How do I set the TARGETDIR programtaically?

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C#

Visual-Studio

msi

I am creating an MSI package for my application. The requirement is such that if application A is stalled at client machine, this new set up should install in that application folder. Otherwise it should create its own folder.

I have created custom installer. I have overriden the methods like OnInsall, OnBeforeInstall etc.
But no effect on TARGETDIR.

Even though I write this :
this.Context.Parameters["TARGETDIR"] = new_path;

It still installs it to oldpath which is set as DefaultLocation property of ApplicationFolder Tab of set up project.

Please send the resolution asap.