Why Icon element require Id attribute to end with '.exe' or '.ico'

Question

Why Icon for Shortcut element should have Id attribute ending with '.ico' or '.exe'?

So this code

<Icon Id="ProgramIcon" SourceFile="Images\MyAppIcon.ico"/>

<Component Guid="MY_GUID" Id="MyAppComponent">
    <File Source="MyApp.exe" Name="MyApp.exe" Id="MyApp.exe">
        <Shortcut Id="MyApp.Shortcut" Directory="ApplicationProgramMenuDir" WorkingDirectory="INSTALLDIR" Name="MyApp" Icon="ProgramIcon" Advertise="yes" />
    </File>
</Component>

generates compile error:

error LGHT0204: ICE50: The extension of Icon 'ProgramIcon' for Shortcut 'MyApp.Shortcut' does not match the extension of the Key File for component 'MyAppComponent'. [D:\APPS\MyProject.wixproj]

but this code

<Icon Id="ProgramIcon.ico" SourceFile="Images\MyAppIcon.ico"/>

<Component Guid="MY_GUID" Id="MyAppComponent">
    <File Source="MyApp.exe" Name="MyApp.exe" Id="MyApp.exe">
        <Shortcut Id="MyApp.Shortcut" Directory="ApplicationProgramMenuDir" WorkingDirectory="INSTALLDIR" Name="MyApp" Icon="ProgramIcon.ico" Advertise="yes" />
    </File>
</Component>

compiles fine.

It's first time I see such limitations for Id attribute of any element and it looks very strange. I don't see any reason for this even to be a warning. So I don't understand some basics of this functionality. Please help me understand the reasons of described behavior.

Answer 1

Looks like it's because of this: http://msdn.microsoft.com/en-us/library/aa369210.aspx (see the remarks section).

I'm guessing it's failing validation because Icon="ProgramIcon" does not match the extension of the shortcut.

http://wix.sourceforge.net/manual-wix2/wix_xsd_shortcut.htm says

Identifier reference to Icon element. The Icon identifier should have the same extension as the file that it points at. For example, a shortcut to an executable (e.g. "my.exe") should reference an Icon with identifier like "MyIcon.exe"