diff --git a/src/Foundation/NSAttributedString.cs b/src/Foundation/NSAttributedString.cs
index 6a2fdfb275d8..bb0657658531 100644
--- a/src/Foundation/NSAttributedString.cs
+++ b/src/Foundation/NSAttributedString.cs
@@ -25,6 +25,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable enable
+
 using System;
 using CoreFoundation;
 using CoreText;
@@ -36,13 +38,13 @@
 namespace Foundation {
 	public partial class NSAttributedString {
 
-		public string Value {
+		public string? Value {
 			get {
 				return CFString.FromHandle (LowLevelValue);
 			}
 		}
 
-		public NSDictionary GetAttributes (nint location, out NSRange effectiveRange)
+		public NSDictionary? GetAttributes (nint location, out NSRange effectiveRange)
 		{
 			return Runtime.GetNSObject<NSDictionary> (LowLevelGetAttributes (location, out effectiveRange));
 		}
@@ -58,21 +60,21 @@ public IntPtr LowLevelGetAttributes (nint location, out NSRange effectiveRange)
 		}
 #endif
 
-		public NSAttributedString (string str, CTStringAttributes attributes)
-			: this (str, attributes != null ? attributes.Dictionary : null)
+		public NSAttributedString (string str, CTStringAttributes? attributes)
+			: this (str, attributes?.Dictionary)
 		{
 		}
 
-		public CTStringAttributes GetCoreTextAttributes (nint location, out NSRange effectiveRange)
+		public CTStringAttributes? GetCoreTextAttributes (nint location, out NSRange effectiveRange)
 		{
 			var attr = GetAttributes (location, out effectiveRange);
-			return attr == null ? null : new CTStringAttributes (attr);
+			return attr is null ? null : new CTStringAttributes (attr);
 		}
 
-		public CTStringAttributes GetCoreTextAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
+		public CTStringAttributes? GetCoreTextAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
 		{
 			var attr = GetAttributes (location, out longestEffectiveRange, rangeLimit);
-			return attr == null ? null : new CTStringAttributes (attr);
+			return attr is null ? null : new CTStringAttributes (attr);
 		}
 
 		public NSAttributedString Substring (nint start, nint len)
@@ -81,52 +83,52 @@ public NSAttributedString Substring (nint start, nint len)
 		}
 
 #if !MONOMAC
-		public NSAttributedString (string str, UIStringAttributes attributes)
-			: this (str, attributes != null ? attributes.Dictionary : null)
+		public NSAttributedString (string str, UIStringAttributes? attributes)
+			: this (str, attributes?.Dictionary)
 		{
 		}
 
-		public UIStringAttributes GetUIKitAttributes (nint location, out NSRange effectiveRange)
+		public UIStringAttributes? GetUIKitAttributes (nint location, out NSRange effectiveRange)
 		{
 			var attr = GetAttributes (location, out effectiveRange);
-			return attr == null ? null : new UIStringAttributes (attr);
+			return attr is null ? null : new UIStringAttributes (attr);
 		}
 
-		public UIStringAttributes GetUIKitAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
+		public UIStringAttributes? GetUIKitAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
 		{
 			var attr = GetAttributes (location, out longestEffectiveRange, rangeLimit);
-			return attr == null ? null : new UIStringAttributes (attr);
+			return attr is null ? null : new UIStringAttributes (attr);
 		}
 
