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Add comprehensive examples to BC42328.md to improve clarity
Co-authored-by: BillWagner <[email protected]>
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‎docs/visual-basic/misc/bc42328.md‎

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@@ -24,29 +24,46 @@ Visual Basic allows you to assign methods to delegates even when their signature
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## When this warning occurs
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This warning appears when Visual Basic can handle the assignment through relaxed conversion, making the explicit `AddressOf` unnecessary. For example:
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This warning appears when Visual Basic can handle the assignment through relaxed conversion, making the explicit `AddressOf` unnecessary.
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```vb
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' Event delegate expects (sender As Object, e As EventArgs)
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Public Event SomeEvent As EventHandler
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' Method with exact signature - no warning
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Sub Handler1(sender As Object, e As EventArgs)
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' Implementation
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End Sub
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' Method with omitted parameters - causes BC42328
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Sub Handler2()
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' Implementation
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End Sub
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### Complete example showing the warning
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' Using AddHandler with exact match - no warning
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AddHandler SomeEvent, AddressOf Handler1
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Here's a realistic scenario where you'll encounter BC42328:
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' Using AddHandler with relaxed conversion - causes BC42328
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AddHandler SomeEvent, AddressOf Handler2
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```vb
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Public Class FileProcessor
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' Define an event using the standard EventHandler delegate
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' EventHandler expects: Sub(sender As Object, e As EventArgs)
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Public Event ProcessingComplete As EventHandler
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' Method with exact signature - no warning
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Private Sub OnProcessingComplete(sender As Object, e As EventArgs)
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Console.WriteLine($"Processing completed by {sender}")
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End Sub
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' Method with simplified signature - this causes BC42328 warning
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Private Sub OnSimpleComplete()
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Console.WriteLine("Processing is done!")
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End Sub
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Public Sub StartProcessing()
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' This works fine - exact signature match
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AddHandler ProcessingComplete, AddressOf OnProcessingComplete
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' This generates BC42328 warning:
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' "The 'AddressOf' expression has no effect in this context because
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' the method argument to 'AddressOf' requires a relaxed conversion
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' to the delegate type of the event"
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AddHandler ProcessingComplete, AddressOf OnSimpleComplete
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' Simulate completion
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RaiseEvent ProcessingComplete(Me, EventArgs.Empty)
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End Sub
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End Class
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```
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When you compile this code, Visual Basic shows the BC42328 warning for the second `AddHandler` statement because `OnSimpleComplete` doesn't match the `EventHandler` signature exactly.
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## Why AddressOf appears to have "no effect"
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The warning message states that `AddressOf` "has no effect" because Visual Basic must create a wrapper delegate for the relaxed conversion regardless. The `AddressOf` operator doesn't change how the compiler handles the conversion: the same result occurs whether you use `AddressOf` or not in relaxed conversion scenarios.
@@ -57,36 +74,163 @@ However, `AddressOf` is still syntactically required in `AddHandler` and `Remove
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## To correct this error
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You have several options depending on your needs:
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You have several options depending on your needs. Using the `FileProcessor` example from above, here are the different ways to resolve the BC42328 warning:
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- **Assign to a variable first (preserves exact semantics):**
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### Option 1: Assign to a variable first (preserves exact semantics)
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```vb
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Dim handler As EventHandler = AddressOf Handler2
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AddHandler SomeEvent, handler
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```
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```vb
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Public Sub StartProcessing()
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' Create delegate variable first - this eliminates the warning
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Dim handler As EventHandler = AddressOf OnSimpleComplete
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AddHandler ProcessingComplete, handler
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End Sub
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```
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- **Match the delegate signature exactly:**
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### Option 2: Match the delegate signature exactly
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```vb
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Sub Handler2(sender As Object, e As EventArgs)
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' Implementation - use parameters or ignore them
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End Sub
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```
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```vb
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' Change the method signature to match EventHandler exactly
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Private Sub OnSimpleComplete(sender As Object, e As EventArgs)
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' You can ignore the parameters if you don't need them
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Console.WriteLine("Processing is done!")
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End Sub
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- **Use a lambda expression:**
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Public Sub StartProcessing()
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' Now this works without warning
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AddHandler ProcessingComplete, AddressOf OnSimpleComplete
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End Sub
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```
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```vb
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AddHandler SomeEvent, Sub() Handler2()
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```
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### Option 3: Use a lambda expression
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- **Use the `Handles` clause if appropriate:**
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```vb
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Public Sub StartProcessing()
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' Wrap the method call in a lambda that matches the signature
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AddHandler ProcessingComplete, Sub(sender, e) OnSimpleComplete()
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' Or create a more complex lambda inline
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AddHandler ProcessingComplete,
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Sub(sender, e)
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Console.WriteLine($"Event from {sender}")
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OnSimpleComplete()
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End Sub
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End Sub
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```
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```vb
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Sub Handler2() Handles someObject.SomeEvent
