🚀 go-pugleaf

>Public Function GetLongName(FName As String) As String
>
>    Dim IpFileName      As String * 255
>    Dim nBufferLength   As Long
>    Dim IpBuffer        As String * 255
>    Dim IpFilePart      As Long
>    Dim ret             As Long
>
>    IpFileName = ""
>    nBufferLength = 50
>    IpFilePart = 1
>    IpBuffer = FName
>
>    ret = GetFullPathName(IpFileName, nBufferLength, IpBuffer, IpFilePart)
>
>    MsgBox IpFileName
>    MsgBox nBufferLength
>    MsgBox IpBuffer
>    MsgBox IpFilePart
>
>End Function

パラメータが違いますね
下のコードで試してみてください

Declare Function GetFullPathName Lib "kernel32" Alias "GetFullPathNameA"
(ByVal lpFileName As String, ByVal nBufferLength As Long, ByVal lpBuffer As
String, ByVal lpFilePart As String) As Long

Public Function GetLongName(FName As String) As String

    Dim IpFileName      As String
    Dim nBufferLength   As Long
    Dim IpBuffer        As String
    Dim IpFilePart      As String
    Dim ret             As Long

    IpFileName = FName                     '対象ファイル
    IpBuffer = String$(255,CHR$(0))      'バッファー初期化
    nBufferLength = Len(IpBuffer)        'バッファーサイズ設定

    ret = GetFullPathName(IpFileName, nBufferLength, IpBuffer, IpFilePart)

    If ret<>0 then
        lpBuffer=Left$(lpBuffer,ret)
        MsgBox IpFileName
        MsgBox nBufferLength
        MsgBox IpBuffer
        MsgBox IpFilePart
    Else
        MsgBox "エラー"
    End If
End Function