diff --git a/array.rs b/array.rs
index 099b0de..35652db 100644
--- a/array.rs
+++ b/array.rs
@@ -3,6 +3,7 @@ use base::{
     CFAllocatorRef,
     CFIndex,
     CFRange,
+    CFTypeID,
     CFTypeRef,
     CFWrapper,
     kCFAllocatorDefault,
@@ -27,6 +28,7 @@ pub type CFArrayRef = *__CFArray;
 
 pub impl CFArrayRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFArrayGetTypeID() }
 }
 
 pub type CFArray<ElemRefType: AbstractCFTypeRef> = CFWrapper<CFArrayRef, ElemRefType, ()>;
@@ -95,9 +97,17 @@ extern {
 
     fn CFArrayCreate(allocator: CFAllocatorRef, values: **c_void,
                      numValues: CFIndex, callBacks: *CFArrayCallBacks) -> CFArrayRef;
-
-    fn CFArrayGetValueAtIndex(theArray: CFArrayRef, idx: CFIndex) -> *c_void;
+    // CFArrayCreateCopy
+    // CFArrayBSearchValues
+    // CFArrayContainsValue
     fn CFArrayGetCount(theArray: CFArrayRef) -> CFIndex;
+    // CFArrayGetCountOfValue
+    // CFArrayGetFirstIndexOfValue
+    // CFArrayGetLastIndexOfValue
+    // CFArrayGetValues
+    fn CFArrayGetValueAtIndex(theArray: CFArrayRef, idx: CFIndex) -> *c_void;
+    // CFArrayApplyFunction
+    fn CFArrayGetTypeID() -> CFTypeID;
 }
 
 #[test]
diff --git a/base.rs b/base.rs
index 382e482..050487d 100644
--- a/base.rs
+++ b/base.rs
@@ -5,6 +5,7 @@ use libc::c_long;
 // borrow semantics. 
 trait AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef;
+    static pure fn type_id() -> CFTypeID;
 }
 
 pub type Boolean = u8;
@@ -35,6 +36,13 @@ pub type CFTypeRef = *__CFType;
 
 pub impl CFTypeRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self }
+    // this can't be used, because CFType is the supertype and has no type id.
+    static pure fn type_id() -> CFTypeID { fail; }
+}
+
+pub pure fn downcast<T:AbstractCFTypeRef>(r: CFTypeRef) -> T unsafe {
+    assert CFGetTypeID(r) == type_id::<T>();
+    cast::transmute(r)
 }
 
 struct CFWrapper<T:Copy AbstractCFTypeRef, PlaceholderType1, PlaceholderType2> {
@@ -89,6 +97,7 @@ pub impl<T:Copy AbstractCFTypeRef, E1, E2>
     }
 
     static fn from_CFType(wrapper: CFType) -> CFWrapper<T,E1,E2> unsafe {
+        assert wrapper.type_id() == type_id::<T>();
         cast::transmute(move wrapper)
     }
 
diff --git a/boolean.rs b/boolean.rs
index ca20c7c..11f053f 100644
--- a/boolean.rs
+++ b/boolean.rs
@@ -1,14 +1,18 @@
 use base::{
     AbstractCFTypeRef,
     CFTypeRef,
+    CFTypeID,
     CFWrapper,
 };
 
+pub type Boolean = u32;
+
 struct __CFBoolean { private: () }
 pub type CFBooleanRef = *__CFBoolean;
 
 pub impl CFBooleanRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFBooleanGetTypeID() }
 }
 
 pub type CFBoolean = CFWrapper<CFBooleanRef, (), ()>;
@@ -28,5 +32,8 @@ pub impl CFBoolean {
 extern {
     const kCFBooleanTrue: CFBooleanRef;
     const kCFBooleanFalse: CFBooleanRef;
+
+    fn CFBooleanGetValue(boolean: CFBooleanRef) -> Boolean;
+    fn CFBooleanGetTypeID() -> CFTypeID;
 }
 
