new interface: include not found / base class undefined

Hey guys!
i created a simple new interface ( following the tutorial in the wiki ).
but i can not inherit from it. the compiler cannot find the include file “Usable.h”
Every source file is in the MyGame directory. If i omit the include, the base class if
undefined. i also tried to predeclarate it, but with no success. whats wrong?

// Usable.cpp
#pragma once
#include "Usable.generated.h"

UINTERFACE(MinimalAPI)
class UUsable :public UInterface
{
	GENERATED_UINTERFACE_BODY()
};

class IUsable
{
	GENERATED_IINTERFACE_BODY()
	virtual void OnUsed(AController * user);
};

// Usable.cpp
#include "MyGame.h"


UUsable::UUsable(const class FPostConstructInitializeProperties& PCIP)
: Super(PCIP)
{

}

void IUsable::OnUsed(AController * user)
{

}

// Crate.h
#pragma once
#include "Usable.h"
#include "Crate.generated.h"

UCLASS()
class ACrate : public AActor, public IUsable
{
	GENERATED_UCLASS_BODY()

	virtual void OnUsed(AController* c) OVERRIDE;
};

Solved it. The files were in the wrong directory.
Hint to myself:
If you create files in Visual Studio, make sure they are created in the right directory!

Note: I ran into the problem of
‘UInterface’: base class undefined
The undocumented solution for 4.26 was to go to the top of your interface header file and add:

#include "UObject/Interface.h"