将接口变量分配给实现它的 class 失败

Question

1. What is the problem?

I am working on a bigger project with a lot of code in between (.Net 4.5), but I will try to give you my current overview of the problem. I was trying to search for the similar question without success (sorry if it is a re-post).

Project A (class Library)

namespace nameA
{
    public interface ILog
    {
        bool GetFile(string a, string b, bool c);
        bool SetFile(string d);
    }
}

Project B (class Library) - Depends on project A

using nameA;

namespace nameB
{
    public class ImplementInterface : ILog
    {
        // implementation of the interface + other methods
    }
}

Project C (class library) depends on projectA and projectB

using nameA;
using nameB;

namespace nameC
{
    public class SomeClass : IDisposable
    {
        public ImplementInterface _implementation { get; }

        public SomeClass()
        {
            _implementation = new ImplementInterface()
        }

        // unrelated implementation code ...

        private method Start()
        {
            ILog tmp = _implementation; // -> error CS0266: Cannot implicitly convert type 'nameB.ImplementInterface' to 'nameA.ILog'
            // ILog tmp = (ILog)_implementation; -> this will compile but it fails during runtime (usage of 'as' returns null)
        }
    }
}

As you can see from the sample above I have a problem that code will not compile due to conversion from child class to interface.

If I do this example inside the same project everything will compile and code will work ok, also it will work ok if I implment SomeClass inside projectB and nameB namespace. What I do not understand why it is not working.

2. Following things I tried:

• Check is there any other implementation of ILog or ImplementInterface so that name resolution is the problem -> found none
• Check Assembly names and versions (so that I use correct assembly when running projectC) -> seems ok
• Check that all Assembly's are loaded -> yep they are loaded
• Use refleaction to check for interface methods inside ImplementInterface -> methods seems to be there and reflection can find the interface implementation inside the class.
• Implement SomeClass in project B and nameB -> seems that the problem is gone then, but it does not answer the question why is the problem there in the first place?

3. Assumptions

• Well I have read about covariance contravariance but I do not think this can play any role here
• I think it is some small thing about name resolution or dll's loading

I am not so familiar with this code base so it can also be something that I missed, but at this point I do not know what?

EDIT: At the time I did not understand the problem correctly. Here is small description what was the root cause of the above code not working.

Project B form above that contains a class ImplementInterface which implements interface ILog, also used ILMerge tool (a .NET static linker library) ILMerge put all of the code from ProjectA (together with the interface from project A) to the.dll from the Project B. Since project C also used Project A and Project B, ILog interface was implemented twice and compiler refused to compile that with the error (error CS0266: Cannot implicitly convert type). When I printed out all of the used assemblies non of them were showed twice since ILMerge merged the assemble of the project A to the project B.

Answer 1

It is working fine for me. Can you clean the code and change return type proper for private method Start(). Also implement IDisposable interface.