'T' 不包含定义

Question

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?)

Answer 1

No, Generic types must be known at compile time. Think about it for a minute, how could compiler know that it is guaranteed that type T has property SpreadsheetLineNumbers ? What if T is of primitive type such as int or object ?

What prevents us from calling the method like this: GetGenericTableContent(ref _, 999) with T as int here ?

To fix it you could first add an interface that contains the 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 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 type T has implemented SpreadsheetLineNumbers
{
    outputTableContent.Append("<td>" + item.SpreadsheetLineNumbers + "</td>");
}

Answer 2

If you can't get make an interface for your type (or a common one between several types):

private void GetGenericTableContant<T>(ref StringBuilder outputTableContent, T item, Func<T, string> lineNumberAccessor)
{
     outputTableContent.Append("<td>" + lineNumberAccessor(item) + "</td>");
}

Usage:

GetGenericTableContent(ref outputTableContent, item, x => x.SpreadsheetLineNumbers);

(Or you could just pass the SpreadSheetLineNumbers property if you don't really need the item reference in your method: void GetGenericTableContant<T>(ref StringBuilder outputTableContent, string lineNumbers) )

Answer 3

Actually it is possible if you know for sure that the generic T has the exact property, using the where (generic type constraint) for a specified class with where T : MyClass .

---

For instance, if you have two entities Foo and Boo :

class Foo 
{
    public Guid Id {get; set;}
    public DateTime CreateDate {get; set;}
    public int FooProp {get; set;}
}

class Boo 
{
    public Guid Id {get; set;}
    public DateTime CreateDate {get; set;}
    public int BooProp {get; set;}
}

With a little refactor we can create a BaseClass that will hold the common properties:

class BaseModel
{
    public Guid Id {get; set;}
    public DateTime CreateDate {get; set;}        
}

And modify Foo and Boo to be:

class Boo : BaseModel
{
    public int BooProp {get; set;}
}

class Foo : BaseModel
{
    public int FooProp {get; set;}
}

And If you have a generic service with the constraint type of where T : BaseModel the compiler will allow you to get or set the BaseModel 's properties.

Lets say that you want that for every entity ( Foo or Boo ) added to DB you will want to set the CreateDate and Id properties from code (and not from server default value):

public interface IGenericService<T>
{
    void Insert(T obj);
}

public class GenericService<T> : IGenericService<T> where T : BaseModel
{
    public void Insert(T obj)
    {
        obj.Id = Guid.NewGuid();
        obj.CreateDate = DateTime.UtcNow;
        this._repository.Insert(obj);           
    } 
}