C++.NET equivalent code of C#.NET GrayScale code

[tlemcenvisit]

Hello
I translated this code (witch convert an image to grayscale) from C#.NET to
C++.NET
The C#.NET code is :

private void GrayScale(Bitma p b)
{
BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width,
b.Height),Image LockMode.ReadWr ite,PixelFormat .Format24bppRgb );
int stride = bmData.Stride;
System.IntPtr Scan0 = bmData.Scan0;
unsafe
{
byte* p = (byte*)(void*)S can0;
int nOffset = stride - b.Width * 3;
byte red, green, blue;
for (int y = 0; y < b.Height; ++y)
{
for (int x = 0; x < b.Width; ++x)
{
blue = p[0];
green = p[1];
red = p[2];
p[0] = p[1] = p[2] = (byte)(.299 * red + .587 * green + .114 * blue);
p += 3;
}
p += nOffset;
}
}
b.UnlockBits(bm Data);
}
(code of Christian Graus)

And the C++.NET code is:

private: System::Void GrayScale(Bitma p ^b)
{
BitmapData ^bmData = b->LockBits(*ne w Rectangle(0, 0, b->Width,
b->Height),ImageL ockMode::ReadWr ite,PixelFormat ::Format24bppRg b);
int stride = bmData->Stride;
System::IntPtr Scan0 = bmData->Scan0;
unsigned char* p = (unsigned char*)(void*)Sc an0;
int nOffset = stride - b->Width * 3;
unsigned char red, green, blue;
for (int y = 0; y < b->Height; ++y)
{
for (int x = 0; x < b->Width; ++x)
{
blue = p[0];
green = p[1];
red = p[2];
p[0] = p[1] = p[2] = (unsigned char)(.299 * red + .587 * green + .114 *
blue);
p += 3;
}
p += nOffset;
}
b->UnlockBits(bmD ata);
}

The problem is that the C++.NETcode take about 10 seconds in execution
and the C#.NET take about only 1 second !
Please, What shall I do to correct this problem?