Debugging and Diagnostics

Option Strict On

Module UnhandledDemo
    Public Sub Run()
        AddHandler AppDomain.CurrentDomain.UnhandledException,
            Sub(sender As Object, e As UnhandledExceptionEventArgs)
                Console.WriteLine("UnhandledException: " & CType(e.ExceptionObject, Exception).Message & ", terminating=" & e.IsTerminating.ToString())
            End Sub
        Throw New InvalidOperationException("nobody caught this")
    End Sub
End Module

UnhandledException: nobody caught this, terminating=True
Unhandled exception. System.InvalidOperationException: nobody caught this
   at ShelfDocs.UnhandledDemo.Run() in /w/UnhandledDemo.vb:line 9
   at ShelfDocs.Program.Main(String[] args) in /w/Program.vb:line 14

UnhandledException: nobody caught this, terminating=True
Unhandled exception. System.InvalidOperationException: nobody caught this
   at ShelfDocs.UnhandledDemo.Run()
   at ShelfDocs.Program.Main(String[] args)

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <RootNamespace>ShelfDocs</RootNamespace>
    <OptionStrict>On</OptionStrict>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <NuGetAudit>false</NuGetAudit>
  </PropertyGroup>
</Project>

Option Strict On

Module Program
    Sub Main(args As String())
        Dim which As String = If(args.Length > 0, args(0), "")
        Select Case which
            Case "trace" : TraceDemo.Run()
            Case "assert" : AssertDemo.Run()
            Case "assertdefault" : DefaultAssertDemo.Run()
            Case "cond" : ConditionalDemo.Run()
            Case "stop" : StopDemo.Run()
            Case "attrs" : AttrDemo.Run()
            Case "first" : FirstChanceDemo.Run()
            Case "unhandled" : UnhandledDemo.Run()
            Case "metrics" : MetricsDemo.Run()
            Case Else : Console.WriteLine("unknown example")
        End Select
    End Sub
End Module

Option Strict On

Imports System.Diagnostics

Module ConditionalDemo
    Private _calls As Integer

    Private Function Expensive() As String
        _calls += 1
        Return "computed"
    End Function

    <Conditional("VERBOSE")>
    Private Sub Note(message As String)
        Console.WriteLine("note: " & message)
    End Sub

    Public Sub Run()
        Debug.WriteLine(Expensive())
        Trace.WriteLine(Expensive())
        Note(Expensive())
        Console.WriteLine("Expensive() ran " & _calls.ToString() & " time(s)")
    End Sub
End Module

== Debug
Expensive() ran 2 time(s)
== Release
Expensive() ran 1 time(s)
== Release with DefineConstants=VERBOSE
note: computed
Expensive() ran 2 time(s)

Option Strict On

Imports System.Diagnostics

Module StopDemo
    Public Sub Run()
        Console.WriteLine("Debugger.IsAttached = " & Debugger.IsAttached.ToString())
        Console.WriteLine("before Stop")
        Stop
        Console.WriteLine("after Stop")
        Debugger.Break()
        Console.WriteLine("after Debugger.Break")
    End Sub
End Module

Debugger.IsAttached = False
before Stop
after Stop
after Debugger.Break

Option Strict On

Imports System.Diagnostics
Imports System.Reflection

<DebuggerDisplay("{Title} ({Copies} copies)")>
Public Class BookStock
    Public Property Title As String = "Dune"
    Public Property Copies As Integer = 3

    <DebuggerBrowsable(DebuggerBrowsableState.Never)>
    Private ReadOnly _cache As New Dictionary(Of String, String)()

    <DebuggerStepThrough>
    Public Function Total() As Integer
        Return Copies * 2
    End Function

    <DebuggerHidden>
    Public Sub Plumbing()
    End Sub
End Class

<DebuggerTypeProxy(GetType(LoanProxy))>
Public Class Loan
    Public ReadOnly Property Due As Date = New Date(2026, 10, 19)
    Friend ReadOnly Property Note As String = "internal detail"
End Class

Public Class LoanProxy
    Private ReadOnly _loan As Loan

    Public Sub New(loan As Loan)
        _loan = loan
    End Sub

    Public ReadOnly Property DueText As String
        Get
            Return _loan.Due.ToString("yyyy-MM-dd")
        End Get
    End Property
End Class

Module AttrDemo
    Public Sub Run()
        Dim display As DebuggerDisplayAttribute = GetType(BookStock).GetCustomAttribute(Of DebuggerDisplayAttribute)()
        Console.WriteLine("DebuggerDisplay: " & display.Value)