-		static internal NSDictionary ToDictionary (
-						  UIFont font,
-						  UIColor foregroundColor,
-						  UIColor backgroundColor,
-						  UIColor strokeColor,
-						  NSParagraphStyle paragraphStyle,
+		static internal NSDictionary? ToDictionary (
+						  UIFont? font,
+						  UIColor? foregroundColor,
+						  UIColor? backgroundColor,
+						  UIColor? strokeColor,
+						  NSParagraphStyle? paragraphStyle,
 						  NSLigatureType ligature,
 						  float kerning,
 						  NSUnderlineStyle underlineStyle,
 #if !WATCH
-						  NSShadow shadow,
+						  NSShadow? shadow,
 #endif
 						  float strokeWidth,
 						  NSUnderlineStyle strikethroughStyle)
 		{
 			var attr = new UIStringAttributes ();
-			if (font != null) {
+			if (font is not null) {
 				attr.Font = font;
 			}
-			if (foregroundColor != null) {
+			if (foregroundColor is not null) {
 				attr.ForegroundColor = foregroundColor;
 			}
-			if (backgroundColor != null) {
+			if (backgroundColor is not null) {
 				attr.BackgroundColor = backgroundColor;
 			}
-			if (strokeColor != null) {
+			if (strokeColor is not null) {
 				attr.StrokeColor = strokeColor;
 			}
-			if (paragraphStyle != null) {
+			if (paragraphStyle is not null) {
 				attr.ParagraphStyle = paragraphStyle;
 			}
 			if (ligature != NSLigatureType.Default) {
@@ -139,7 +141,7 @@ static internal NSDictionary ToDictionary (
 				attr.UnderlineStyle = underlineStyle;
 			}
 #if !WATCH
-			if (shadow != null) {
+			if (shadow is not null) {
 				attr.Shadow = shadow;
 			}
 #endif
@@ -154,16 +156,16 @@ static internal NSDictionary ToDictionary (
 		}
 
 		public NSAttributedString (string str,
-					   UIFont font = null,
-					   UIColor foregroundColor = null,
-					   UIColor backgroundColor = null,
-					   UIColor strokeColor = null,
-					   NSParagraphStyle paragraphStyle = null,
+					   UIFont? font = null,
+					   UIColor? foregroundColor = null,
+					   UIColor? backgroundColor = null,
+					   UIColor? strokeColor = null,
+					   NSParagraphStyle? paragraphStyle = null,
 					   NSLigatureType ligatures = NSLigatureType.Default,
 					   float kerning = 0,
 					   NSUnderlineStyle underlineStyle = NSUnderlineStyle.None,
 #if !WATCH
-					   NSShadow shadow = null,
+					   NSShadow? shadow = null,
 #endif
 					   float strokeWidth = 0,
 					   NSUnderlineStyle strikethroughStyle = NSUnderlineStyle.None)
@@ -183,7 +185,7 @@ public NSAttributedString (string str,
 		// note: we cannot reuse the same method name - as it would break compilation of existing apps
 		public static NSAttributedString CreateFrom (NSTextAttachment attachment)
 		{
-			return (null as NSAttributedString).FromTextAttachment (attachment);
+			return (null as NSAttributedString)!.FromTextAttachment (attachment);
 		}
 #endif
 #endif
diff --git a/src/Foundation/NSAttributedString.iOS.cs b/src/Foundation/NSAttributedString.iOS.cs
index 10359c6dcbc4..73811f4c9199 100644
--- a/src/Foundation/NSAttributedString.iOS.cs
+++ b/src/Foundation/NSAttributedString.iOS.cs
@@ -25,6 +25,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable enable
+
 using System;
 
 using ObjCRuntime;
@@ -39,7 +41,7 @@ namespace Foundation {
 
 #if !MONOMAC && !COREBUILD
 	public partial class NSAttributedString {
-		static NSDictionary ignore;
+		static NSDictionary? ignore;
 
 		public NSAttributedString (NSUrl url, NSAttributedStringDocumentAttributes documentAttributes, ref NSError error)
 		: this (url, documentAttributes, out ignore, ref error) { }
@@ -48,14 +50,14 @@ public NSAttributedString (NSData data, NSAttributedStringDocumentAttributes doc
 		: this (data, documentAttributes, out ignore, ref error) { }
 
 		public NSAttributedString (NSUrl url, ref NSError error)
-		: this (url, (NSDictionary) null, out ignore, ref error) { }
+		: this (url, new NSDictionary (), out ignore, ref error) { }
 
 		public NSAttributedString (NSData data, ref NSError error)
-		: this (data, (NSDictionary) null, out ignore, ref error) { }
+		: this (data, new NSDictionary (), out ignore, ref error) { }
 