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' Implementation
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End Sub
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```
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### Option 4: Use the `Handles` clause (if appropriate)
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```vb
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Public Class FileProcessor
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' Declare the event source with WithEvents
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Private WithEvents processor As FileProcessor
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' Use Handles clause - no AddHandler needed
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Private Sub OnSimpleComplete() Handles processor.ProcessingComplete
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Console.WriteLine("Processing is done!")
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End Sub
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End Class
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```
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### Option 5: Direct assignment to delegate variable
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```vb
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Public Sub StartProcessing()
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' Assign to delegate variable then use with AddHandler
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Dim handler As EventHandler = AddressOf OnSimpleComplete
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AddHandler ProcessingComplete, handler
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End Sub
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```
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## Complete working example
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Here's a full example demonstrating the warning and multiple solutions:
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```vb
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Imports System
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Public Class DocumentProcessor
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' Standard .NET event using EventHandler
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Public Event DocumentProcessed As EventHandler
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' Custom event with different signature
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Public Event StatusChanged As Action(Of String)
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' Handlers with different signatures
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' Exact match for EventHandler - no warning
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Private Sub OnDocumentProcessed_Exact(sender As Object, e As EventArgs)
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Console.WriteLine("Document processed (exact signature)")
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End Sub
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' Simplified handler - causes BC42328 with EventHandler
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Private Sub OnDocumentProcessed_Simple()
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Console.WriteLine("Document processed (simple)")
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End Sub
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' Handler for custom event - exact match
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Private Sub OnStatusChanged_Exact(message As String)
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Console.WriteLine($"Status: {message}")
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End Sub
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' Handler with ignored parameters - causes BC42328 with custom event
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Private Sub OnStatusChanged_Simple()
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Console.WriteLine("Status changed")
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End Sub
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Public Sub DemonstrateWarnings()
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Console.WriteLine("Setting up event handlers...")
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' These work without warnings (exact matches)
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AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Exact
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AddHandler StatusChanged, AddressOf OnStatusChanged_Exact
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' These generate BC42328 warnings (relaxed conversions)
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AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Simple
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AddHandler StatusChanged, AddressOf OnStatusChanged_Simple
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' Fire the events
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RaiseEvent DocumentProcessed(Me, EventArgs.Empty)
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RaiseEvent StatusChanged("Processing complete")
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End Sub
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Public Sub DemonstrateSolutions()
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Console.WriteLine("Using solutions to avoid warnings...")
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' Solution 1: Assign to variable first
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Dim handler1 As EventHandler = AddressOf OnDocumentProcessed_Simple
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AddHandler DocumentProcessed, handler1
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' Solution 2: Use lambda expression
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AddHandler DocumentProcessed, Sub(s, e) OnDocumentProcessed_Simple()
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' Solution 3: Direct assignment to delegate variable
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Dim handler2 As Action(Of String) = AddressOf OnStatusChanged_Simple
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AddHandler StatusChanged, handler2
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' Fire the events
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RaiseEvent DocumentProcessed(Me, EventArgs.Empty)
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RaiseEvent StatusChanged("All solutions work")
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End Sub
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End Class
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Module Program
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Sub Main()
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Dim processor As New DocumentProcessor()
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Console.WriteLine("=== Demonstrating BC42328 Warning ===")
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processor.DemonstrateWarnings()
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Console.WriteLine()
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Console.WriteLine("=== Demonstrating Solutions ===")
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processor.DemonstrateSolutions()
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Console.WriteLine("Press any key to exit...")
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Console.ReadKey()
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End Sub
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End Module
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```
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This example shows how the warning occurs in realistic scenarios and demonstrates multiple ways to resolve it.
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## When you can ignore this warning
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- Your handler method intentionally omits parameters it doesn't need.
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- You're using a simpler method signature for cleaner code.
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- The performance impact is negligible for your application.
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- You're prototyping or writing quick utility code where clarity matters more than efficiency.
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The relaxed conversion works correctly; this warning just indicates a minor efficiency consideration.
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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.
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### Example of when ignoring is appropriate
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```vb
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Public Class QuickLogger
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Public Event LogMessage As Action(Of String, DateTime)
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' Simple handler that ignores the timestamp parameter
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Private Sub WriteToConsole()
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Console.WriteLine("Something was logged")
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End Sub
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Public Sub SetupLogging()
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' This generates BC42328, but it's fine for simple scenarios
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AddHandler LogMessage, AddressOf WriteToConsole
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End Sub
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End Class
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```
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## See also
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‎docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vb‎

