If you are using Visual Studio Online and setup your project using TFS doing a code review is straightforward in visual studio.

However ff you are using Visual Studio Online and setup your project Git you might wonder how you can conduct a code review. You may do a quick search and then try ‘new pull request’ . You might get a weird error message you don’t understand.

TF401189: The source branch has been modified since the last merge attempt

Then you do another search (bing or google) you won’t find anything useful.

After trial and error I found that the root cause is that I didn’t use a topic branch. It is that simple.

Two useful links as below

https://www.visualstudio.com/get-started/code/git/pullrequest
https://blogs.msdn.microsoft.com/visualstudioalm/2014/06/10/conduct-a-git-pull-request-on-visual-studio-online/ (unfortunately all the images are missing)