 #if IOS // not TVOS or WATCH
 		// use the best selector based on the OS version
-		public NSAttributedString (NSUrl url, NSDictionary options, out NSDictionary resultDocumentAttributes, ref NSError error)
+		public NSAttributedString (NSUrl url, NSDictionary? options, out NSDictionary resultDocumentAttributes, ref NSError error)
 		{
 			if (SystemVersion.CheckiOS (9,0))
 				Handle = InitWithURL (url, options, out resultDocumentAttributes, ref error);
@@ -73,12 +75,12 @@ public NSAttributedString (NSUrl url, NSDictionary options, out NSDictionary res
 	public partial class NSAttributedStringDocumentAttributes : DictionaryContainer {
 #if !MONOMAC && !COREBUILD
 		public NSAttributedStringDocumentAttributes () : base (new NSMutableDictionary ()) { }
-		public NSAttributedStringDocumentAttributes (NSDictionary dictionary) : base (dictionary) { }
+		public NSAttributedStringDocumentAttributes (NSDictionary? dictionary) : base (dictionary) { }
 
 		public NSStringEncoding? StringEncoding {
 			get {
 				var value = GetInt32Value (UIStringAttributeKey.NSCharacterEncodingDocumentAttribute);
-				if (value == null)
+				if (value is null)
 					return null;
 				else
 					return (NSStringEncoding) value.Value;
@@ -88,7 +90,7 @@ public NSStringEncoding? StringEncoding {
 			}
 		}
 
-		public NSString WeakDocumentType {
+		public NSString? WeakDocumentType {
 			get {
 				return GetNSStringValue (UIStringAttributeKey.NSDocumentTypeDocumentAttribute);
 			}
@@ -139,7 +141,7 @@ public CGSize? PaperSize {
 				return null;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSPaperSizeDocumentAttribute);
 				else
 					Dictionary [UIStringAttributeKey.NSPaperSizeDocumentAttribute] = NSValue.FromCGSize (value.Value);
@@ -156,7 +158,7 @@ public UIEdgeInsets? PaperMargin {
 				return null;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSPaperMarginDocumentAttribute);
 				else
 					Dictionary [UIStringAttributeKey.NSPaperMarginDocumentAttribute] = NSValue.FromUIEdgeInsets (value.Value);
@@ -173,7 +175,7 @@ public CGSize? ViewSize {
 				return null;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSViewSizeDocumentAttribute);
 				else
 					Dictionary [UIStringAttributeKey.NSViewSizeDocumentAttribute] = NSValue.FromCGSize (value.Value);
@@ -185,7 +187,7 @@ public float? ViewZoom {
 				return GetFloatValue (UIStringAttributeKey.NSViewZoomDocumentAttribute);
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSViewZoomDocumentAttribute);
 				else
 					SetNumberValue (UIStringAttributeKey.NSViewZoomDocumentAttribute, value);
@@ -195,13 +197,13 @@ public float? ViewZoom {
 		public NSDocumentViewMode? ViewMode {
 			get {
 				var value = GetInt32Value (UIStringAttributeKey.NSViewModeDocumentAttribute);
-				if (value == null)
+				if (value is null)
 					return null;
 				else
 					return (NSDocumentViewMode) value.Value;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSViewModeDocumentAttribute);
 				else
 					SetNumberValue (UIStringAttributeKey.NSViewModeDocumentAttribute, (int) value.Value);
@@ -211,7 +213,7 @@ public NSDocumentViewMode? ViewMode {
 		public bool ReadOnly {
 			get {
 				var value = GetInt32Value (UIStringAttributeKey.NSReadOnlyDocumentAttribute);
-				if (value == null || value.Value <= 0)
+				if (value is null || value.Value <= 0)
 					return false;
 				return true;
 			}
@@ -220,14 +222,14 @@ public bool ReadOnly {
 			}
 		}
 
-		public UIColor BackgroundColor {
+		public UIColor? BackgroundColor {
 			get {
-				NSObject value;
+				NSObject? value;
 				Dictionary.TryGetValue (UIStringAttributeKey.NSBackgroundColorDocumentAttribute, out value);
 				return value as UIColor;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSBackgroundColorDocumentAttribute);
 				else
 					Dictionary [UIStringAttributeKey.NSBackgroundColorDocumentAttribute] = value;
@@ -239,7 +241,7 @@ public float? HyphenationFactor {
 				return GetFloatValue (UIStringAttributeKey.NSReadOnlyDocumentAttribute);
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSReadOnlyDocumentAttribute);
 				else {
 					if (value < 0 || value > 1.0f)
@@ -254,7 +256,7 @@ public float? DefaultTabInterval {
 				return GetFloatValue (UIStringAttributeKey.NSDefaultTabIntervalDocumentAttribute);
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSDefaultTabIntervalDocumentAttribute);
 				else {
 					if (value < 0 || value > 1.0f)
@@ -264,14 +266,14 @@ public float? DefaultTabInterval {
 			}
 		}
 
