Add IComparable<T> to NSString (#7468)

Fixes #5522

src/Foundation/NSString2.cs

@@ -29,7 +29,7 @@ using ObjCRuntime;
 
 namespace Foundation {
 
-	public partial class NSString {
+	public partial class NSString : IComparable<NSString> {
 		const string selDataUsingEncodingAllow = "dataUsingEncoding:allowLossyConversion:";
 
 #if MONOMAC
@@ -91,6 +91,11 @@ namespace Foundation {
 			return ret;
 		}
 
+		public int CompareTo (NSString other)
+		{
+			return (int)Compare (other);
+		}
+
 		// [Export ("initWithContentsOfURL:encoding:error")]
 		// IntPtr Constructor (NSUrl url, NSStringEncoding encoding, out NSError error);
 		// 

tests/apitest/src/Foundation/NSString.cs

@@ -54,6 +54,23 @@ namespace Xamarin.Mac.Tests
 			Assert.IsTrue (rect.Width > 0);
 			Assert.IsTrue (rect.Height > 0);
 		}
+
+		[Test]
+		public void NSString_CompareTo ()
+		{
+			using (var a = new NSString ("a"))
+			using (var b = new NSString ("b"))
+			using (var c = new NSString ("c"))
+			{
+				var tests = new NSString[] { c, a, b };
+
+				Array.Sort (tests);
+
+				Assert.AreSame (a, tests[0], "0");
+				Assert.AreSame (b, tests[1], "1");
+				Assert.AreSame (c, tests[2], "2");
+			}
+		}
 	}
 
 	[TestFixture]