Finding the Concrete Type behind an Interface instance

To cut a long story short I have a C# function that performs a task on a given Type that is passed in as an Object instance. All works fine when a class instance is passed in. However, when the object is declared as an interface I'd really like to find the concrete class and perform the action upon that class type.

Here is the ubiquitous bad example (resplendent with incorrect property casing etc):

public interface IA
{
    int a { get; set; }
}
public class B : IA
{
    public int a { get; set; }
    public int b { get; set; }
}
public class C : IA
{
    public int a { get; set; }
    public int c { get; set; }
}

// snip

IA myBObject = new B();
PerformAction(myBObject);

IA myCObject = new C();
PerformAction(myCObject);

// snip

void PerformAction(object myObject)
{
    Type objectType = myObject.GetType();   // Here is where I get typeof(IA)
    if ( objectType.IsInterface )
    {
        // I want to determine the actual Concrete Type, i.e. either B or C
        // objectType = DetermineConcreteType(objectType);
    }
    // snip - other actions on objectType
}

I'd like the code in PerformAction to use Reflection against it's parameter and see that it's not just an instance of IA but that it's an instance of B and to see the property "b" via GetProperties(). If I use .GetType() I get the Type of IA - not what I want.

How can PerformAction determine the underlying Concrete Type of the instance of IA?

Some might be tempted to suggest using an Abstract class but that is just the limitation of my bad example. The variable will be originally declared as an interface instance.

Type objectType = myObject.GetType();

Should still give you the concrete type, according to your example.

What are you doing is really bed design but you don't have to use reflection you can check it like this

void PerformAction(object myObject)
{
    B objectType = myObject as B;   // Here is where I get typeof(IA)
    if ( objectType != null )
    {
        //use objectType.b
    }
    else
    {
       //Same with A 
    }
    // snip - other actions on objectType
}

I have to agree about the bad design. If you have an interface, it should be because you need to utilize some common functionality without caring what the concrete implementation is. Given you're example, it sounds like the PerformAction method should actually be a part of the interface:

public interface IA
{
    int a { get; set; }
    void PerformAction();
}

public class B: IA
{
    public int a { get; set; }
    public int b { get; set; }

    public void PerformAction()
    {
        // perform action specific to B
    }
}

public class C : IA
{
    public int a { get; set; }
    public int c { get; set; }

    public void PerformAction()
    {
        // perform action specific to C
    }
}

void PerformActionOn(IA instance)
{
    if (instance == null) throw new ArgumentNullException("instance");

    instance.PerformAction();

    // Do some other common work...
}


B b = new B();
C c = new C();

PerformActionOn(b);
PerformActionOn(c);

You can never have instances of an Interface. So determining whether you are dealing with an interface or a concrete type is not possible since you will always be dealing with a concrete type. So I am not sure you question makes any sense. What exactly are you trying to do and why?

Maybe you are looking for the is operator

void PerformAction(object myObject)
{
    if (myObject is B)
    {
    	B myBObject = myObject as B;
		myBObject.b = 1;
    }

	if (myObject is C)
	{
		C myCObject = myObject as C;
		myCObject.c = 1;
	}

    // snip - other actions on objectType
}