Не получается разобраться с исключениями - C#

[SecuritySafeCritical]
internal void Init(string path, FileMode mode, FileAccess access, int rights, bool useRights, FileShare share, int bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, string msgPath, bool bFromProxy, bool useLongPath)
{
    int num1;
    if (path == null)
    {
        throw new ArgumentNullException("path", Environment.GetResourceString("ArgumentNull_Path"));
    }
    if (path.Length == 0)
    {
        throw new ArgumentException(Environment.GetResourceString("Argument_EmptyPath"));
    }
    FileSystemRights fileSystemRight = rights;
    this._fileName = msgPath;
    this._exposedHandle = false;
    FileShare fileShare = share & (FileShare.Read | FileShare.Write | FileShare.ReadWrite | FileShare.Delete);
    string str1 = null;
    if (mode < 1 || mode > 6)
    {
        str1 = "mode";
    }
    if (str1 != null)
    {
        throw new ArgumentOutOfRangeException(str1, Environment.GetResourceString("ArgumentOutOfRange_Enum"));
    }
    if (options != FileOptions.None && (options & 67092479) != FileOptions.None)
    {
        throw new ArgumentOutOfRangeException("options", Environment.GetResourceString("ArgumentOutOfRange_Enum"));
    }
    if (bufferSize <= 0)
    {
        throw new ArgumentOutOfRangeException("bufferSize", Environment.GetResourceString("ArgumentOutOfRange_NeedPosNum"));
    }
    if ((!useRights && (access & FileAccess.Write) == 0 || useRights && (fileSystemRight & FileSystemRights.Write) == 0) && (mode == FileMode.Truncate || mode == FileMode.CreateNew || mode == FileMode.Create) || mode == FileMode.Append)
    {
        if (!useRights)
        {
            throw new ArgumentException(Environment.GetResourceString(object[] objArray1[1] = access, objArray1));
        }
        throw new ArgumentException(Environment.GetResourceString(object[] objArray2[1] = fileSystemRight, objArray2));
    }
    if (useRights && mode == FileMode.Truncate)
    {
        if (fileSystemRight == FileSystemRights.Write)
        {
            useRights = 0;
            access = FileAccess.Write;
        }
        throw new ArgumentException(Environment.GetResourceString(object[] objArray3[1] = fileSystemRight, objArray3));
    }
    if (!useRights)
    {
        num1 = access == FileAccess.Read;
    }
    else
    {
        num1 = rights;
    }
    int num2 = (useLongPath ? Path.MaxLongPath : Path.MaxPath);
    string str2 = Path.NormalizePath(path, true, num2);
    this._fileName = str2;
    if (str2.StartsWith("\\\\.\\", StringComparison.Ordinal))
    {
        throw new ArgumentException(Environment.GetResourceString("Arg_DevicesNotSupported"));
    }
    FileIOPermissionAccess fileIOPermissionAccess = FileIOPermissionAccess.NoAccess;
    if (!useRights && (access & FileAccess.Read) != 0 || useRights && (fileSystemRight & FileSystemRights.ReadAndExecute) != 0)
    {
        if (mode == FileMode.Append)
        {
            throw new ArgumentException(Environment.GetResourceString("Argument_InvalidAppendMode"));
        }
        fileIOPermissionAccess = fileIOPermissionAccess | FileIOPermissionAccess.Read;
    }
    if ((useRights || (access & FileAccess.Write) == 0) && (useRights || (fileSystemRight & (FileSystemRights.WriteData | FileSystemRights.CreateFiles | FileSystemRights.AppendData | FileSystemRights.CreateDirectories | FileSystemRights.WriteExtendedAttributes | FileSystemRights.DeleteSubdirectoriesAndFiles | FileSystemRights.WriteAttributes | FileSystemRights.Delete | FileSystemRights.ChangePermissions | FileSystemRights.TakeOwnership | FileSystemRights.Write)) == 0) && (useRights || (fileSystemRight & FileSystemRights.Synchronize) == 0 || mode != FileMode.OpenOrCreate) || mode == FileMode.Append)
