类型参数声明必须是标识符而不是类型 [英] Type parameter declaration must be identifier not a type

问题描述

  public abstract class BaseDocumentLayer：IDocumentLayer 
 {
 public IDocumentLayer NextLayer {get; set;} 
 
 public virtual Document< T， U> ProcessDocument< T，U>（Document< T，U> document）{
 if（NextLayer！= null）
 document = NextLayer.ProcessDocument（document）; 
退货凭证; 
} 
} 
 
 public override Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType> ProcessDocument< GenericDatabaseDTO，GenericDatabaseConstants.ActionType>（Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType> document）
 {
 return base.ProcessDocument< GenericDatabaseDTO，
} 
 
 public static类GenericDatabaseConstants 
 {
 public enum ActionType 
 {
 CreateGeneric，
 ReadGeneric，
 UpdateGeneric，
 DeleteGeneric，
} 
 
 
 
 $ b $ p 
 $ b为什么会出现这样的错误：类型参数声明必须是一个不是类型的标识符？ 
 
 
 
  
 
 
 
 Error1： 
 
 
  public override Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType> ProcessDocument（Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType> document）
 {
 
 return base.ProcessDocument（document）; 
 
  
错误1'GenericDocumentLayer2.Document.GenericDatabase.ProcessLayer.ProcessDocument（GenericDocumentLayer2.TypeDefinition .Document）'：找不到合适的方法覆盖
 
 
 
我没有改变我的原点BaseLayer.ProcessDocument方法
 
 
 
  UPDATE  
  public void Add（GenericDatabaseField field）{
 var dto = new GenericDatabaseDTO （）; 
 dto.Field = field; 
 dto.Id =test; 
 
 var document = new Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType>（dto，GenericDatabaseConstants.ActionType.CreateGeneric，null）; 
 IDocumentLayer layer = DocumentFactory.CreateDocumentLayers（）; 
 //通用ProcessDocument永远不会被称为基本版本
 document = layer.ProcessDocument< GenericDatabaseDTO，GenericDatabaseConstants.ActionType>（document）; 
 
  
 
 
解决方案
一种特定类型的通用方法。对于C ++的模板，这是可能的，但对于C＃的泛型是不可能的。
 
 
 
为了让你的代码能够编译，把它改为：
  public override Document< TDTO，TActionType> ProcessDocument< TDTO，TActionType>（文档< TDTO，TActionType>文档）
  
将覆盖所有泛型参数的基类型方法。
 
 
 
如果您想为一种类型提供特殊实现，则可以添加非泛型重载和内部调用在特殊处理后的通用版本： 
 
 
  public Document< GenericDatabaseDTO，GenericDatabaseConstants.ActionType> ProcessDocument（文档< GenericDatabaseDTO，GenericDatabaseConstants.ActionType>文档）
 {
 //在这里对这个特殊情况进行一些特殊处理... 
 
 return base.ProcessDocument（document）; 
 
 $ / code> 
然而，请注意，这将会破坏像这样的多态： / p> 
 
 
假设上述方法在 Derived 中实现。 派生源自 Base  
  GenericDatabaseDTO dto = /*...*/; 
 GenericDatabaseConstants.ActionType actionType = /*...*/; 
 
派生的d = new Derived（）; 
基数b = d; 
 
 //通过Derived特殊处理调用非通用版本：
 d.ProcessDocument（dto，actionType）; 
 //从Base调用通用版本：
 b.ProcessDocument（dto，actionType）; 
  
 
public abstract class BaseDocumentLayer : IDocumentLayer
{                                                       
   public IDocumentLayer NextLayer { get;set;}

   public virtual Document<T, U> ProcessDocument<T, U>(Document<T, U> document) {
      if(NextLayer != null)
         document = NextLayer.ProcessDocument(document);
      return document;          
   }
}

public override Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)
{       
    return base.ProcessDocument<GenericDatabaseDTO, 
}

public static class GenericDatabaseConstants 
{    
    public enum ActionType 
    {                
       CreateGeneric,
       ReadGeneric,
       UpdateGeneric,
       DeleteGeneric,               
    }
}
Why do I get the error: Type paramter declaration must be an identifier not a type?




Error1:
public override Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)
{   

    return base.ProcessDocument(document);
}
Error   1   'GenericDocumentLayer2.Document.GenericDatabase.ProcessLayer.ProcessDocument(GenericDocumentLayer2.TypeDefinition.Document)': no suitable method found to override

I have not changed my origin BaseLayer.ProcessDocument method

UPDATE
public void Add(GenericDatabaseField field) {
            var dto = new GenericDatabaseDTO();
            dto.Field = field;
            dto.Id = "test";

            var document = new Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(dto, GenericDatabaseConstants.ActionType.CreateGeneric, null);
            IDocumentLayer layer = DocumentFactory.CreateDocumentLayers();
                                          // The generic ProcessDocument is never called just the base version
        document = layer.ProcessDocument<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(document);                
    }

解决方案
You can't override a generic method for a specific type. This is possible with C++'s templates but not with C#'s generics.

To make your code compile, change it to this:
public override Document<TDTO, TActionType> ProcessDocument<TDTO, TActionType>(Document<TDTO, TActionType> document) 
This will compile, but it will override the base type method for all generic arguments.

If you want to provide a special implementation just for one type, you can add a non-generic overload and internally call the generic version after the special handling:
public Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)  
{
    // some special handling for this special case here...

    return base.ProcessDocument(document);
}
However, please note that this will break in polymorphic scenarios like this one:

Assume the methods above are implemented in Derived. Derived is derived from Base
GenericDatabaseDTO dto = /*...*/;
GenericDatabaseConstants.ActionType actionType = /*...*/;

Derived d = new Derived();
Base b = d;

// calls the non-generic version with special handling from Derived:
d.ProcessDocument(dto, actionType);
// calls the generic version from Base:
b.ProcessDocument(dto, actionType);


                        
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