Implying method return type

eggie5

I have a class Categroy that derives from it's base RESTResource.

public class Category : RESTResource
{
}

public class RESTResource
{
public < derived type > Get()
{
}
}

The above example should evaluate to ... public Category Get() ... at runtime. Depending on the Type of the derviced class from RESTResource, how Can I set the return type of RESTResource.Get()?

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Christian Graus

I'm not sure if you can write generic properties. If not, all you can do is return the base type, and users would have to cast it. I don't think you can override a property and change it's return type.

Christian Graus - Microsoft MVP - C++ "I am working on a project that will convert a FORTRAN code to corresponding C++ code.I am not aware of FORTRAN syntax" ( spotted in the C++/CLI forum )

eggie5

I guess ruby got my hopes up...

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TJoe

It really shouldn't matter that you can't change the return type. Assuming that you could, you would still need to declare a variable to hold it using the dervied type. This implies that you know the type like:

RESTResource r = /* set from somewhere else, but is actually a Category */;

//...

// Assume Get()'s return type is Category
Category c = r.Get();

Since you need to know the return type anyway, you can simply set the return type of Get to RESTResource then cast it like so:

RESTResource r = /* set from somewhere else, but is actually a Category */;

//...

// Assume Get()'s return type is RESTResource
Category c = r.Get() as Category;
if (null != c)
// c is a Category

Now, you may be referring to virtual functions, which work like this:

public class RESTResource {
public virtual RESTResource Get() {
return new RESTResource();
}
}

public class Category : RESTResource {
public override RESTResource Get() {
return new Category();
}
}

// ... some other method ...
RESTResource r = new RESTResource();
RESTResource r2 = r.Get(); // returns a RESTResource

Category c = new Category();
Category c2 = c.Get() as Category; // returns a RESTResource, which is actually an instance of Category so we need to cast it.

Take care, Tom ----------------------------------------------- Check out my blog at http://tjoe.wordpress.com

eggie5

Thanks. I think I'm going to head in a different direction with this... Now, is this the right use of generics? Category has a static string endpoint_url. I would like to get that from T1 in the Create method.

Create<Category>();
.
.
.

public Response Create<T1>()
{
    string url=//How can I get Category.endpoint\_url here?
}

public class Category
{
    public static string endpoint\_url = "/categories";
    public int id;
    public string name;
    public int content\_provider\_id;

}

-- modified at 1:50 Tuesday 2nd October, 2007

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TJoe

Your code isn't complete, so I can't really tell what you are doing (or want to do). Can you provide a little more info or more complete example?

Take care, Tom ----------------------------------------------- Check out my blog at http://tjoe.wordpress.com

TJoe

Did you perhaps for get to escape a < in your code?

Take care, Tom ----------------------------------------------- Check out my blog at http://tjoe.wordpress.com

eggie5

I have this class structure:

public class Test
{
  public Response Create<T1>()
  {
    string url=//How can I get Category.endpoint\_url here?
  }
}

public class Category
{
    public static string endpoint\_url = "/categories";
    public int id;
    public string name;
    public int content\_provider\_id;

}

My Category class has a static string endpoint_url. I want to get that string from inside of create using the Type T1 and save it to url. My expected results are to have url set to "/categories"

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eggie5

just fixed it.

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TJoe

You can either create an abstract base class or an interface (from which Category and others would derive). If Category and any possible other classes have common functionality, then you should go with an abstract base class, because you can put that common functionality in there. If you go with an interface, then your classes (e.g. Category) really won't share code. You won't be able to get the static endpoint_url without creating an instance of Category. Both methods above require an instance of Category. Assuming you are willing to create an instance of Category, the code would be like this:

public Response Create<T>() where T : MyBaseClass {
T myClass = new T();
String url = myClass.GetURL();
// ....
}

// ...

public abstract class MyBaseClass {
public abstract String GetURL();

// Other method implementations can go here

}

public class Category : MyBaseClass {
public static string endpoint_url = "/categories";

public override String GetURL() {
    return Category.endpoint\_url;
}

}

// Then you can call Create like this, where t is of type Test
Response r = t.Create();

Take care, Tom ----------------------------------------------- Check out my blog at http://tjoe.wordpress.com

eggie5

TJoe wrote:

If Category and any possible other classes have common functionality, then you should go with an abstract base class

Yeah, I have a few other classes... I was just using Category as an example. I'll try out what you've worked out first thing tomorrow. Thanks, Alex

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eggie5

I get a compile error on this line: T myClass = new T(); Error 1 Cannot create an instance of the variable type 'T' because it does not have the new() constraint

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Pete OHanlon

This means that the constraint for the generic is missing the new() keyword. For example:

public T Create() where T : ISomething, new()
{
  T item = new T();
  return item;
}

The new constraint can only be used with a zero-arg constructor.

Deja View - the feeling that you've seen this post before.

DavidNohejl

"Because not all objects are guaranteed to have public default constructors, the compiler does not allow you to call the default constructor on the type parameter. To override this compiler restriction, you add the text new() after all other constraints are specified. This text is a constructor constraint, and it forces the type parameter decorated with the constructor constraint to have a default constructor." (http://www.codeproject.com/books/EssentialCS20.asp[^]) Fix is easy, public Response Create() where T : MyBaseClass, New()

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Pete OHanlon

This will do it.

public class Category : RESTResource<Category>
{
}

public class RESTResource<T> where T : RESTResource, new()
{
  public <T> Get()
  {
    T item = new T();
    // do some work;
    return item;
  }
}

Deja View - the feeling that you've seen this post before.

TJoe

Sorry, forgot about the new() keyword. Pete's response describes how to use it.

Take care, Tom ----------------------------------------------- Check out my blog at http://tjoe.wordpress.com

eggie5

Thanks

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Pete OHanlon

No problem. I hope it helped.

Deja View - the feeling that you've seen this post before.