        Dim cache As FieldInfo = GetType(BookStock).GetField("_cache", BindingFlags.Instance Or BindingFlags.NonPublic)
        Console.WriteLine("Browsable state of _cache: " & cache.GetCustomAttribute(Of DebuggerBrowsableAttribute)().State.ToString())

        Dim total As MethodInfo = GetType(BookStock).GetMethod("Total")
        Console.WriteLine("Total has DebuggerStepThrough: " & (total.GetCustomAttribute(Of DebuggerStepThroughAttribute)() IsNot Nothing).ToString())
        Dim plumbing As MethodInfo = GetType(BookStock).GetMethod("Plumbing")
        Console.WriteLine("Plumbing has DebuggerHidden: " & (plumbing.GetCustomAttribute(Of DebuggerHiddenAttribute)() IsNot Nothing).ToString())

        Dim proxy As DebuggerTypeProxyAttribute = GetType(Loan).GetCustomAttribute(Of DebuggerTypeProxyAttribute)()
        Console.WriteLine("Proxy type: " & proxy.ProxyTypeName)
    End Sub
End Module

DebuggerDisplay: {Title} ({Copies} copies)
Browsable state of _cache: Never
Total has DebuggerStepThrough: True
Plumbing has DebuggerHidden: True
Proxy type: ShelfDocs.LoanProxy, Core, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null

Option Strict On

Imports System.Runtime.ExceptionServices

Module FirstChanceDemo
    Private _firstChance As Integer

    Private Function LogAndDecline(ex As Exception) As Boolean
        Console.WriteLine("  filter sees: " & ex.GetType().Name & " from " & ex.TargetSite.Name)
        Return False
    End Function

    Private Sub Inner()
        Throw New FormatException("bad ISBN")
    End Sub

    Private Sub Outer()
        Try
            Inner()
        Catch ex As FormatException When LogAndDecline(ex)
            Console.WriteLine("never reached")
        End Try
    End Sub

    Private Sub Swallowed()
        Try
            Integer.Parse("x")
        Catch ex As FormatException
            ' swallowed on purpose: still a first-chance exception
        End Try
    End Sub

    Public Sub Run()
        AddHandler AppDomain.CurrentDomain.FirstChanceException,
            Sub(sender As Object, e As FirstChanceExceptionEventArgs)
                _firstChance += 1
                Console.WriteLine("first chance #" & _firstChance.ToString() & ": " & e.Exception.GetType().Name)
            End Sub

        Swallowed()
        Swallowed()
        Console.WriteLine("swallowed twice, counted " & _firstChance.ToString())

        Try
            Outer()
        Catch ex As FormatException
            Console.WriteLine("caught outside: " & ex.Message)
            Dim trace As New System.Diagnostics.StackTrace(ex, True)
            Console.WriteLine("top frame: " & trace.GetFrame(0).GetMethod().Name & " line " & trace.GetFrame(0).GetFileLineNumber().ToString())
        End Try
    End Sub
End Module

first chance #1: FormatException
first chance #2: FormatException
swallowed twice, counted 2
first chance #3: FormatException
  filter sees: FormatException from Inner
caught outside: bad ISBN
top frame: Inner line 14

Option Strict On

Imports Microsoft.Extensions.Logging

Public Class LoanService
    Private Shared ReadOnly LoanCreated As Action(Of ILogger, String, String, Exception) =
        LoggerMessage.Define(Of String, String)(LogLevel.Information, New EventId(1001, "LoanCreated"),
                                                "Loan created for {Member} of {Isbn}")

    Private Shared ReadOnly LoanFailed As Action(Of ILogger, String, Exception) =
        LoggerMessage.Define(Of String)(LogLevel.Error, New EventId(1002, "LoanFailed"), "Could not lend {Isbn}")

    Private ReadOnly _logger As ILogger(Of LoanService)

    Public Sub New(logger As ILogger(Of LoanService))
        _logger = logger
    End Sub

    Public Sub Lend(member As String, isbn As String)
        Using _logger.BeginScope("request {RequestId}", "r-7")
            _logger.LogDebug("Checking availability of {Isbn}", isbn)
            If isbn.EndsWith("0", StringComparison.Ordinal) Then
                Try
                    Throw New InvalidOperationException("no copies left")
                Catch ex As InvalidOperationException
                    LoanFailed(_logger, isbn, ex)
                End Try
            Else
                LoanCreated(_logger, member, isbn, Nothing)
            End If
        End Using
    End Sub
End Class

