Field xxx is never assigned to, and will always have its default value null

Question

Anyone know whats this problem?

I got this warning Field xxx is never assigned to, and will always have its default value null on private static Quantizer quantit;

I dont know what to do to fix, cause when I try to use quantit.Quantize() debug says: "Object reference not set to an instance of an object." and point to au = quantit.Quantize();

The code:

public class Quantization : System.Windows.Forms.Form
{ 
    private static Quantizer quantit;

    private Button btnLoad;
    private PictureBox imgPhoto;

    public Quantization()
    {

        btnLoad = new Button();
        btnLoad.Text = "&Load";
        btnLoad.Left = 10;
        btnLoad.Top = 10;
        btnLoad.Click += new System.EventHandler(this.OnLoadClick);

        imgPhoto = new PictureBox();
        imgPhoto.BorderStyle = System.Windows.Forms.BorderStyle.Fixed3D;
        imgPhoto.Width = this.Width / 2;
        imgPhoto.Height = this.Height / 2;
        imgPhoto.Left = (this.Width - imgPhoto.Width) / 2;
        imgPhoto.Top = (this.Height - imgPhoto.Height) / 2;
        imgPhoto.SizeMode = PictureBoxSizeMode.StretchImage;

        this.Controls.Add(btnLoad);
        this.Controls.Add(imgPhoto);  
    }

    protected void OnLoadClick(object sender, System.EventArgs e)
    {
      OpenFileDialog dlg = new OpenFileDialog();

      dlg.Title = "Open Image";
      dlg.Filter = "jpg files (*.jpg)|*.jpg|All files (*.*)|*.*" ;

      if (dlg.ShowDialog() == DialogResult.OK)
      {
          Bitmap au;

          //Image bmp = Image.FromFile("D:\\Documents and Settings\\kiosk.suprisul\\My Documents\\foto1.jpg");

          au = quantit.Quantize();
          imgPhoto.Image = au;
          //imgPhoto.Image = bmp;
          //imgPhoto.Image = au;
          //new Bitmap(dlg.OpenFile());
      }

      dlg.Dispose();
    }
    [STAThread]
    static void Main(string[] args)
    {

        //Image bmp;

        //bmp = Image.FromFile("teste.jpg");
        //PaintEventArgs e;
        //teste2.Quantize(bmp);


        Application.Run(new Quantization());

        /*
        System.Console.WriteLine("Hello, World!");
        System.Console.ReadLine();*/

    }
}

The class:

namespace ImageManipulation
{
    public unsafe abstract class Quantizer
    {
        /// <summary>
        /// Construct the quantizer
        /// </summary>
        /// <param name="singlePass">If true, the quantization only needs to loop through the source pixels once</param>
        /// <remarks>
        /// If you construct this class with a true value for singlePass, then the code will, when quantizing your image,
        /// only call the 'QuantizeImage' function. If two passes are required, the code will call 'InitialQuantizeImage'
        /// and then 'QuantizeImage'.
        /// </remarks>
        public Quantizer(bool singlePass)
        {
            _singlePass = singlePass;
        }

        /// <summary>
        /// Quantize an image and return the resulting output bitmap
        /// </summary>
        /// <param name="source">The image to quantize</param>
        /// <returns>A quantized version of the image</returns>
        public Bitmap Quantize()//Image source)
        {
            Image source = Image.FromFile("C:\\Users\\crashboy\\Downloads\\image009.jpg");
            // Get the size of the source image
            int height = source.Height;
            int width = source.Width;

            // And construct a rectangle from these dimensions
            Rectangle bounds = new Rectangle(0, 0, width, height);

            // First off take a 32bpp copy of the image
            Bitmap copy = new Bitmap(width, height, PixelFormat.Format32bppArgb);

            // And construct an 8bpp version
            Bitmap output = new Bitmap(width, height, PixelFormat.Format8bppIndexed);

            // Now lock the bitmap into memory
            using (Graphics g = Graphics.FromImage(copy))
            {
                g.PageUnit = GraphicsUnit.Pixel;

                // Draw the source image onto the copy bitmap,
                // which will effect a widening as appropriate.
                g.DrawImage(source, bounds);
            }

            // Define a pointer to the bitmap data
            BitmapData sourceData = null;

            try
            {
                // Get the source image bits and lock into memory
                sourceData = copy.LockBits(bounds, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

                // Call the FirstPass function if not a single pass algorithm.
                // For something like an octree quantizer, this will run through
                // all image pixels, build a data structure, and create a palette.
                if (!_singlePass)
                    FirstPass(sourceData, width, height);

                // Then set the color palette on the output bitmap. I'm passing in the current palette 
                // as there's no way to construct a new, empty palette.
                output.Palette = this.GetPalette(output.Palette);

                // Then call the second pass which actually does the conversion
                SecondPass(sourceData, output, width, height, bounds);
            }
            finally
            {
                // Ensure that the bits are unlocked
                copy.UnlockBits(sourceData);
            }

            // Last but not least, return the output bitmap
            return output;

        }

        /// <summary>
        /// Execute the first pass through the pixels in the image
        /// </summary>
        /// <param name="sourceData">The source data</param>
        /// <param name="width">The width in pixels of the image</param>
        /// <param name="height">The height in pixels of the image</param>
        protected virtual void FirstPass(BitmapData sourceData, int width, int height)
        {
            // Define the source data pointers. The source row is a byte to
            // keep addition of the stride value easier (as this is in bytes)
            byte* pSourceRow = (byte*)sourceData.Scan0.ToPointer();
            Int32* pSourcePixel;

            // Loop through each row
            for (int row = 0; row < height; row++)
            {
                // Set the source pixel to the first pixel in this row
                pSourcePixel = (Int32*)pSourceRow;

