C# Signature:

[DllImport("kernel32.dll")]
static extern uint FormatMessage(uint dwFlags, IntPtr lpSource,
   uint dwMessageId, uint dwLanguageId, [Out] StringBuilder lpBuffer,
   uint nSize, IntPtr Arguments);

// the version, the sample is built upon:
[DllImport("Kernel32.dll", SetLastError=true)]
static extern uint FormatMessage( uint dwFlags, IntPtr lpSource,
   uint dwMessageId, uint dwLanguageId, ref IntPtr lpBuffer,
   uint nSize, IntPtr pArguments);

// the parameters can also be passed as a string array:
[DllImport("Kernel32.dll", SetLastError=true)]
static extern uint FormatMessage( uint dwFlags, IntPtr lpSource,
   uint dwMessageId, uint dwLanguageId, ref IntPtr lpBuffer,
   uint nSize, string[] Arguments);

// see the sample code
[DllImport("kernel32.dll", SetLastError = true)]
static extern uint FormatMessage(uint dwFlags, IntPtr lpSource, uint dwMessageId, uint dwLanguageId, [Out] StringBuilder lpBuffer, uint nSize, string[] Arguments);

VB.NET Signature:

<DllImport("Kernel32.dll", EntryPoint:="FormatMessageW", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)> _
Public Shared Function FormatMessage(ByVal dwFlags As Integer, ByRef lpSource As IntPtr, ByVal dwMessageId As Integer, ByVal dwLanguageId As Integer, ByRef lpBuffer As [String], ByVal nSize As Integer, ByRef Arguments As IntPtr) As Integer
End Function

VB.NET Signature:

<DllImport("Kernel32.dll", EntryPoint:="FormatMessageW", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)> _
Public Shared Function FormatMessage(ByVal dwFlags As Integer, ByVal lpSource As IntPtr, ByVal dwMessageId As Integer, ByVal dwLanguageId As Integer, <MarshalAs(UnmanageType.LPWStr)> ByRef lpBuffer As String, ByVal nSize As Integer, ByVal Arguments As IntPtr) As Integer
End Function

VB.NET Signature:

<DllImport("Kernel32.dll", EntryPoint:="FormatMessageW", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)> _
Public Shared Function FormatMessage(ByVal dwFlags As Integer, ByVal lpSource As IntPtr, ByVal dwMessageId As Integer, ByVal dwLanguageId As Integer, ByRef lpBuffer As StringBuilder, ByVal nSize As Integer, ByVal Arguments As IntPtr) As Integer
End Function

User-Defined Types:

None.

Notes:

To have FormatMessage substitute %1, %2, etc. with arguments you pass it, declare FormatMessage with a string[] for the last parameter and just pass a normal C# string array. You need to include FORMAT_MESSAGE_ARGUMENT_ARRAY in the dwFlags parameter for this to work.

Tips & Tricks:

If you are trying use FormatMessage to "decode" an error from Marshal.GetLastWin32Error, go take a look at Marshal.ThrowExceptionForHR (http://msdn.microsoft.com/en-us/library/ckxcyk36(v=VS.80).aspx).

ThrowExceptionForHR is for HRESULT values returned directly from NT functions, is it not?

For Win32Errors you most likely want new Win32Exception() (http://msdn.microsoft.com/en-us/library/asy2azk0.aspx) which gives you a ready made message for the error code returned by Marshal.GetLastWin32Error().

Sample Code:

    // from header files
    const uint FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100;
    const uint FORMAT_MESSAGE_IGNORE_INSERTS  = 0x00000200;
    const uint FORMAT_MESSAGE_FROM_SYSTEM    = 0x00001000;
    const uint FORMAT_MESSAGE_ARGUMENT_ARRAY = 0x00002000;
    const uint FORMAT_MESSAGE_FROM_HMODULE = 0x00000800;
    const uint FORMAT_MESSAGE_FROM_STRING = 0x00000400;

    int nLastError= Marshal.GetLastWin32Error();

    IntPtr lpMsgBuf= IntPtr.Zero;

    uint dwChars= FormatMessage(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
        IntPtr.Zero,
        (uint)nLastError,
        0, // Default language
        ref lpMsgBuf,
        0,
        IntPtr.Zero);
    if (dwChars==0)
    {
        // Handle the error.
        int le= Marshal.GetLastWin32Error();
        return null;
    }

    string sRet= Marshal.PtrToStringAnsi(lpMsgBuf);

    // Free the buffer.
    lpMsgBuf= LocalFree( lpMsgBuf );
    return sRet;

    // add for the forth signature
    try
    {
        StringBuilder msgBuilder = new StringBuilder(101);

        string formatExpression = "%1,%2%!";
        string[] formatArgs = new string[] { "Hello", "world" };

        IntPtr formatPtr = Marshal.StringToHGlobalAnsi(formatExpression);

        try
        {
            //must specify the FORMAT_MESSAGE_ARGUMENT_ARRAY flag when pass an array
            uint length = FormatMessage(FORMAT_MESSAGE_FROM_STRING | FORMAT_MESSAGE_ARGUMENT_ARRAY, formatPtr, 0, 0, msgBuilder, 101, formatArgs);

            if (length == 0)
            {
                FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, IntPtr.Zero, (uint)Marshal.GetLastWin32Error(), 0, msgBuilder, 101, null);

                Console.WriteLine("Error:" + msgBuilder.ToString());
            }
            else
            {
                Console.WriteLine("Format result:" + msgBuilder.ToString() + ", length:" + length.ToString());
            }
        }
        finally
        {
            Marshal.FreeHGlobal(formatPtr);
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }

Alternative Managed API:

This functionality is also given by System.ComponentModel.Win32Exception:

string errorMessage = new Win32Exception(Marshal.GetLastWin32Error()).Message;
Console.WriteLine(errorMessage);

Yet the sample might still be useful for messages not provided by the system.

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