When I edit Web application project, Visual Studio 2017 (15.3.1) adds <Use64BitIISExpress /> element under Project/PropertyGroup in csproj file.
I can't find any documentation, what is the purpose and if it affects something, when presented (as it is, without any attributes).
Only result I was able to find was mention about registry value of the same name.
Does someone know what this element serves for? Was it introduced in some of recent updates of Visual Studio 2017?
I noticed this entry, not surprisingly, after I made a change to the Properties page of the my Project. Under the Web section of the Properties page, you'll see a section called "Servers". After I changed the "Project URL" to use the correct port number for debugging, this entry appeared (not because I changed that option specifically, but it's when I noticed it appearing).
<Use64BitIISExpress />
In this section you can select either "IIS Express" or "External Host". Next to that dropdownlist, there is another dropdownlist for "Bitness". Mine was currently set to "Default", which displays the entry in the Project file as an empty element. After changing the "Bitness" to "x64", my Project file entry changed to:
<Use64BitIISExpress>true</Use64BitIISExpress>
Changing my "Bitness" to "x86" results in:
<Use64BitIISExpress>false</Use64BitIISExpress>
Returning "Bitness" to "Default" makes it again an empty element:
<Use64BitIISExpress>
</Use64BitIISExpress>
I understand this doesn't address your question of "where is the documentation?". I, too, could not find any relevant MSBuild documentation for this attribute. But, I thought it worth noting where the attribute is coming from and how it acts based on selected options from the Project properties while we anxiously await some formal, official documentation.