具有可选通用参数的通用方法 [英] Generic Method with optional generic parameter

问题描述

如何使用可选的泛型类型创建泛型方法？这就是我现在所拥有的，它可以工作

  public GridViewColumn< T> Column< D>（String HeaderText，D decorator）其中D：IColumnDecorator，new（）
 {
 GridViewColumn< T> column = new GridViewColumn< T>（）; 
 column.HeaderText = HeaderText; 
 column.Decorator = new D（）; 
返回列; 
 
 
 
 
 
 $ b 
正如你所看到的，我需要实例化类型D（实现IColumnDecorator）在Column（）方法中。  
 
 
问题是，类型D是可选的。如果为空，我想明确地使用我拥有的默认ColumnDecorator。类似于 
 
 
  public GridViewColumn< T> Column< D>（String HeaderText，D decorator）其中D：IColumnDecorator，new（）
 {
 GridViewColumn< T> column = new GridViewColumn< T>（）; 
 column.HeaderText = HeaderText; 
 if（decorator == null）
 {
 column.Decorator = new DefaultColumnDecorator（）; 
} 
 else {
 column.Decorator = new D（）; 
} 
返回列; 
} 
  
请帮忙。谢谢！
 
 
 
 。 
 
 
 
如果我有一个自定义的IColumnDecorator实现，我想如何在剃刀MVC中使用它 
 
 
  @Model。[IEnumerable] .Grid（grid => {
 .. 
 ... 
 grid.columns（
 grid.Column< MyOwnColumnDecorator>（FirstColumn）
）
}）; 
  
如果我没有任何要使用的默认值，那么我希望能够做到类似于
  @Model。[IEnumerable] .Grid（grid => {
 .. 
 ... 
 grid.columns（
 grid.Column（FirstColumn，null）或者只是grid.Column（FirstColumn）; 
）
}）; 
  
 
 
解决方案
在您当前的代码中，您不需要 decorator 参数，因为您创建了 D 的新实例并使用它。
  public GridViewColumn< T> Column< D>（String HeaderText）其中D：IColumnDecorator，new（）
 {
GridViewColumn< T> column = new GridViewColumn< T>（）; 
 column.HeaderText = HeaderText; 
 column.Decorator = new D（）; 
返回列; 
 
 
 
 $ b $ p 
 $ b 
如果你有一个默认的参数类型可供使用，你不需要使用泛型： 
 
 
  public GridViewColumn< DefaultColumnDecorator> Column（String headerText）
 {
 return Column< DefaultColumnDecorator>（headerText）; 
} 
  
另外，您可以保留参数并删除 new （）约束：
  public GridViewColumn< T> Column< D>（String HeaderText，D装饰器）其中D：IColumnDecorator 
 {
GridViewColumn< T> column = new GridViewColumn< T>（）; 
 column.HeaderText = HeaderText; 
 column.Decorator =装饰者; 
返回列; 
} 
 
 public GridViewColumn< DefaultColumnDecorator> Column（String headerText）
 {
 return Column（headerText，new DefaultColumnDecorator（））; 
} 
  
 
How do I create a generic method with an optional generic type? This is what I have right now, which works
 public GridViewColumn<T> Column<D>(String HeaderText, D decorator) where D: IColumnDecorator, new()
        {
            GridViewColumn<T> column = new GridViewColumn<T>();
            column.HeaderText = HeaderText;
            column.Decorator = new D();
            return column;
        }
As you can see, I need to instantiate the type D (implements IColumnDecorator) inside the Column() method. 

The issue is, the type "D" is optional. If Null, I want to explicitly use a default ColumnDecorator that I have. something like 
 public GridViewColumn<T> Column<D>(String HeaderText, D decorator) where D: IColumnDecorator, new()
        {
            GridViewColumn<T> column = new GridViewColumn<T>();
            column.HeaderText = HeaderText;
            if(decorator ==null)
            {
               column.Decorator = new DefaultColumnDecorator();
            }
            else{
               column.Decorator = new D();
            }
            return column;
        }
Please help. Thanks!

[Edit]. 

Here is how I want to use it in razor MVC if I have a custom IColumnDecorator implementation
@Model.[IEnumerable].Grid(grid=>{
      ..
      ...
      grid.columns(
         grid.Column<MyOwnColumnDecorator>("FirstColumn")
      )
});
If I don't have any and want to use default, then I want to be able to do something like
@Model.[IEnumerable].Grid(grid=>{
          ..
          ...
          grid.columns(
             grid.Column("FirstColumn",null) or simply grid.Column("FirstColumn"); 
          )
    });

解决方案
In your current code, you don't need the decorator parameter since you create a new instance of D and use that instead.
public GridViewColumn<T> Column<D>(String HeaderText) where D: IColumnDecorator, new()
{
    GridViewColumn<T> column = new GridViewColumn<T>();
    column.HeaderText = HeaderText;
    column.Decorator = new D();
    return column;
}
If you have a default parameter type to use, you don't need to use generics:
public GridViewColumn<DefaultColumnDecorator> Column(String headerText)
{
    return Column<DefaultColumnDecorator>(headerText);
}
Alternatively you could keep the parameter and remove the new() constraint:
public GridViewColumn<T> Column<D>(String HeaderText, D decorator) where D : IColumnDecorator
{
    GridViewColumn<T> column = new GridViewColumn<T>();
    column.HeaderText = HeaderText;
    column.Decorator = decorator;
    return column;
}

public GridViewColumn<DefaultColumnDecorator> Column(String headerText)
{
    return Column(headerText, new DefaultColumnDecorator());
}


                        
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