VC++ 2010 exp. error C3225: generic type argument for 'T' cannot be '...

Question

My code of structures is:

	struct Item
    {
		//map reader
        int id;
        int count;
    };
	struct Container {
	    int bp_id;
	    System::Collections::Generic::List<Item> items = gcnew System::Collections::Generic::List<Item>;
    };

and error:

error C3225: generic type argument for 'T' cannot be 'Project::Item', it must be a value type or a handle to a reference type

What's wrong?

Answer 1

You need to define this as a handle to an Item:

System::Collections::Generic::List<Item^>^ items = gcnew System::Collections::Generic::List<Item^>();

When working with ref class types, you always work with a managed handle (^).

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Reed Copsey, Jr. - http://reedcopsey.com
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Answer 2

Note that you are mixing C++/CLI code with C++ code. In particular, you are adding a managed member variable to an unmanaged struct.

Most newcomers to Visual C++ may not appreciate that C++/CLI is a language separate and distinct from C++ that is used to support .NET concepts. What is particularly confusing is that both C++ and C++/CLI code can be intermixed in the same source file. Just because this is possible, doesn't mean it is desirable. If you are just learning C++, for example, I strongly urge you to stick with just C++ code (also called unmanaged code) until you gain further experience and confidence with the language. Once you are comfortable with C++ you can then take on C++/CLI later. As a general rule, C++/CLI is typically used to bridge .NET client applications with unmanaged DLL's.

In the case of your example, instead of using System::Collections::Generic::List<>, use std::vector<> instead.

Answer 3

error C3699: '^' : cannot use this indirection on type 'Project::Item'
error C3225: generic type argument for 'T' cannot be 'Project::Item *', it must be a value type or a handle to a reference type

Answer 4

List<T> is a C++/CLI type, which means you can only store "ref struct" or "ref class" types (C++/CLI types) within it.  As Brian said, if you're trying to use all native code, you should avoid C++/CLI and don't compile with /clr.  In that case, you can use std::vector to store these items.

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Reed Copsey, Jr. - http://reedcopsey.com
If a post answers your question, please click "Mark As Answer" on that post and "Mark as Helpful".

Answer 5

Hello Dubler,

Visual C++ Express Edition Forum is for question about getting started with VC++ Express.

I moved your thread to Visual C++ Language Forum for better support.

Thanks for your understanding.

Regards,

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Damon Zheng [MSFT]
MSDN Community Support | Feedback to us

Answer 6

correct answer