Module Program
    Sub Main()
        Using factory As ILoggerFactory = LoggerFactory.Create(
            Sub(builder As ILoggingBuilder)
                builder.SetMinimumLevel(LogLevel.Debug)
                builder.AddSimpleConsole(Sub(o) o.IncludeScopes = True)
            End Sub)
            Dim service As New LoanService(factory.CreateLogger(Of LoanService)())
            service.Lend("ada", "9780441013593")
            service.Lend("ada", "9780441172710")
        End Using
    End Sub
End Module

dbug: ShelfDocs.LoanService[0]
      => request r-7
      Checking availability of 9780441013593
info: ShelfDocs.LoanService[1001]
      => request r-7
      Loan created for ada of 9780441013593
dbug: ShelfDocs.LoanService[0]
      => request r-7
      Checking availability of 9780441172710
fail: ShelfDocs.LoanService[1002]
      => request r-7
      Could not lend 9780441172710
      System.InvalidOperationException: no copies left
         at ShelfDocs.LoanService.Lend(String member, String isbn) in /w/Program.vb:line 24

Option Strict On

Imports System.Diagnostics.Metrics

Module Program
    Sub Main()
        Dim meter As New Meter("ShelfDocs.Loans")
        Dim created As Counter(Of Integer) = meter.CreateCounter(Of Integer)("shelf.loans.created")

        Using listener As New MeterListener()
            listener.InstrumentPublished =
                Sub(instrument As Instrument, l As MeterListener)
                    l.EnableMeasurementEvents(instrument)
                End Sub
            listener.SetMeasurementEventCallback(Of Integer)(
                Sub(instrument As Instrument, value As Integer, tags As ReadOnlySpan(Of KeyValuePair(Of String, Object)), state As Object)
                    Console.WriteLine(instrument.Name & " += " & value.ToString())
                End Sub)
            listener.Start()
            created.Add(1)
        End Using
    End Sub
End Module

Program.vb(16,73): error BC30668: 'ReadOnlySpan(Of KeyValuePair(Of String, Object))' is obsolete: 'Types with embedded references are not supported in this version of your compiler.'.

Option Strict On

Imports System.Diagnostics
Imports System.Diagnostics.Metrics
Imports System.Diagnostics.Tracing
Imports System.Threading

Public NotInheritable Class MetricsPrinter
    Inherits EventListener

    Protected Overrides Sub OnEventSourceCreated(eventSource As EventSource)
        If eventSource.Name = "System.Diagnostics.Metrics" Then
            Dim args As New Dictionary(Of String, String)() From {
                {"SessionId", "shelf-session"},
                {"Metrics", "ShelfDocs.Loans"},
                {"RefreshInterval", "1"}
            }
            EnableEvents(eventSource, EventLevel.Informational, EventKeywords.All, args)
        End If
    End Sub

    Protected Overrides Sub OnEventWritten(eventData As EventWrittenEventArgs)
        If eventData.EventName = "CounterRateValuePublished" Then
            Dim names As IList(Of String) = eventData.PayloadNames
            Dim values As IList(Of Object) = eventData.Payload
            Console.WriteLine(Convert.ToString(values(names.IndexOf("instrumentName"))) &
                              ": rate=" & Convert.ToString(values(names.IndexOf("rate"))) &
                              " value=" & Convert.ToString(values(names.IndexOf("value"))))
        End If
    End Sub
End Class

Module MetricsDemo
    Public Sub Run()
        Dim meter As New Meter("ShelfDocs.Loans")
        Dim created As Counter(Of Integer) = meter.CreateCounter(Of Integer)("shelf.loans.created", "loans", "Loans created")
        Using printer As New MetricsPrinter()
            created.Add(1)
            created.Add(2)
            Thread.Sleep(2500)
        End Using

        Dim source As New ActivitySource("ShelfDocs.Loans")
        Using al As New ActivityListener()
            al.ShouldListenTo = Function(s As ActivitySource) s.Name = "ShelfDocs.Loans"
            al.Sample = Function(ByRef o As ActivityCreationOptions(Of ActivityContext)) ActivitySamplingResult.AllData
            al.ActivityStopped = Sub(a As Activity) Console.WriteLine("activity " & a.OperationName & " tag loan=" & CStr(a.GetTagItem("loan")))
            ActivitySource.AddActivityListener(al)
            Using a As Activity = source.StartActivity("CreateLoan")
                a.SetTag("loan", "L-42")
            End Using
        End Using
    End Sub
End Module

shelf.loans.created: rate=3 value=3
shelf.loans.created: rate=0 value=3
activity CreateLoan tag loan=L-42

