{"id":108141,"date":"2023-05-04T07:00:00","date_gmt":"2023-05-04T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108141"},"modified":"2023-05-04T09:21:20","modified_gmt":"2023-05-04T16:21:20","slug":"20230504-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230504-00\/?p=108141","title":{"rendered":"Why does XAML complain that none of the overloads of winrt::to_hstring<\/CODE> could be used?"},"content":{"rendered":"

A customer was having trouble compiling their C++\/WinRT project that used XAML binding.<\/p>\n

They had a data template that binds the Content<\/code> property of the AwesomeThing<\/code> to a TextBlock<\/code>‘s text:<\/p>\n

<!-- XAML -->\r\n<DataTemplate x:DataType=\"local:AwesomeThing\">\r\n    <TextBlock Text=\"{x:Bind Content}\" \/>\r\n<\/DataTemplate>\r\n<\/pre>\n
But when they tried to compile this, they got a C++\/WinRT error:<\/p>\n
MainPage.xaml.g.hpp(200): error C2665: 'winrt::to_hstring': none of the 13 overloads could convert all the argument types\r\n    while trying to match the argument list '(winrt::Windows::Foundation::IInspectable)'\r\n    while compiling class template member function 'void winrt::Contoso::implementation::MainPageT<winrt::Contoso::implementation::MainPage>::MainPage_obj1_Bindings::Update_Content(winrt::Windows::Foundation::IInspectable,int32_t)'\r\n    see reference to class template instantiation 'winrt::Contoso::implementation::MainPageT<winrt::Contoso::implementation::MainPage>::MainPage_obj1_Bindings' being compiled\r\nMainPage.xaml.g.hpp(200): error C2660: 'winrt::Contoso::implementation::MainPageT<winrt::Contoso::implementation::MainPage>::MainPage_obj1_Bindings::Set_Windows_UI_Xaml_Controls_TextBlock_Text': function does not take 1 arguments\r\n    see declaration of 'winrt::Contoso::implementation::MainPageT<winrt::Contoso::implementation::MainPage>::MainPage_obj1_Bindings::Set_Windows_UI_Xaml_Controls_TextBlock_Text'\r\n<\/pre>\n
What’s going on here?<\/p>\n
Let’s see if we can decode the error message.<\/p>\n
The first error says that somebody is trying to call winrt::to_hstring<\/code> with something that can’t be converted to a string. How curious.<\/p>\n
The Visual Studio Error List window shows only the first line of each error message.  To see the important supplemental information, you have to switch to the Output window<\/a>. That’s what tells us that the argument that couldn’t be converted to a string is an IInspectable<\/code>.<\/p>\n
The second error says that somebody passed only one parameter to the Set_Windows_UI_Xaml_Controls_TextBlock_Text<\/code> function. This is probably a cascade failure: The function takes two parameters, but one of them ended up being an error, so there was only one valid parameter left.<\/p>\n
Looking at the source code confirms this diagnosis:<\/p>\n
void Update_Content(::winrt::Windows::Foundation::IInspectable obj, int32_t phase)\r\n{\r\n    if((phase & ((1 << 0) | NOT_PHASED )) != 0)\r\n    {\r\n        Set_Windows_UI_Xaml_Controls_TextBlock_Text(obj7, ::winrt::to_hstring(obj));\r\n    }\r\n}\r\n<\/pre>\n
What we’ve learned so far is that the XAML compiler generated some code that takes an IInspectable<\/code> and tries to convert to a string (via to_hstring<\/code>), so it can set the Text<\/code> property of a TextBlock<\/code>.<\/p>\n
Gosh, where in our XAML markup could we be trying to assign an IInspectable<\/code> to a Text<\/code> property?<\/p>\n
Well, I made it easy because it is in fact the only binding in the entire XAML fragment: It’s the Text=\"{x:Bind Content}\"<\/code>. (The other clue is that the method is named Update_Content<\/code>.)<\/p>\n
The Content<\/code> property was declared like this:<\/p>\n
\/\/ Contoso.idl\r\n\r\nnamespace Contoso\r\n{\r\n    runtimeclass AwesomeThing\r\n    {\r\n        Object Name{ get; };\r\n        Object Content{ get; };\r\n        \/* and other properties *\/\r\n    }\r\n}\r\n<\/pre>\n
Notice that the Content<\/code> property is declared as an Object<\/code>, which is the MIDL compiler’s name for what in the ABI is called IInspectable<\/code>.<\/p>\n
And that’s why we have the error. The XAML markup is trying to bind the TextBlock.Text<\/code> property (which is a string) to the Awesome\u00adThing.Content<\/code> property (which is an IInspectable<\/code>). The XAML compiler sees that this is a type mismatch and tries to coerce the conversion by calling to_hstring<\/code> and hoping for the best.<\/p>\n
