I solved this issue by disabled all of the below Visual Studio Marketplace Extensions:

• Visual Studio Bitbucket Extension
• Web Compiler

I actually disabled all my extensions and enabled them one-by-one, and the issue was caused by both of these extensions.

Uninstall BitBucket Extension to get your projects working. Create Issues here on their page: https://github.com/MistyKuu/bitbucket-for-visual-studio/issues/64 So they can fix asap.

Hi everyone, and thanks for taking the time to post each one of your experiences here.

We have determined this to be an issue with older versions of the Rx assemblies clashing with the one used in Xamarin bits. So far we have identified at least two extensions that use this: GitHub for VS and BitBucket. If you happen to have one of these installed and you disable it, this should fix the issue.

We are contacting these teams so they can update their Rx bits so this doesn't clash anymore. At the same time, we have updated our code to not use the type that was conflicting so going forward this shouldn't be an issue if these or other extensions happen to bundle Rx assemblies.

If anyone is still experiencing this after disabling the mentioned extensions please do ping me here (or via email VGA at microsoft dot com) so we can properly troubleshoot your install and see if there is another extension using Rx that may be causing this.

thanks,
-VGA.

Remove Github for VS and/or BitBucket, then repair VS, reboot.

Thank you for your feedback! This issue has been fixed in Visual Studio 2017 version 15.7.2. Thank you for helping us build a better Visual Studio!

New update are coming, hope they fixed it.