使用一些简单的mod

多谢各位

bindingContext.ValueProvider
Count = 2
[0]: {Microsoft.AspNetCore.Mvc.ModelBinding.RouteValueProvider}
[1]: {Microsoft.AspNetCore.Mvc.ModelBinding.QueryStringValueProvider}