-		public NSDictionary WeakDefaultAttributes {
+		public NSDictionary? WeakDefaultAttributes {
 			get {
-				NSObject value;
+				NSObject? value;
 				Dictionary.TryGetValue (UIStringAttributeKey.NSDefaultAttributesDocumentAttribute, out value);
 				return value as NSDictionary;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (UIStringAttributeKey.NSDefaultAttributesDocumentAttribute);
 				else
 					Dictionary [UIStringAttributeKey.NSDefaultAttributesDocumentAttribute] = value;
@@ -290,13 +292,13 @@ public NSDictionary WeakDefaultAttributes {
 		[Mac (10, 15)]
 		[iOS (13, 0)]
 #endif
-		public NSUrl ReadAccessUrl {
+		public NSUrl? ReadAccessUrl {
 			get {
 				Dictionary.TryGetValue (NSAttributedStringDocumentReadingOptionKeys.ReadAccessUrlKey, out var value);
 				return value as NSUrl;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (NSAttributedStringDocumentReadingOptionKeys.ReadAccessUrlKey);
 				else
 					Dictionary [NSAttributedStringDocumentReadingOptionKeys.ReadAccessUrlKey] = value;
diff --git a/src/Foundation/NSAttributedString.mac.cs b/src/Foundation/NSAttributedString.mac.cs
index 2edc7e0ff903..ae1a42d44487 100644
--- a/src/Foundation/NSAttributedString.mac.cs
+++ b/src/Foundation/NSAttributedString.mac.cs
@@ -6,6 +6,8 @@
 //
 // Copyright 2013 Xamarin Inc
 
+#nullable enable
+
 #if MONOMAC
 
 using System;
@@ -18,39 +20,39 @@ namespace Foundation
 {
 	public partial class NSAttributedString
 	{
-		public NSAttributedString (string str, NSStringAttributes attributes)
-			: this (str, attributes != null ? attributes.Dictionary : null)
+		public NSAttributedString (string str, NSStringAttributes? attributes)
+			: this (str, attributes?.Dictionary)
 		{
 		}
 
 		public NSAttributedString (string str,
-			NSFont font = null,
-			NSColor foregroundColor = null,
-			NSColor backgroundColor = null,
-			NSColor strokeColor = null,
-			NSColor underlineColor = null,
-			NSColor strikethroughColor = null,
+			NSFont? font = null,
+			NSColor? foregroundColor = null,
+			NSColor? backgroundColor = null,
+			NSColor? strokeColor = null,
+			NSColor? underlineColor = null,
+			NSColor? strikethroughColor = null,
 			NSUnderlineStyle underlineStyle = NSUnderlineStyle.None,
 			NSUnderlineStyle strikethroughStyle = NSUnderlineStyle.None,
-			NSParagraphStyle paragraphStyle = null,
+			NSParagraphStyle? paragraphStyle = null,
 			float strokeWidth = 0,
-			NSShadow shadow = null,
-			NSUrl link = null,
+			NSShadow? shadow = null,
+			NSUrl? link = null,
 			bool superscript = false,
-			NSTextAttachment attachment = null,
+			NSTextAttachment? attachment = null,
 			NSLigatureType ligature = NSLigatureType.Default,
 			float baselineOffset = 0,
 			float kerningAdjustment = 0,
 			float obliqueness = 0,
 			float expansion = 0,
-			NSCursor cursor = null,
-			string toolTip = null,
+			NSCursor? cursor = null,
+			string? toolTip = null,
 			int characterShape = 0,
-			NSGlyphInfo glyphInfo = null,
-			NSArray writingDirection = null,
+			NSGlyphInfo? glyphInfo = null,
+			NSArray? writingDirection = null,
 			bool markedClauseSegment = false,
 			NSTextLayoutOrientation verticalGlyphForm = NSTextLayoutOrientation.Horizontal,
-			NSTextAlternatives textAlternatives = null,
+			NSTextAlternatives? textAlternatives = null,
 			NSSpellingState spellingState = NSSpellingState.None) : this (str, NSStringAttributes.ToDictionary (
 				font: font,
 				foregroundColor: foregroundColor,
@@ -127,16 +129,16 @@ public static NSAttributedString CreateWithDocFormat (NSData wordDocFormat, out
 			return new NSAttributedString (wordDocFormat, NSAttributedStringDataType.DocFormat, out docAttributes);
 		}
 
