The other evening I was playing around with an ASP.NET Core application. When I started debugging it, I immediately saw an error message pop up that said 

Unable to start program 'http://localhost:59676/'. An operation is not legal in the current state.

It was a bit of a head scratcher, so I decided to restart the debug session and see if the problem resolved itself. This didn't help and I saw the same error again.

I also saw that my Chrome browser had started up, but it wasn't showing my web page. It was stuck on a notification page telling me that JavaScript debugging is now enabled in Visual Studio 2017 for Chrome.

This got me thinking.... if it is a feature, it must be able to be switched off.

Disabling JavaScript debugging

I then went to Tools > Options. In the search box, I typed javascript.

The option Enable JavaScript debugging for ASP.NET (Chrome and IE) was checked. I went ahead and unchecked this and clicked on the OK button. I then started another debug session.

Sure enough, the error message was gone.

Not a fix

While I agree that this isn't a fix, but more of a workaround... there is an investigation into the issue at the moment. Have a look at Chrome debugging crashes VS with "An operation is not legal in the current state." on the Developer Community Forum. Client-side debugging of web applications in Chrome is really a very nice feature and something I will definitely use. For now however, it seems that for my ASP.NET Core application I will have to wait a bit longer.

To read up more on debugging web applications client-side in Visual Studio 2017, have a look at an article introducing the feature: Client-side debugging of ASP.NET projects in Google Chrome