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From WiX Wiki at MindCapers

MSIExec is the executable that provides the means to install, modify, and perform operations on Windows Installer from the command line.

Question

In the project I'm working on, we've hit a pain point and we're not certain how to overcome it. Right now if we want to do an upgrade,the user has to uninstall before installing the new version.

Answer

Does it complain about a different version of the product already being installed? If so, you need to install the MSI via the msiexec.exe command like this:

  msiexec.exe /I your_msi.msi REINSTALL=All REINSTALLMODE=vomus 

What are you trying to achieve? Are you trying to make it so you don't have to do that from the command-line? One thing that I think that you can do is set a property in your wxs file that is:

  <Property Id="REINSTALLMODE" Value="vomus" />

If you look at the MSDN documentation for the REINSTALLMODE property, 'you're not supposed to put the "v" in "vomus" on the initial installation. This is because "v" means any time you do a repair operation with the MSI, that the original source media will be required. Users hate that.