-		public NSStringAttributes GetAppKitAttributes (nint location, out NSRange effectiveRange)
+		public NSStringAttributes? GetAppKitAttributes (nint location, out NSRange effectiveRange)
 		{
 			var attr = GetAttributes (location, out effectiveRange);
-			return attr == null ? null : new NSStringAttributes (attr);
+			return attr is null ? null : new NSStringAttributes (attr);
 		}
 
-		public NSStringAttributes GetAppKitAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
+		public NSStringAttributes? GetAppKitAttributes (nint location, out NSRange longestEffectiveRange, NSRange rangeLimit)
 		{
 			var attr = GetAttributes (location, out longestEffectiveRange, rangeLimit);
-			return attr == null ? null : new NSStringAttributes (attr);			
+			return attr is null ? null : new NSStringAttributes (attr);
 		}
 	}
 
@@ -144,27 +146,27 @@ public partial class NSAttributedStringDocumentAttributes : DictionaryContainer
 		public NSAttributedStringDocumentAttributes () : base (new NSMutableDictionary ()) {}
 		public NSAttributedStringDocumentAttributes (NSDictionary dictionary) : base (dictionary) {}
 
-		public WebPreferences WebPreferences { 
+		public WebPreferences? WebPreferences {
 			get {
 				NSObject value;
 				Dictionary.TryGetValue (NSStringAttributeKey.NSWebPreferencesDocumentOption, out value);
 				return value as WebPreferences;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (NSStringAttributeKey.NSWebPreferencesDocumentOption);
 				else
 					Dictionary [NSStringAttributeKey.NSWebPreferencesDocumentOption] = value;
 			}
 		}
-		public NSObject WebResourceLoadDelegate { 
+		public NSObject? WebResourceLoadDelegate {
 			get {
 				NSObject value;
 				Dictionary.TryGetValue (NSStringAttributeKey.NSWebResourceLoadDelegateDocumentOption, out value);
 				return value;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (NSStringAttributeKey.NSWebResourceLoadDelegateDocumentOption);
 				else
 					Dictionary [NSStringAttributeKey.NSWebResourceLoadDelegateDocumentOption] = value;
@@ -174,7 +176,7 @@ public NSObject WebResourceLoadDelegate {
 		public NSStringEncoding? StringEncoding {
 			get {
 				var value = GetInt32Value (NSStringAttributeKey.NSCharacterEncodingDocumentOption);
-				if (value == null)
+				if (value is null)
 					return null;
 				else
 					return (NSStringEncoding) value.Value;
@@ -184,7 +186,7 @@ public NSStringEncoding? StringEncoding {
 			}
 		}
 
-		public NSString WeakDocumentType {
+		public NSString? WeakDocumentType {
 			get {
 				return GetNSStringValue (NSStringAttributeKey.NSDocumentTypeDocumentOption);
 			}
@@ -256,21 +258,21 @@ public NSDocumentType DocumentType {
 			}
 		}
 
-		public NSDictionary WeakDefaultAttributes {
+		public NSDictionary? WeakDefaultAttributes {
 			get {
 				NSObject value;
 				Dictionary.TryGetValue (NSStringAttributeKey.NSDefaultAttributesDocumentOption, out value);
 				return value as NSDictionary;
 			}
 			set {
-				if (value == null)
+				if (value is null)
 					RemoveValue (NSStringAttributeKey.NSDefaultAttributesDocumentOption);
 				else
 					Dictionary [NSStringAttributeKey.NSDefaultAttributesDocumentOption] = value;
 			}
 		}
 
-		public NSUrl BaseUrl {
+		public NSUrl? BaseUrl {
 			get { 
 				return GetNativeValue <NSUrl> (NSStringAttributeKey.NSBaseURLDocumentOption);
 			} 
@@ -279,7 +281,7 @@ public NSUrl BaseUrl {
 			} 
 		}
 
-		public string TextEncodingName { 
+		public string? TextEncodingName {
 			get {
 				return (string)GetNSStringValue (NSStringAttributeKey.NSTextEncodingNameDocumentOption);
 			}