diff --git a/docs/visual-basic/misc/bc42328.md b/docs/visual-basic/misc/bc42328.md index 89ba3c35ddf91..d5d3e0072b052 100644 --- a/docs/visual-basic/misc/bc42328.md +++ b/docs/visual-basic/misc/bc42328.md @@ -24,28 +24,15 @@ Visual Basic allows you to assign methods to delegates even when their signature ## When this warning occurs -This warning appears when Visual Basic can handle the assignment through relaxed conversion, making the explicit `AddressOf` unnecessary. For example: +This warning appears when Visual Basic can handle the assignment through relaxed conversion, making the explicit `AddressOf` unnecessary. -```vb -' Event delegate expects (sender As Object, e As EventArgs) -Public Event SomeEvent As EventHandler +### Complete example showing the warning -' Method with exact signature - no warning -Sub Handler1(sender As Object, e As EventArgs) - ' Implementation -End Sub +The following example shows one scenario that produces BC42328: -' Method with omitted parameters - causes BC42328 -Sub Handler2() - ' Implementation -End Sub +:::code language="vb" source="snippets/bc42328/AddressOfExample.vb" id="DocumentProcessor"::: -' Using AddHandler with exact match - no warning -AddHandler SomeEvent, AddressOf Handler1 - -' Using AddHandler with relaxed conversion - causes BC42328 -AddHandler SomeEvent, AddressOf Handler2 -``` +When you compile this code, Visual Basic shows the BC42328 warning for the `AddHandler` statements that use relaxed delegate conversion. ## Why AddressOf appears to have "no effect" @@ -57,36 +44,72 @@ However, `AddressOf` is still syntactically required in `AddHandler` and `Remove ## To correct this error -You have several options depending on your needs: +You have several options depending on your needs. Using the `DocumentProcessor` example from previously, here are the different ways to resolve the BC42328 warning: -- **Assign to a variable first (preserves exact semantics):** +### Option 1: Assign to a variable first (preserves exact semantics) - ```vb - Dim handler As EventHandler = AddressOf Handler2 - AddHandler SomeEvent, handler - ``` +```vb +Public Sub DemonstrateHandler() + ' Create delegate variable first - this eliminates the warning + Dim handler As EventHandler = AddressOf OnDocumentProcessed_Simple + AddHandler DocumentProcessed, handler +End Sub +``` -- **Match the delegate signature exactly:** +### Option 2: Match the delegate signature exactly - ```vb - Sub Handler2(sender As Object, e As EventArgs) - ' Implementation - use parameters or ignore them - End Sub - ``` +```vb +' Change the method signature to match EventHandler exactly +Private Sub OnDocumentProcessed_Simple(sender As Object, e As EventArgs) + ' You can ignore the parameters if you don't need them + Console.WriteLine("Document processed (simple)") +End Sub -- **Use a lambda expression:** +Public Sub DemonstrateHandler() + ' Now this works without warning + AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Simple +End Sub +``` - ```vb - AddHandler SomeEvent, Sub() Handler2() - ``` +### Option 3: Use a lambda expression -- **Use the `Handles` clause if appropriate:** +```vb +Public Sub DemonstrateHandler() + ' Wrap the method call in a lambda that matches the signature + AddHandler DocumentProcessed, Sub(sender, e) OnDocumentProcessed_Simple() + + ' Or create a more complex lambda inline + AddHandler DocumentProcessed, + Sub(sender, e) + Console.WriteLine($"Event from {sender}") + OnDocumentProcessed_Simple() + End Sub +End Sub +``` + +### Option 4: Use the `Handles` clause (if appropriate) - ```vb - Sub Handler2() Handles someObject.SomeEvent - ' Implementation - End Sub - ``` +```vb +Public Class DocumentHandler + ' Declare the event source with WithEvents + Private WithEvents processor As DocumentProcessor + + ' Use Handles clause - no AddHandler needed + Private Sub OnDocumentComplete() Handles processor.DocumentProcessed + Console.WriteLine("Document processing is done!") + End Sub +End Class +``` + +### Option 5: Direct assignment to delegate variable + +```vb +Public Sub DemonstrateHandler() + ' Assign to delegate variable then use with AddHandler + Dim handler As EventHandler = AddressOf OnDocumentProcessed_Simple + AddHandler DocumentProcessed, handler +End Sub +``` ## When you can ignore this warning @@ -95,8 +118,27 @@ This warning is often safe