{"id":108017,"date":"2023-04-05T07:00:00","date_gmt":"2023-04-05T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108017"},"modified":"2023-04-04T19:00:07","modified_gmt":"2023-04-05T02:00:07","slug":"20230405-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230405-00\/?p=108017","title":{"rendered":"How do I modify the contents of a boxed Windows Runtime value?"},"content":{"rendered":"

When you box a value in the Windows Runtime, it’s boxed for good. You can’t change the value inside a box. Most of the time, it’s pretty obvious that you can’t do it:<\/p>\n

\/\/ C#\r\nvoid UpdateX(Nullable<Point> pt)\r\n{\r\n  if (pt != null) {\r\n    \/\/ Error: Cannot modify the return value of Point?.Value\r\n    \/\/ because it is not a variable\r\n    pt.Value.X = 42.0;\r\n  }\r\n}\r\n\r\n\/\/ C++\/WinRT\r\nvoid UpdateX(IReference<Point> pt)\r\n{\r\n  if (pt != nullptr) {\r\n    \/\/ Error: expression must be a modifiable lvalue\r\n    pt.Value().X = 42.0;\r\n  }\r\n}\r\n\r\n\/\/ C++\/CX\r\nvoid UpdateX(IBox<Point>^ pt)\r\n{\r\n  if (pt != nullptr) {\r\n    \/\/ Error: lvalue required as left operand of assignment\r\n    pt->Value.X = 42.0;\r\n  }\r\n}\r\n<\/pre>\n
In the case of C++\/WinRT, I think the error is unlikely to occur because you’re clearly modifying the result of a function call.<\/p>\n
But for C# and C++\/CX, the property syntax looks a lot like a member variable access, and you may not realize that the property value is secretly the result of a function call.<\/p>\n
Boxed values are read-only. If you look at  IReference<T><\/code><\/a> (which is the interface at the ABI layer that is used for Windows Runtime boxed values), you’ll see that there is a read-only Value<\/code> property, but no method for setting a new value. Once a value is boxed, you can’t change it.<\/p>\n
But what if you want to change it?<\/p>\n
You’ll have to box up a new value and then ask everybody to switch over to it.<\/p>\n
\/\/ C#\r\nNullable<Point> UpdateX(Nullable<Point> pt)\r\n{\r\n  if (pt != null) {\r\n    var value = pt.Value;\r\n    value.X = 42.0;\r\n    pt = value; \/\/ box up a new value\r\n  }\r\n  return pt;\r\n}\r\n\r\n\/\/ C++\/WinRT\r\nIReference<Point> UpdateX(IReference<Point> pt)\r\n{\r\n  if (pt != nullptr) {\r\n    auto value = pt.Value();\r\n    value.X = 42.0;\r\n    pt = winrt::box_value(value); \/\/ box up a new value\r\n  }\r\n  return pt;\r\n}\r\n\r\n\/\/ C++\/CX\r\nIBox<Point> UpdateX(IBox<Point>^ pt)\r\n{\r\n  if (pt != nullptr) {\r\n    auto value = pt->Value;\r\n    value.X = 42.0;\r\n    pt = value; \/\/ box up a new value\r\n  }\r\n  return pt;\r\n}\r\n<\/pre>\n
Of course, you now have to take the updated boxed value and update wherever you got it from.<\/p>\n
\/\/ C#\r\nflyoutShowOptions.Position = UpdateX(flyoutShowOptions.Position);\r\n\r\n\/\/ C++\/WinRT\r\nflyoutShowOptions.Position(UpdateX(flyoutShowOptions.Position()));\r\n\r\n\/\/ C++\/CX\r\nflyoutShowOptions->Position = UpdateX(flyoutShowOptions->Position);\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Once it’s in the box, you can’t change it.<\/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-108017","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Once it’s in the box, you can’t change it.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108017","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=108017"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108017\/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=108017"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108017"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108017"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}