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Imports System
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Public Class DocumentProcessor
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' Standard .NET event using EventHandler
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Public Event DocumentProcessed As EventHandler
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' Custom event with different signature
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Public Event StatusChanged As Action(Of String)
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' Handlers with different signatures
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' Exact match for EventHandler - no warning
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Private Sub OnDocumentProcessed_Exact(sender As Object, e As EventArgs)
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Console.WriteLine("Document processed (exact signature)")
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End Sub
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' Simplified handler - causes BC42328 with EventHandler
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Private Sub OnDocumentProcessed_Simple()
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Console.WriteLine("Document processed (simple)")
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End Sub
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' Handler for custom event - exact match
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Private Sub OnStatusChanged_Exact(message As String)
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Console.WriteLine($"Status: {message}")
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End Sub
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' Handler with ignored parameters - causes BC42328 with custom event
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Private Sub OnStatusChanged_Simple()
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Console.WriteLine("Status changed")
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End Sub
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Public Sub DemonstrateWarnings()
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Console.WriteLine("Setting up event handlers...")
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' These work without warnings (exact matches)
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AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Exact
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AddHandler StatusChanged, AddressOf OnStatusChanged_Exact
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' These generate BC42328 warnings (relaxed conversions)
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AddHandler DocumentProcessed, AddressOf OnDocumentProcessed_Simple
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AddHandler StatusChanged, AddressOf OnStatusChanged_Simple
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' Fire the events
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RaiseEvent DocumentProcessed(Me, EventArgs.Empty)
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RaiseEvent StatusChanged("Processing complete")
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End Sub
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Public Sub DemonstrateSolutions()
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Console.WriteLine("Using solutions to avoid warnings...")
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' Solution 1: Assign to variable first
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Dim handler1 As EventHandler = AddressOf OnDocumentProcessed_Simple
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AddHandler DocumentProcessed, handler1
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' Solution 2: Use lambda expression
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AddHandler DocumentProcessed, Sub(s, e) OnDocumentProcessed_Simple()
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' Solution 3: Direct assignment to delegate variable
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Dim handler2 As Action(Of String) = AddressOf OnStatusChanged_Simple
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AddHandler StatusChanged, handler2
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' Fire the events
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RaiseEvent DocumentProcessed(Me, EventArgs.Empty)
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RaiseEvent StatusChanged("All solutions work")
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End Sub
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End Class
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Module Program
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Sub Main()
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Dim processor As New DocumentProcessor()
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Console.WriteLine("=== Demonstrating BC42328 Warning ===")
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processor.DemonstrateWarnings()
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Console.WriteLine()
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Console.WriteLine("=== Demonstrating Solutions ===")
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processor.DemonstrateSolutions()
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Console.WriteLine("Press any key to exit...")
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Console.ReadKey()
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End Sub
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End Module

‎docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vbproj‎

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net8.0</TargetFramework>
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<RootNamespace>BC42328Example</RootNamespace>
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</PropertyGroup>
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</Project>

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