Unfortunately, it didn’t work.<\/p>\n
You can fix this in a few ways.<\/p>\n
The simplest way is to change the type of the Content<\/code> property to String<\/code>. Then you’re binding a string to a string, and everything lines up.<\/p>\n
On the other hand, maybe your Content<\/code> property is an Object<\/code> on purpose, say, because it can hold multiple things. Maybe it’s a string. Maybe it’s an integer. Maybe it’s a XAML element tree.<\/p>\n
In that case, you’ll have to teach XAML how to convert that mysterious object to a string. You can do this with a converter, or since you’re already invested in x:Bind<\/code>, you can use a function binding. Write a function to convert that mysterious object into a string, using an algorithm that describes how you want the conversion to occur. For example, maybe you’ll write it like this:<\/p>\n
\/\/ Contoso.idl\r\n\r\nnamespace Contoso\r\n{\r\n    runtimeclass AwesomeThing\r\n    {\r\n        Object Name{ get; };\r\n        Object Content{ get; };\r\n        \/* and other properties *\/\r\n\r\n        static String ContentToString(Object o);<\/span>\r\n    }\r\n}\r\n\r\n\/\/ C++\/WinRT implementation\r\n\r\nwinrt::hstring AwesomeThing::ContentToString(IInspectable const& o)\r\n{\r\n    if (auto s = o.try_as<winrt::hstring>()) {\r\n        return s.value();\r\n    }\r\n    if (auto i = o.try_as<int>()) {\r\n        \/\/ assume that \"%d\" is acceptable\r\n        return winrt::to_hstring(i.value());\r\n    }\r\n    if (auto e = o.try_as<winrt::FrameworkElement>()) {\r\n        \/\/ ?? figure out what to return here\r\n        return L\"\";\r\n    }\r\n    return L\"\";\r\n}\r\n\r\n<!-- XAML -->\r\n<DataTemplate x:DataType=\"local:AwesomeThing\">\r\n    <TextBlock Text=\"{x:Bind local:AwesomeThing.ContentToString(Content)}\" \/>\r\n<\/DataTemplate>\r\n<\/pre>\n
You might have thought of adding a new property that does the conversion:<\/p>\n
\/\/ Contoso.idl\r\n\r\nnamespace Contoso\r\n{\r\n    runtimeclass AwesomeThing\r\n    {\r\n        Object Name{ get; };\r\n        Object Content{ get; };\r\n        \/* and other properties *\/\r\n\r\n        String ContentAsString{ get; };<\/span>\r\n    }\r\n}\r\n\r\n\/\/ C++\/WinRT implementation\r\n\r\nwinrt::hstring AwesomeThing::ContentAsString()<\/span>\r\n{\r\n    auto o = Content();<\/span>\r\n\r\n    if (auto s = o.try_as<winrt::hstring>()) {\r\n        return s.value();\r\n    }\r\n    if (auto i = o.try_as<int>()) {\r\n        \/\/ assume that \"%d\" is acceptable\r\n        return winrt::to_hstring(i.value());\r\n    }\r\n    if (auto e = o.try_as<winrt::FrameworkElement>()) {\r\n        \/\/ ?? figure out what to return here\r\n        return L\"\";\r\n    }\r\n    return L\"\";\r\n}\r\n\r\n<!-- XAML -->\r\n<DataTemplate x:DataType=\"local:AwesomeThing\">\r\n    <TextBlock Text=\"{x:Bind ContentAsString<\/span>}\" \/>\r\n<\/DataTemplate>\r\n<\/pre>\n
This works, but only because x:Bind<\/code> defaults to one-time binding. If you switch to one-way binding:<\/p>\n
<!-- XAML -->\r\n<DataTemplate x:DataType=\"local:AwesomeThing\">\r\n    <TextBlock Text=\"{x:Bind ContentAsString, Mode=OneWay<\/span>}\" \/>\r\n<\/DataTemplate>\r\n<\/pre>\n
then changes to the Content<\/code> property will not trigger a recalculation of ContentAsString<\/code>. You have to remember to raise a PropertyChanged<\/code> event for ContentAsString<\/code> whenever the Content<\/code> property changes.<\/p>\n
The function binding avoids this need to remember to raise extra property change notifications: Passing Content<\/code> as a parameter to the function binding not only provides a parameter to the ContentAsString<\/code> function, the XAML compiler will see that the parameter is a property and recalculate the binding whenever Content<\/code> changes.<\/p>\n","protected":false},"excerpt":{"rendered":"
Look at what you are converting from and converting to.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-108141","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Look at what you are converting from and converting to.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108141","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=108141"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108141\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=108141"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108141"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108141"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}