Option Strict On

Imports System.Diagnostics.Metrics
Imports System.Threading

Module Program
    Sub Main()
        Dim meter As New Meter("ShelfDocs.Loans")
        Dim created As Counter(Of Integer) = meter.CreateCounter(Of Integer)("shelf.loans.created", "loans")
        Console.WriteLine("pid " & Environment.ProcessId.ToString())
        For i As Integer = 1 To 120
            created.Add(1)
            Thread.Sleep(100)
        Next
    End Sub
End Module

dotnet-counters collect -p $PID --counters ShelfDocs.Loans,System.Runtime[cpu-time,alloc-rate] --format csv -o /tmp/counters --refresh-interval 1 --duration 00:00:05

Timestamp,Provider,Counter Name,Counter Type,Mean/Increment
10/19/2026 22:29:07,ShelfDocs.Loans,shelf.loans.created (loans / 1 sec),Rate,9
10/19/2026 22:29:07,System.Runtime,Allocation Rate (B / 1 sec),Rate,343016
10/19/2026 22:29:08,System.Runtime,Allocation Rate (B / 1 sec),Rate,40488
10/19/2026 22:29:08,ShelfDocs.Loans,shelf.loans.created (loans / 1 sec),Rate,10
10/19/2026 22:29:09,ShelfDocs.Loans,shelf.loans.created (loans / 1 sec),Rate,10

Thread (0xC5):
  [Native Frames]
  System.Private.CoreLib!System.Threading.Thread.Sleep(int32)
  Ctr!ShelfDocs.Program.Main()

Option Strict On

Imports System.Text
Imports BenchmarkDotNet.Attributes
Imports BenchmarkDotNet.Running

<MemoryDiagnoser>
<ShortRunJob>
Public Class JoinBenchmarks
    Private ReadOnly _titles As String() = Enumerable.Range(1, 100).Select(Function(i) "Title " & i.ToString()).ToArray()

    <Benchmark(Baseline:=True)>
    Public Function Concatenate() As String
        Dim result As String = ""
        For Each t As String In _titles
            result = result & t & ";"
        Next
        Return result
    End Function

    <Benchmark>
    Public Function StringBuilderAppend() As String
        Dim sb As New StringBuilder()
        For Each t As String In _titles
            sb.Append(t).Append(";"c)
        Next
        Return sb.ToString()
    End Function

    <Benchmark>
    Public Function StringJoin() As String
        Return String.Join(";", _titles) & ";"
    End Function
End Class

Module Program
    Sub Main(args As String())
        BenchmarkRunner.Run(Of JoinBenchmarks)()
    End Sub
End Module

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.5 LTS (Noble Numbat) (container)
.NET SDK 10.0.402
  [Host]   : .NET 10.0.13 (10.0.13, 10.0.1326.50504), Arm64 RyuJIT armv8.0-a
  ShortRun : .NET 10.0.13 (10.0.13, 10.0.1326.50504), Arm64 RyuJIT armv8.0-a

Job=ShortRun  IterationCount=3  LaunchCount=1
WarmupCount=3

| Method              | Mean       | Error     | StdDev   | Ratio | RatioSD | Gen0    | Allocated | Alloc Ratio |
|-------------------- |-----------:|----------:|---------:|------:|--------:|--------:|----------:|------------:|
| Concatenate         | 3,749.6 ns | 906.20 ns | 49.67 ns |  1.00 |    0.02 | 21.9154 |  89.52 KB |        1.00 |
| StringBuilderAppend |   388.5 ns |  66.67 ns |  3.65 ns |  0.10 |    0.00 |  1.0424 |   4.26 KB |        0.05 |
| StringJoin          |   333.7 ns |  47.94 ns |  2.63 ns |  0.09 |    0.00 |  0.8640 |   3.53 KB |        0.04 |

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Launch VB program",
      "type": "coreclr",
      "request": "launch",
      "preLaunchTask": "build",
      "program": "${workspaceFolder}/bin/Debug/net10.0/Core.dll",
      "args": [ "first" ],
      "cwd": "${workspaceFolder}",
      "console": "internalConsole",
      "stopAtEntry": false
    }
  ]
}