cancel XAML completion in attribute values, if '{' is pressed and we are in an empty attribute value

src/AddIns/BackendBindings/XamlBinding/XamlBinding/XamlCodeCompletionBinding.cs

@@ -397,7 +397,9 @@ namespace ICSharpCode.XamlBinding
 			if (context.Description == XamlContextDescription.InMarkupExtension && context.AttributeValue != null && !context.AttributeValue.IsString) {
 				if (!XamlBindingOptions.UseExtensionCompletion)
 					return false;
-				XamlCompletionItemList completionList = generator.CreateMarkupExtensionCompletion(context) as XamlCompletionItemList;
+				var completionList = generator.CreateMarkupExtensionCompletion(context) as XamlCompletionItemList;
+				if (completionList == null)
+					return false;
 				string word = context.Editor.GetWordBeforeCaretExtended();
 				if (context.PressedKey != '.' && context.PressedKey != '=' && !word.EndsWith(".", StringComparison.Ordinal) && completionList.PreselectionLength == 0)
 					completionList.PreselectionLength = word.Length;

src/AddIns/BackendBindings/XamlBinding/XamlBinding/XamlCompletionItemList.cs

@@ -41,9 +41,13 @@ namespace ICSharpCode.XamlBinding
 			if (key == ':' || key == '/')
 				return CompletionItemListKeyResult.NormalKey;
 			
-			if (key == '.' && (context.InAttributeValueOrMarkupExtension && context.Attribute.Name.StartsWith("xmlns")))
+			if (key == '.' && (context.InAttributeValueOrMarkupExtension && context.Attribute.Name.StartsWith("xmlns", StringComparison.Ordinal)))
 				return CompletionItemListKeyResult.NormalKey;
 			
+			// cancel completion if user might want to start a markup extension and value is still empty
+			if (key == '{' && (context.InAttributeValueOrMarkupExtension && string.IsNullOrEmpty(context.RawAttributeValue)))
+				return CompletionItemListKeyResult.Cancel;
+			
 			return base.ProcessInput(key);
 		}