Declaring Explicit interfaces and the does not contain a definition error C#

Question

I am having an issue with an explicit interface that I created and am getting the exception,

'x' does not contain a definition for 'y' and no extension method 'y' accepting a first argument of type 'x' could be found

I have a series of classes. The base class:

public interface IFactoryResponse
{
    object instance { get; set; }
    string instanceconfig { get; set; }
}

The class that explicitly implements it:

public class FactoryResponseImpl : IFactoryResponse
{
    object IFactoryResponse.instance {
        get { return ((IFactoryResponse)this).instance; }
        set { ((IFactoryResponse)this).instance = value; }
    }

    string IFactoryResponse.instanceconfig   {
        get { return ((IFactoryResponse)this).instanceconfig; }
        set { ((IFactoryResponse)this).instanceconfig = value; }
    }
}

and in another class I get the above error. Visual studio can find the interface and class ok, but it can't resolve the instance property. What am I missing here. I am probably missing one of the more refined rules of explicit inheritance.

if (facconfig.useabstract) {
    response.instance = Activator.CreateInstance(m_entassembly.GetType(entconfig.Classname, true, true));
    response.instanceconfig = facconfig.config;
} else {
    Assembly assem = Assembly.LoadFrom(facconfig.assemblyfile);
    object Obj = Activator.CreateInstance(assem.GetType(facconfig.Classname, true, true));
    response.instance = Obj;
    response.instanceconfig = facconfig.config;
}

you need to cast the object as that interface then you can see the properties — Jake Rote, Jan 26 '14 at 18:13
Isn't this a stack overflow exception waiting to happen? `return ((IFactoryResponse)this).instance;` — Lasse V. Karlsen, Jan 26 '14 at 18:14
Correct me if I'm wrong, but looks like your property call itself. — MarcinJuraszek, Jan 26 '14 at 18:14
I haven't really dealt with explicit interface implementation before. If there is a better way to do it then by all means let me know. — SoftwareSavant, Jan 26 '14 at 18:15
How can I referrence the instance variable in the base class? — SoftwareSavant, Jan 26 '14 at 18:16
The class is an instance of the IFactoryResponse method. As such it should have its instance and instanceconfig members set. How can I do this? — SoftwareSavant, Jan 26 '14 at 18:19

score 2 · Accepted Answer · answered Jan 26 '14 at 18:19

Your implementation is incorrect. It will cause StackOverflowException because property calls itself. You can easily implement the properties using autoproperties:

public class FactoryResponseImpl : IFactoryResponse
{
    object IFactoryResponse.instance { get; set; }

    string IFactoryResponse.instanceconfig { get; set; }
}

When interface member is implemented explicitly you have to look at variable as the interface, either by casting your class instance to that interface or assigning it into a variable types as that interface.

if (facconfig.useabstract) {
    ((IFactoryResponse)response).instance = Activator.CreateInstance(m_entassembly.GetType(entconfig.Classname, true, true));
    ((IFactoryResponse)response).instanceconfig = facconfig.config;
} else {
    Assembly assem = Assembly.LoadFrom(facconfig.assemblyfile);
    object Obj = Activator.CreateInstance(assem.GetType(facconfig.Classname, true, true));
    ((IFactoryResponse)response).instance = Obj;
    ((IFactoryResponse)response).instanceconfig = facconfig.config;
}

Why do you need the interface to be implemented explicitly? You shouldn't do that unless you have very good reason. With implicit implementation everything is much easier:
```
public class FactoryResponseImpl : IFactoryResponse
{
    public object instance { get; set; }

    public string instanceconfig { get; set; }
}
```
And your other code should work just fine.

This is a very good answer. Every time I think I know all I need to know I run into a problem like this. I need to go over this a bit more but I think your answer is the best one. I am just going to eliminate the explicit interfaces implementations. — SoftwareSavant, Jan 26 '14 at 18:24

score 0 · Answer 2 · answered Jan 26 '14 at 18:19

Your explicit implementations are referencing themselves. You should be referencing a private field or the public implementation. E.g.:

public class FactoryResponseImpl : IFactoryResponse
{
    DatabaseFactoryResponseInstance _instance;

    public FactoryResponseImpl()
    {
        _instance = new DatabaseFactoryResponseInstance();
    }

    object IFactoryResponse.instance {
    get { return (object)_instance; }
    set { 
            if (value != null)
            {
                DatabaseFactoryResponseInstance dbInstance;
                dbInstance = value as DatabaseFactoryResponseInstance;
                if (dbInstance == null)
                    throw new InvalidOperationException();

                _instance = dbInstance;
            }
         }
}

Where did the DatabaseFactoryResponseInstance come from? – SoftwareSavant Jan 26 '14 at 18:20 — SoftwareSavant, Jan 26 '14 at 18:20
It is one possible implementation of the `instance`. – Keith Payne Jan 26 '14 at 18:22 — Keith Payne, Jan 26 '14 at 18:22
duh, Silly me. I thought I missed something. – SoftwareSavant Jan 26 '14 at 18:30 — SoftwareSavant, Jan 26 '14 at 18:30

score 0 · Answer 3 · answered Jan 26 '14 at 18:19

Is this what you where aiming for?

public interface IFactoryResponse
        {
            object instance { get; set; }
            string instanceconfig { get; set; }
        }

        public class FactoryResponseImpl : IFactoryResponse
        {
            object IFactoryResponse.instance { get; set; }
            string IFactoryResponse.instanceconfig { get; set; }
        }

        class Test
        {
            public void TestMethod()
            {

                IFactoryResponse response = new FactoryResponseImpl();
                response.instance = null;
            } 
        }

score 0 · Answer 4 · answered Jan 26 '14 at 18:20

0

If one uses explicit interface implementations like IFactoryResponse.instance, then these methods are not publicly visible. Either you need to cast to IFactoryResponse to access them or define the methods as public: public object instance { ... }.

answered Jan 26 '14 at 18:20

Pieter

201
2
3

Declaring Explicit interfaces and the does not contain a definition error C#

4 Answers4