I've recently started having this issue for the latest version of Xamarin Forms with .NET standard 2.0 as the code sharing strategy.
I am able to place a breakpoint and stop in any code that lives in the UWP project but if I place a breakpoint in the shared DLL code, the breakpoint turns from solid red to a red outlined circle filled in with white. When I put my mouse over the breakpoint, it says the symbols have not been loaded for that location.
What am I doing wrong to be able to debug the shared code for Xamarin Forms UWP? Android and iOS work.
What am I doing wrong to be able to debug the shared code for Xamarin Forms UWP? Android and iOS work.
It's known issue in Visual Studio. Currently, there is a workaround that you could refer.
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
<DebugType>Full</DebugType>
</PropertyGroup>
Place the above code in the .NET Standard Library csproj file.
Select the UWP project as startup project and then open Build menu and choose Configuration Manager. There make sure that the .NET Standard library is selected in the Build column.
If that does not help, delete the bin and obj folder from both shared library and UWP project and try again. Sometimes also uninstalling the UWP app from your PC manually helps.
Not sure if it helps, but under Debug,Options,Debugging (General) I would disable the Enable Just My Code option
来源：https://stackoverflow.com/questions/48374187/xamarin-forms-uwp-unable-to-debug-shared-code