Dotcover coverage of xx.exe in the test source

In the .net C# program we want to use test coverage of exe file by using the test source of C#(Nunit3).

Coverage of the test source is normal and the exe file is successful excuted. but ,it wasn't able to make the coverage of exe file. 

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In the test.dll there is calling of XXX.exe ,and below command of dotcover is excuted.

>dotcover.exe cover --targetExecutable="C:\Program Files\NUnit 2.6\bin\nunit-console.exe" --targetArguments="C:\test\test.dll /xml="C:\Coverage\Cover.xml" --output=CoverageReport.html --reportType=HTML

please help me ,thank you very much.