Sorry, that didn’t work – Debugging AL from VS Code

Sorry that didn't work
So you’ve installed your NAV 2018 developer environment, fired up VS Code, pressed F5 (publish and debug), the Web Client starts to load and…

Sorry, that didn’t work
An error has occurred

If you look in the Windows Event Viewer and find the message below, read on for the solution:

Server instance: DynamicsNAV110
Tenant ID:
<ii>User: BLANKED\Dan.Kinsella
Type: System.InvalidOperationException
Message: <ii>Dynamic operations can only be performed in homogenous AppDomain.</ii>
at System.Runtime.CompilerServices.CallSiteBinder.BindCore[T](CallSite`1 site, Object[] args)
at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
at Microsoft.Dynamics.Nav.Service.Dev.Web.Controllers.DebuggerHub.OnDebuggeeConnected(Object sender, DebugSessionStartEventArgs eventArgs)
at Microsoft.Dynamics.Nav.Runtime.Debugger.DebugRuntimeManager.OnDebuggingSessionStarted(DebugSessionStartEventArgs e)
at Microsoft.Dynamics.Nav.Runtime.Debugger.DebugRuntimeManager.CreateRuntime(String debuggingContext, NavSession session, Boolean waitForDebugAdapterConfiguration)
at Microsoft.Dynamics.Nav.Runtime.NavSession.StartDebuggingSession(String debuggingContext, Boolean waitForDebugAdapterConfiguration)
at Microsoft.Dynamics.Nav.Service.NSService.OpenConnection(ConnectionRequest connectionRequest)
at SyncInvokeOpenConnection(Object , Object[] , Object[] )
at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
at Microsoft.Dynamics.Nav.Service.ServiceOperationInvoker.ErrorMappingCombinator(ServiceOperation innerOperation, NSServiceBase serviceInstance, MethodBase syncMethod, Object[] inputs, Object[]& outputs)
Source: System.Core
HResult: -2146233079

The solution

To enable AL debugging from VS Code we need to change the NetFx40_LegacySecurityPolicy setting in the Microsoft.Dynamics.Nav.Server.exe.config file to false.

The Microsoft.Dynamics.Nav.Server.exe.config file is found in the Dynamics NAV service install directory, which (if you accepted the default) will be in the following location:

C:\Program Files\Microsoft Dynamics NAV\110\Service\Microsoft.Dynamics.Nav.Server.exe.config

Search the config file for the NetFx40_LegacySecurityPolicy setting, and change the enabled property to false:

<NetFx40_LegacySecurityPolicy enabled="false"/>
After restarting the service tier, try again and the Web Client should now open with a debug session.

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