'T'不包含定义 [英] 'T' does not contain a definition

问题描述

是否可以执行以下操作(如果这样，我似乎无法使其正常工作..................................

如果推断出类型(因为它被忽略了)，那是什么问题?

private void GetGenericTableContent<T>(ref StringBuilder outputTableContent, T item)
{
    outputTableContent.Append("<td>" + item.SpreadsheetLineNumbers + "</td>");
}

// 'item' is either DuplicateSpreadsheetRowModel class or SpreadsheetRowModel class

使用上面的代码，我得到以下错误:

'T'不包含'SpreadsheetLineNumbers'的定义，也找不到找到接受类型为'T'的第一个参数的扩展方法'SpreadsheetLineNumbers'(您是否缺少using指令或程序集引用?)

解决方案

否，这是不可能的.泛型类型必须在编译时知道. 想一想，编译器如何才能知道保证T类型具有SpreadsheetLineNumbers属性?如果T是原始类型(例如int或object)怎么办?

是什么阻止了我们使用ref _, 999参数调用方法(此处T为int)?

仅当我们添加一个包含此属性的接口时，它才起作用:

public interface MyInterface 
{
    string SpreadsheetLineNumbers { get; set; }
}

让您的类从该接口继承

public class MyClass : MyInterface
{
    public string SpreadsheetLineNumbers { get; set; }
}

然后我们可以使用泛型类型约束让编译器知道T类型是从此接口派生的，因此它必须包含并实现其所有成员.

private void GetGenericTableContent<T>(ref StringBuilder outputTableContent, T item) 
    where T : IMyInterface // now compiler knows that T type has implemented all members of the interface
{
    outputTableContent.Append("<td>" + item.SpreadsheetLineNumbers + "</td>");
}

Is it possible to do the following (If so I can't seem to get it working.. forgoing constraints for the moment)...

If the type (because it's ommitted) is inferred, what's the problem?

private void GetGenericTableContent<T>(ref StringBuilder outputTableContent, T item)
{
    outputTableContent.Append("<td>" + item.SpreadsheetLineNumbers + "</td>");
}

// 'item' is either DuplicateSpreadsheetRowModel class or SpreadsheetRowModel class

With the above code I get the following error:

'T' does not contain a definition for 'SpreadsheetLineNumbers' and no extension method 'SpreadsheetLineNumbers' accepting a first argument of type 'T' could be found (are you missing a using directive or an assembly reference?)

解决方案

No, it's not possible. Generic types must be known at compile time. Think about it for a minute, how could compiler know that it is guaranteed that the type T has SpreadsheetLineNumbers property? What if T is primitive type such as int or object ?

What prevents us from calling method with ref _, 999 parameters (T is int here) ?

It'd only work if we add an interface that contains this property :

public interface MyInterface 
{
    string SpreadsheetLineNumbers { get; set; }
}

And let your class inherit from this interface

public class MyClass : MyInterface
{
    public string SpreadsheetLineNumbers { get; set; }
}

Then we could use generic type constraints to let compiler know that the type T derives from this interface and therefore it has to contain and implement all its members:

private void GetGenericTableContent<T>(ref StringBuilder outputTableContent, T item) 
    where T : IMyInterface // now compiler knows that T type has implemented all members of the interface
{
    outputTableContent.Append("<td>" + item.SpreadsheetLineNumbers + "</td>");
}

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