    {
        fileIOPermissionAccess = fileIOPermissionAccess | FileIOPermissionAccess.Append;
    }
    AccessControlActions accessControlAction = ((secAttrs == null ? AccessControlActions.None : secAttrs.pSecurityDescriptor != (UIntPtr)0) ? AccessControlActions.Change : AccessControlActions.None);
    new FileIOPermission(string[] strArrays1 = new string[1], strArrays1[0] = str2, strArrays1, false, false).Demand();
    share = share & (FileShare.Read | FileShare.Write | FileShare.ReadWrite | FileShare.Delete);
    if (mode == FileMode.Append)
    {
        mode = FileMode.OpenOrCreate;
    }
    if (FileStream._canUseAsync)
    {
        if ((options & FileOptions.Asynchronous) != FileOptions.None)
        {
            this._isAsync = true;
        }
        else
        {
            options = options & (FileOptions.WriteThrough | FileOptions.RandomAccess | FileOptions.DeleteOnClose | FileOptions.SequentialScan | FileOptions.Encrypted);
        }
    }
    int num3 = options;
    num3 = num3 | 1048576;
    int num4 = Win32Native.SetErrorMode(1);
    try
    {
        string str3 = str2;
        if (useLongPath)
        {
            str3 = Path.AddLongPathPrefix(str3);
        }
        this._handle = Win32Native.SafeCreateFile(str3, num1, share, secAttrs, mode, num3, Win32Native.NULL);
        if (this._handle.IsInvalid)
        {
            int lastWin32Error = Marshal.GetLastWin32Error();
            if (lastWin32Error == 3 && str2.Equals(Directory.InternalGetDirectoryRoot(str2)))
            {
                lastWin32Error = 5;
            }
            bool flag1 = false;
            if (!bFromProxy)
            {
                new FileIOPermission(string[] strArrays2[0] = this._fileName, strArrays2, false, false).Demand();
                flag1 = true;
            }
            try
            {
            }
            catch (SecurityException)
            {
            }
            if (flag1)
            {
                __Error.WinIOError(lastWin32Error, this._fileName);
            }
            else
            {
                __Error.WinIOError(lastWin32Error, msgPath);
            }
        }
    }
    finally
    {
        Win32Native.SetErrorMode(num4);
    }
    int fileType = Win32Native.GetFileType(this._handle);
    if (fileType != 1)
    {
        this._handle.Close();
        throw new NotSupportedException(Environment.GetResourceString("NotSupported_FileStreamOnNonFiles"));
    }
    if (this._isAsync)
    {
        bool flag2 = false;
        new SecurityPermission(SecurityPermissionFlag.UnmanagedCode).Assert();
        try
        {
            flag2 = ThreadPool.BindHandle(this._handle);
        }
        finally
        {
            CodeAccessPermission.RevertAssert();
            if (!flag2)
            {
                this._handle.Close();
            }
        }
        if (!flag2)
        {
            throw new IOException(Environment.GetResourceString("IO.IO_BindHandleFailed"));
        }
    }
    if (!useRights)
    {
        this._canRead = (access & FileAccess.Read) != 0;
        this._canWrite = (access & FileAccess.Write) != 0;
    }
    else
    {
        this._canRead = (fileSystemRight & FileSystemRights.ReadData) != 0;
        this._canWrite = (fileSystemRight & FileSystemRights.WriteData ? FileSystemRights.ReadData : (fileSystemRight & FileSystemRights.AppendData) != 0);
    }
    this._canSeek = true;
    this._isPipe = false;
    this._pos = (long)0;
    this._bufferSize = bufferSize;
    this._readPos = 0;
    this._readLen = 0;
    this._writePos = 0;
    if (mode == FileMode.Append)
    {
        this._appendStart = this.SeekCore((long)0, SeekOrigin.End);
        return;
    }
    this._appendStart = (long)-1;
}