Visual Basic On Error Resume Next behavior inside async methods

Behavior of On Error Resume Next in Async Contexts

On Error Resume Next does not suppress exceptions thrown inside an Async method. While it can intercept synchronous runtime errors occurring within the same method body before an Await point, it cannot swallow exceptions that propagate from an asynchronous operation.

Exception Propagation and Task State

When an Async method encounters an exception, the .NET runtime captures that exception and stores it within the returned Task object. Because the exception is wrapped in the Task, On Error Resume Next is bypassed. The behavior is as follows:

• Awaited Tasks: If the caller Awaits the task, the exception is re-thrown at the point of the Await. On Error Resume Next in the calling method will not catch this exception.
• Unawaited Tasks: If the task is not awaited (fire-and-forget), the exception remains stored in the Task.Exception property. It will not be suppressed by On Error Resume Next, but it may remain unobserved until the Task is garbage collected, potentially triggering the UnobservedTaskException event.

Compile-Time Errors and Option Explicit

On Error Resume Next is a runtime directive and has no impact on the compilation process. When Option Explicit is enabled, the compiler enforces variable declarations regardless of any error-handling statements. Undeclared identifiers within Async methods will still trigger compile-time errors.

Verification Steps

To verify this behavior in your environment (compatible with .NET Framework 4.5+ and .NET Core/5+), you can use the following scoped test:

Option Explicit On

Async Function TestAsync() As Task
    Throw New Exception("Async Error")
End Function

Async Sub RunTest()
    On Error Resume Next
    ' This will NOT swallow the exception upon await
    Await TestAsync()
    Console.WriteLine("This line may not be reached.")
End Sub

Summary Table

Scenario	Effect of On Error Resume Next	Result
Synchronous Error	Active	Execution continues to next line
Async Exception (Awaited)	Inactive	Exception propagates to caller
Undeclared Variable	Inactive	Compile-time error (Option Explicit)

Diagnostic Note: To provide a more specific recommendation, please clarify if you are using Async Sub (event handlers) or Async Function (Task-returning), as Async Sub handles exceptions differently by re-throwing them on the synchronization context.