OutputDebugStringA not working at all

Question

OutputDebugStringA not working at all

Andrew Mercer 546

Neither OutputDebugStringA nor OutputDebugStringB (ODS) are working in my environment, Windows 11, dell intel machine, Visual Studio Community 2022 64-bit. The symptom is that the call is made and returns without error, but the output does not show up in the debug pane of the output window. My objective is to insert ODS calls in a C dll (release config) P/Invoked from a VB forms program (debug or release config), and if running in the VS debugger, I want to see the output. If not in the debugger, I don't care what happens, but best is to noop the call. I don't mind checking this - the C dll can tell if a debugger is attached or not.

After a lot of trial and error with the C dll, I found that the VB program could not successfully use ODS either. A minimal test program exhibiting the problem is shown below. There are 5 Trace.Writeline lines and 4 different hacks at ODS. All that makes it to the output window are the Trace.Writeline calls. Same effect if I run VS as administrator, so I doubt it is a security issue.

So, what I want is to call a function from the dll (not C++, and no clr) that will do what Trace.Writeline actually does, or equivalently, does what OutputDebugStringA is documented to actually do.

Public Class Form1

  Private Declare Sub OutputDebugStringA Lib "kernel32" (ByRef OutputString As Byte)
  Private Declare Sub OutputDebugStringW Lib "kernel32" (ByRef OutputString As Char)

  Private Declare Sub OutputDebugStringS1 Lib "kernel32" Alias "OutputDebugStringW" (ByVal OutputString As String)

  <Runtime.InteropServices.DllImport("kernel32.dll", CharSet:=Runtime.InteropServices.CharSet.Unicode, EntryPoint:="OutputDebugStringW")>
  Private Shared Sub OutputDebugStringS2(ByVal OutputString As String)
  End Sub

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    Dim s As String = "yyyy" & ChrW(0)
    Dim a() As Byte = System.Text.Encoding.ASCII.GetBytes(s) ' s as ascii bytes
    Dim w() As Char = s.ToCharArray ' s as unicode chars
    ' only Trace.Writeline shows up in the debug pane output window
    Trace.WriteLine("xxxx")
    OutputDebugStringA(a(0))
    Trace.WriteLine("xxxx")
    OutputDebugStringW(w(0))
    Trace.WriteLine("xxxx")
    OutputDebugStringS1(s) ' message CA2101, P/Invoke marshaling
    Trace.WriteLine("xxxx")
    OutputDebugStringS2(s)
    Trace.WriteLine("xxxx")
  End Sub

End Class

Answer accepted by question author

1 additional answer

Your answer

Answer 1

Viorel 126.8K

In order to view the output of OutputDebugStringA, try to select the “Enable native code debugging” option in Project Properties (Debug tab). It is available for both .NET and .NET Framework. This seems to work:

<DllImport("kernel32", CharSet:=CharSet.Ansi, EntryPoint:="OutputDebugStringA")>
Private Shared Sub OutputDebugStringA(ByVal OutputString As String)
End Sub

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

   OutputDebugStringA("My Test 1" & vbCrLf)

   Debug.WriteLine("My Test 2") ' works too

End Sub

Should work in case of C DLL too.

If the program is launched without debugging, then the output of OutputDebugStringA can be viewed using the Debug View tool (dbgview64.exe) from Sysinternals Suite, even in case of Release configuration.

Andrew Mercer 546 Reputation points

2026-03-14T20:54:47.84+00:00

It worked as you indicated, and many thanks. Looking for such a thing never occurred to me because on the managed side (VB in my case), Debug.Writeline and Trace.Writeline always worked as advertised.

Answer 2

OutputDebugString is working as designed, but Visual Studio is not the only consumer of that API, and the way it is called from VB in the sample is incorrect.

Key points from the platform behavior:

OutputDebugStringA/W send a string to the debugger of the calling process. If no debugger is attached and no system debugger is active, the call is effectively a no-op.
In user mode, OutputDebugString simply sends a null-terminated string to the debugger; it does not format or buffer like printf.
If there is no debugger, and the system debugger is not active, OutputDebugString does nothing.

Relevant details:

The official prototype is:
```
  VOID OutputDebugString(
     LPCTSTR lpOutputString
  );
```
which is a pointer to a null-terminated string.
In VB, the P/Invoke must pass a pointer to a null-terminated string, not a single Byte or Char by reference.

In the VB sample:

Private Declare Sub OutputDebugStringA Lib "kernel32" (ByRef OutputString As Byte)
Private Declare Sub OutputDebugStringW Lib "kernel32" (ByRef OutputString As Char)

These signatures pass a single element (a(0) or w(0)) by reference, which does not match the expected LPSTR/LPWSTR pointer to a null-terminated string. The marshaler is not providing a proper C-style string pointer, so the native function does not receive valid text to send to the debugger.

The correct approach in VB is to declare the function with a String parameter and let the runtime marshal it as a null-terminated ANSI or Unicode string, for example:

<DllImport("kernel32.dll", CharSet:=CharSet.Ansi, EntryPoint:="OutputDebugStringA", _
           SetLastError:=False)>
Private Shared Sub OutputDebugStringA(ByVal lpOutputString As String)
End Sub

<DllImport("kernel32.dll", CharSet:=CharSet.Unicode, EntryPoint:="OutputDebugStringW", _
           SetLastError:=False)>
Private Shared Sub OutputDebugStringW(ByVal lpOutputString As String)
End Sub

Then call:

OutputDebugStringA("Test from ANSI")
OutputDebugStringW("Test from Unicode")

With a debugger attached to the VB process, these strings are sent as an OUTPUT_DEBUG_STRING_EVENT to the debugger and should appear in the debugger’s output view (subject to how the specific Visual Studio version chooses to display them).

For the C DLL scenario:

In the DLL, call OutputDebugStringA/OutputDebugStringW directly with a valid null-terminated string.
If the DLL should only emit output when a debugger is attached, call IsDebuggerPresent first and skip the call when it returns false.

If Visual Studio is not attached, but debug output still needs to be captured, a tool such as DebugView can be used. DebugView captures:

Win32 OutputDebugString
Kernel-mode DbgPrint and its variants

This allows viewing debug output without a debugger attached.

References:

Andrew Mercer 546 Reputation points

2026-03-14T14:03:03.1866667+00:00

edit - I meant OutputDebugStringW, not OutputDebugStringB early in my question.
Andrew Mercer 546 Reputation points

2026-03-14T14:16:10.9566667+00:00

Tried two variants (below) of the AI suggestion, neither worked. Result is the same as all other tries - call, successful return, data is not visible anywhere.

<Runtime.InteropServices.DllImport("kernel32.dll", CharSet:=Runtime.InteropServices.CharSet.Unicode, EntryPoint:="OutputDebugStringW", SetLastError:=False)>

Private Shared Sub OutputDebugStringS3(ByVal lpOutputString As String)

End Sub

<Runtime.InteropServices.DllImport("kernel32.dll", CharSet:=Runtime.InteropServices.CharSet.Ansi, EntryPoint:="OutputDebugStringA", SetLastError:=False)>

Private Shared Sub OutputDebugStringS4(ByVal lpOutputString As String)

End Sub
Andrew Mercer 546 Reputation points

2026-03-14T22:52:40.7333333+00:00

My parting shot on this issue... AI assist missed the point, Viorel had the answer. I had already tried everything AI suggested (AI had no way of knowing that). AI was long and wordy. I'm guessing AI did not know the secret sauce of enabling native code debugging. Issue is closed as far as I'm concerned.

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OutputDebugStringA not working at all

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