diff --git a/data.rs b/data.rs
index 2a07cd5..f647b07 100644
--- a/data.rs
+++ b/data.rs
@@ -3,8 +3,8 @@ use base::{
     CFAllocatorRef,
     CFIndex,
     CFRange,
-    CFType,
     CFTypeRef,
+    CFTypeID,
     CFWrapper,
     kCFAllocatorDefault,
 };
@@ -16,6 +16,7 @@ pub type CFDataRef = *__CFData;
 
 impl CFDataRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFDataGetTypeID() }
 }
 
 type CFData = CFWrapper<CFDataRef, (), ()>;
@@ -66,4 +67,6 @@ extern {
     fn CFDataGetBytePtr(theData: CFDataRef) -> *u8;
     fn CFDataGetBytes(theData: CFDataRef, range: CFRange, buffer: *u8);
     fn CFDataGetLength(theData: CFDataRef) -> CFIndex;
+
+    fn CFDataGetTypeID() -> CFTypeID;
 }
diff --git a/dictionary.rs b/dictionary.rs
index ca8de07..937a533 100644
--- a/dictionary.rs
+++ b/dictionary.rs
@@ -43,6 +43,7 @@ pub type CFDictionaryRef = *__CFDictionary;
 
 impl CFDictionaryRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFDictionaryGetTypeID() }
 }
 
 pub type CFDictionary<KeyRefType, ValueRefType> = CFWrapper<CFDictionaryRef, KeyRefType, ValueRefType>;
diff --git a/number.rs b/number.rs
index 657fb2b..1b7d702 100644
--- a/number.rs
+++ b/number.rs
@@ -36,6 +36,7 @@ pub type CFNumberRef = *__CFNumber;
 
 pub impl CFNumberRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFNumberGetTypeID() }
 }
 
 pub impl CFNumberRef {
@@ -159,3 +160,13 @@ extern {
     //fn CFNumberCompare
     fn CFNumberGetTypeID() -> CFTypeID;
 }
+
+fn should_fail_on_bad_downcast() {
+    #[test];
+    #[should_fail];
+
+    use boolean::CFBooleanRef;
+
+    let one = CFWrapper::to_CFType(CFNumber::new(1_i32));
+    let casted = base::downcast::<CFBooleanRef>(*one.borrow_ref());
+}
\ No newline at end of file
diff --git a/set.rs b/set.rs
index 176b012..6f5a80c 100644
--- a/set.rs
+++ b/set.rs
@@ -30,6 +30,7 @@ pub type CFSetRef = *__CFSet;
 
 impl CFSetRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFSetGetTypeID() }
 }
 
 pub type CFSet<ElemRefType: AbstractCFTypeRef> = CFWrapper<CFSetRef, ElemRefType, ()>;
diff --git a/string.rs b/string.rs
index 0459351..952ad78 100644
--- a/string.rs
+++ b/string.rs
@@ -199,6 +199,7 @@ pub type CFStringRef = *__CFString;
 
 pub impl CFStringRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFStringGetTypeID() }
 }
 
 pub type CFString = CFWrapper<CFStringRef, (), ()>;
diff --git a/test.rs b/test.rs
index b535b6b..be0c2c0 100644
--- a/test.rs
+++ b/test.rs
@@ -22,4 +22,3 @@ fn main() {
     ]);
     d.show();
 }
-
diff --git a/url.rs b/url.rs
index 7c75c60..44e62f2 100644
--- a/url.rs
+++ b/url.rs
@@ -20,6 +20,7 @@ pub type CFURLRef = *__CFURL;
 
 impl CFURLRef : AbstractCFTypeRef {
     pure fn as_type_ref(&self) -> CFTypeRef { *self as CFTypeRef }
+    static pure fn type_id() -> CFTypeID unsafe { CFURLGetTypeID() }
 }
 
 pub type CFURL = CFWrapper<CFURLRef, (), ()>;