How to set an application as the default program of opening a certain type of file programmatically

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C++

Windows

shell

How to set my program to be default viewer for some file type (.pdf) programmatically? Do I need to write some info into registry or to use some windows api (as user can do manually)?

I already (for pdf) set the corresponding registry keys using Inno setup script:

Root: HKCR; Subkey: ".pdf"; ValueType: string; ValueName: ""; ValueData: "myprogramdoc";

Root: HKCR; Subkey: "myprogramdoc"; ValueType: string; ValueName: ""; ValueData: "myprogram PDF Document";

Root: HKCR; Subkey: "myprogramdocument\DefaultIcon"; ValueType: string; ValueName: ""; ValueData: "{app}\myprogram.exe,1";

and these registries work well for win 7. But under XP, if there is already set some other viewer as default (foxit), my icon will not be presented for pdf (I have to do manually - to choose my program as default viewer).