[corefoundation] Cache kCFNull to avoid native calls

src/CoreFoundation/CFArray.cs

@@ -53,6 +53,9 @@ namespace CoreFoundation {
 	// interesting bits: https://github.com/opensource-apple/CF/blob/master/CFArray.c
 	public partial class CFArray : NativeObject {
 
+		// this cache the handle instead of issuing a native call
+		internal static NativeHandle CFNullHandle = _CFNullHandle;
+
 #if !NET
 		internal CFArray (NativeHandle handle)
 			: base (handle, false)

src/corefoundation.cs

@@ -33,7 +33,7 @@ interface CFAllocator {
 	interface CFArray {
 
 		[Internal][Field ("kCFNull")]
-		IntPtr /* CFNullRef */ CFNullHandle { get; }
+		IntPtr /* CFNullRef */ _CFNullHandle { get; }
 	}
 
 	[Partial]

tests/perftest/NativeArrayPerf.cs

@@ -32,5 +32,17 @@ public void Create ()
 			var native = CFArray.Create (array);
 			CFString.ReleaseNative (native); // that's a `CFObject.CFRelease` with a null-check
 		}
+
+		int error;
+
+		[Benchmark]
+		public void ArrayFromHandleFunc ()
+		{
+			var native = CFArray.Create (array);
+			var managed = CFArray.ArrayFromHandle<NSNumber> (native);
+			if (managed.Length != array.Length)
+				error++;
+			CFString.ReleaseNative (native); // that's a `CFObject.CFRelease` with a null-check
+		}
 	}
 }