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Property collectors determine how the server or agent should find finds a tool. The default property collectors describe the locations where the agent might look to find a tool, so for example the NUnit 2.4.8 property collector tells the agent to look for a file named nunit-console.exe in either %PROGRAMFILES%\NUnit 2.4.8\bin\ or %PROGRAMFILES(x86)%\NUnit 2.4.8\bin\ or on the %PATH%. If it finds the file there it will report a property nunit.2.4.8.path, if doesn't find the file then it will not that property.
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