Bitmap.LockBits Method

private: Bitmap^ bmp;
private: Rectangle rect;
private: array<Byte>^rgbValues;
private: int bytes;
private: Byte shade;
private: IntPtr ptr;
private: System::Drawing::Imaging::BitmapData^ bmpData;
private: System::Void button1_Click(System::Object^ sender, System::EventArgs^ e)
{
    // Write pixel to array. Stride * pixel y, gives us the position of next scanline. 
    for ( int counter = 0; counter < rgbValues->Length; counter += 4 )
    {
            rgbValues[ counter ] = 0; // Blue channel.
            rgbValues[ counter + 1 ] = 0; // Green channel.
            rgbValues[ counter + 2 ] = 255; // Red channel.
            rgbValues[ counter + 3 ] = shade; // Alpha channel ( 255 = solid color, 0 = background colour ).
    }

    // Lock the bitmap's bits.
    bmpData = bmp->LockBits( rect, System::Drawing::Imaging::ImageLockMode::ReadWrite, bmp->PixelFormat );
    // Get the address of the first line.
    // Pointer to the start of bitmap memory. It is FLAT and can be treated as a one-dimensional array.
    ptr = bmpData->Scan0;
    // Copy the ARGB values to the bitmap. Remember it will be BGRA.
    // DWORDS or QWORDS written to the bitmap are alot more efficient. This is from windows MSDN so i will stick with this.
    System::Runtime::InteropServices::Marshal::Copy( rgbValues, 0, ptr, bytes );
    // Unlock the bits.
    bmp->UnlockBits( bmpData );
    // Set the PictureBox to display the image.
    // Not sure this is legal but it allows me to use alpha channel to create volumetric fog / smoke, haze, light for free!
    pictureBox1->Image = bmp;
    // Increase alpha channel by 16 everytime button is pressed.
    shade += 16;
}

private: System::Void Form1_Load(System::Object^ sender, System::EventArgs^ e) {
shade = 0;
// Create a new bitmap.
// Bitmap("filename", true) will load a bitmap file into the memory bitmap.
bmp = gcnew Bitmap( pictureBox1->Width,pictureBox1->Height,System::Drawing::Imaging::PixelFormat::Format32bppArgb );

// Lock the bitmap's bits.
rect = Rectangle(0,0,bmp->Width,bmp->Height);
bmpData = bmp->LockBits( rect, System::Drawing::Imaging::ImageLockMode::ReadWrite, bmp->PixelFormat );
// Declare an array to hold the bytes of the bitmap.
// This code is specific to a bitmap with 32 bits per pixels.
// Always use stride * height, not width * height.
bytes = bmpData->Stride * bmp->Height;
rgbValues = gcnew array<Byte>(bytes);
// Unlock the bits.
bmp->UnlockBits( bmpData );
// Display the pixel format in Label1.
label1->Text = String::Format( "Pixel format: {0}", bmp->PixelFormat );
//See if stride equals 640 * 4 = 2560, can also equal -2560.
numericUpDown1->Value = bmpData->Stride;
//Compare to Width * 4
numericUpDown2->Value = bmp->Width * 4;
//If bmp->Width * 4 is equal to bmpData->Stride then we can write to one-dimensional array.
}

See here for example of using LockBits:

http://msdn.microsoft.com/en-us/library/5ey6h79d.aspx