                // And loop through each column
                for (int col = 0; col < width; col++, pSourcePixel++)
                    // Now I have the pixel, call the FirstPassQuantize function...
                    InitialQuantizePixel((Color32*)pSourcePixel);

                // Add the stride to the source row
                pSourceRow += sourceData.Stride;
            }
        }

        /// <summary>
        /// Execute a second pass through the bitmap
        /// </summary>
        /// <param name="sourceData">The source bitmap, locked into memory</param>
        /// <param name="output">The output bitmap</param>
        /// <param name="width">The width in pixels of the image</param>
        /// <param name="height">The height in pixels of the image</param>
        /// <param name="bounds">The bounding rectangle</param>
        protected virtual void SecondPass(BitmapData sourceData, Bitmap output, int width, int height, Rectangle bounds)
        {
            BitmapData outputData = null;

            try
            {
                // Lock the output bitmap into memory
                outputData = output.LockBits(bounds, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

                // Define the source data pointers. The source row is a byte to
                // keep addition of the stride value easier (as this is in bytes)
                byte* pSourceRow = (byte*)sourceData.Scan0.ToPointer();
                Int32* pSourcePixel = (Int32*)pSourceRow;
                Int32* pPreviousPixel = pSourcePixel;

                // Now define the destination data pointers
                byte* pDestinationRow = (byte*)outputData.Scan0.ToPointer();
                byte* pDestinationPixel = pDestinationRow;

                // And convert the first pixel, so that I have values going into the loop
                byte pixelValue = QuantizePixel((Color32*)pSourcePixel);

                // Assign the value of the first pixel
                *pDestinationPixel = pixelValue;

                // Loop through each row
                for (int row = 0; row < height; row++)
                {
                    // Set the source pixel to the first pixel in this row
                    pSourcePixel = (Int32*)pSourceRow;

                    // And set the destination pixel pointer to the first pixel in the row
                    pDestinationPixel = pDestinationRow;

                    // Loop through each pixel on this scan line
                    for (int col = 0; col < width; col++, pSourcePixel++, pDestinationPixel++)
                    {
                        // Check if this is the same as the last pixel. If so use that value
                        // rather than calculating it again. This is an inexpensive optimisation.
                        if (*pPreviousPixel != *pSourcePixel)
                        {
                            // Quantize the pixel
                            pixelValue = QuantizePixel((Color32*)pSourcePixel);

                            // And setup the previous pointer
                            pPreviousPixel = pSourcePixel;
                        }

                        // And set the pixel in the output
                        *pDestinationPixel = pixelValue;
                    }

                    // Add the stride to the source row
                    pSourceRow += sourceData.Stride;

                    // And to the destination row
                    pDestinationRow += outputData.Stride;
                }
            }
            finally
            {
                // Ensure that I unlock the output bits
                output.UnlockBits(outputData);
            }
        }

        /// <summary>
        /// Override this to process the pixel in the first pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <remarks>
        /// This function need only be overridden if your quantize algorithm needs two passes,
        /// such as an Octree quantizer.
        /// </remarks>
        protected virtual void InitialQuantizePixel(Color32* pixel)
        {

        }

        /// <summary>
        /// Override this to process the pixel in the second pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <returns>The quantized value</returns>
        protected abstract byte QuantizePixel(Color32* pixel);

        /// <summary>
        /// Retrieve the palette for the quantized image
        /// </summary>
        /// <param name="original">Any old palette, this is overrwritten</param>
        /// <returns>The new color palette</returns>
        protected abstract ColorPalette GetPalette(ColorPalette original);

        /// <summary>
        /// Flag used to indicate whether a single pass or two passes are needed for quantization.
        /// </summary>
        private bool _singlePass;

        /// <summary>
        /// Struct that defines a 32 bpp colour
        /// </summary>
        /// <remarks>
        /// This struct is used to read data from a 32 bits per pixel image
        /// in memory, and is ordered in this manner as this is the way that
        /// the data is layed out in memory
        /// </remarks>
        [StructLayout(LayoutKind.Explicit)]
        public struct Color32
        {
            /// <summary>
            /// Holds the blue component of the colour
            /// </summary>
            [FieldOffset(0)]
            public byte Blue;
            /// <summary>
            /// Holds the green component of the colour
            /// </summary>
            [FieldOffset(1)]
            public byte Green;
            /// <summary>
            /// Holds the red component of the colour
            /// </summary>
            [FieldOffset(2)]
            public byte Red;
            /// <summary>
            /// Holds the alpha component of the colour
            /// </summary>
            [FieldOffset(3)]
            public byte Alpha;

            /// <summary>
            /// Permits the color32 to be treated as an int32
            /// </summary>
            [FieldOffset(0)]
            public int ARGB;

            /// <summary>
            /// Return the color for this Color32 object
            /// </summary>
            public Color Color
            {
                get { return Color.FromArgb(Alpha, Red, Green, Blue); }
            }
        }
    }
}

Answer 1

The compiler is warning you that quantit is never initialized and will always be null.

You probably should initialize it with an instance of a class that derives from ImageManipulation.Quantizer (you cannot instantiate Quantizer itself because it's an abstract class):

private static Quantizer quantit = new QuantizerImplementation();

Answer 2

You are never assigning an instance of the Quantizer class to your quantit static variable, so it will remain a null reference and will generate that exception when you'll try to use one of its methods. To fix the problem, just initialize that member with a new Quantizer object before using it.

By the way, I'm not sure you want that variable to be static.

Edit

I saw just now that Quantizer is an abstract class... then you can't instantiate it directly, you have first to derive your specific class from it implementing the abstract methods (namely QuantizePixel and GetPalette) or use another ready-made class deriving from Quantizer, and then initialize the quantit field with a new instance of such class.