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124 changes: 83 additions & 41 deletions docs/visual-basic/misc/bc42328.md
Original file line number Diff line number Diff line change
Expand Up @@ -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"

Expand All @@ -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

Expand All @@ -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

Expand Down
84 changes: 84 additions & 0 deletions docs/visual-basic/misc/snippets/bc42328/AddressOfExample.vb
Original file line number Diff line number Diff line change
@@ -0,0 +1,84 @@
Imports System

' <DocumentProcessor>
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
' </DocumentProcessor>

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
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
<Project Sdk="Microsoft.NET.Sdk">

<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net8.0</TargetFramework>
<RootNamespace>BC42328Example</RootNamespace>
</PropertyGroup>

</Project>