to ignore when: - Your handler method intentionally omits parameters it doesn't need. - You're using a simpler method signature for cleaner code. - The performance impact is negligible for your application. +- You're prototyping or writing quick utility code where clarity matters more than efficiency. + +The relaxed conversion works correctly; this warning just indicates a minor efficiency consideration. The compiler creates a wrapper delegate automatically, which adds a small performance overhead but doesn't affect functionality. -The relaxed conversion works correctly; this warning just indicates a minor efficiency consideration. +### Example of when ignoring is appropriate + +```vb +Public Class QuickLogger + Public Event LogMessage As Action(Of String, DateTime) + + ' Simple handler that ignores the timestamp parameter + Private Sub WriteToConsole() + Console.WriteLine("Something was logged") + End Sub + + Public Sub SetupLogging() + ' This generates BC42328, but it's fine for simple scenarios + AddHandler LogMessage, AddressOf WriteToConsole + End Sub +End Class +``` ## See also diff --git a/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vb b/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vb new file mode 100644 index 0000000000000..39ae891067d4f --- /dev/null +++ b/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vb @@ -0,0 +1,84 @@ +Imports System + +' +Public Class DocumentProcessor + ' Standard .NET event using EventHandler + Public Event DocumentProcessed As EventHandler + + ' Custom event with different signature + Public Event StatusChanged As Action(Of String) + + ' Handlers with different signatures + + ' Exact match for EventHandler - no warning + Private Sub OnDocumentProcessed_Exact(sender As Object, e As EventArgs) + Console.WriteLine("Document processed (exact signature)") + End Sub + + ' Simplified handler - causes BC42328 with EventHandler + Private Sub OnDocumentProcessed_Simple() + Console.WriteLine("Document processed (simple)") + End Sub + + ' Handler for custom event - exact match + Private Sub OnStatusChanged_Exact(message As String) + Console.WriteLine($"Status: {message}") + End Sub + + ' Handler with ignored parameters - causes BC42328 with custom event + Private Sub OnStatusChanged_Simple() + Console.WriteLine("Status changed") + End Sub + + Public Sub DemonstrateWarnings() + Console.WriteLine("Setting up event handlers...") + + ' These work without warnings (exact matches) + AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Exact + AddHandler StatusChanged, AddressOf OnStatusChanged_Exact + + ' These generate BC42328 warnings (relaxed conversions) + AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Simple + AddHandler StatusChanged, AddressOf OnStatusChanged_Simple + + ' Fire the events + RaiseEvent DocumentProcessed(Me, EventArgs.Empty) + RaiseEvent StatusChanged("Processing complete") + End Sub + + Public Sub DemonstrateSolutions() + Console.WriteLine("Using solutions to avoid warnings...") + + ' Solution 1: Assign to variable first + Dim handler1 As EventHandler = AddressOf OnDocumentProcessed_Simple + AddHandler DocumentProcessed, handler1 + + ' Solution 2: Use lambda expression + AddHandler DocumentProcessed, Sub(s, e) OnDocumentProcessed_Simple() + + ' Solution 3: Direct assignment to delegate variable + Dim handler2 As Action(Of String) = AddressOf OnStatusChanged_Simple + AddHandler StatusChanged, handler2 + + ' Fire the events + RaiseEvent DocumentProcessed(Me, EventArgs.Empty) + RaiseEvent StatusChanged("All solutions work") + End Sub +End Class +' + +Module Program + Sub Main() + Dim processor As New DocumentProcessor() + + Console.WriteLine("=== Demonstrating BC42328 Warning ===") + processor.DemonstrateWarnings() + + Console.WriteLine() + Console.WriteLine("=== Demonstrating Solutions ===") + processor.DemonstrateSolutions() + + Console.WriteLine("Press any key to exit...") + Console.ReadKey() + End Sub +End Module \ No newline at end of file diff --git a/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vbproj b/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vbproj new file mode 100644 index 0000000000000..5a06255081c8d --- /dev/null +++ b/docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vbproj @@ -0,0 +1,9 @@ + + + + Exe + net8.0 + BC42328Example + + + \ No newline at end of file