c#: how to read parts of a file? (DICOM)

Something like this should read the file, its basic and doesn't handle all cases, but it would be a starting point:

public void ReadFile(string filename)
{
    using (FileStream fs = File.OpenRead(filename))
    {
        fs.Seek(128, SeekOrigin.Begin);
        if ((fs.ReadByte() != (byte)'D' ||
             fs.ReadByte() != (byte)'I' ||
             fs.ReadByte() != (byte)'C' ||
             fs.ReadByte() != (byte)'M'))
        {
            Console.WriteLine("Not a DCM");
            return;
        }
        BinaryReader reader = new BinaryReader(fs);

        ushort g;
        ushort e;
        do
        {
            g = reader.ReadUInt16();
            e = reader.ReadUInt16();

            string vr = new string(reader.ReadChars(2));
            long length;
            if (vr.Equals("AE") || vr.Equals("AS") || vr.Equals("AT")
                || vr.Equals("CS") || vr.Equals("DA") || vr.Equals("DS")
                || vr.Equals("DT") || vr.Equals("FL") || vr.Equals("FD")
                || vr.Equals("IS") || vr.Equals("LO") || vr.Equals("PN")
                || vr.Equals("SH") || vr.Equals("SL") || vr.Equals("SS")
                || vr.Equals("ST") || vr.Equals("TM") || vr.Equals("UI")
                || vr.Equals("UL") || vr.Equals("US"))
               length = reader.ReadUInt16();
            else
            {
                // Read the reserved byte
                reader.ReadUInt16();
                length = reader.ReadUInt32();
            }

            byte[] val = reader.ReadBytes((int) length);

        } while (g == 2);

        fs.Close();
    }

    return ;
}

The code does not actually try and take into account that the transfer syntax of the encoded data can change after the group 2 elements, it also doesn't try and do anything with the actual values read in.

Just some pseudologic

How to I read the header file and verify if it is a DICOM file by checking for the 'D','I','C','M' characters after the 128 byte preamble?

• Open as binary file, using File.OpenRead
• Seek to position 128 and read 4 bytes into the array and compare it againts byte[] value for DICM. You can use ASCIIEncoding.GetBytes() for that

How do I continue to parse the file reading the other parts of the data?

• Continue reading the file using Read or ReadByte using the FileStream object handle that you have earlier
• Use the same method like above to do your comparison.

Dont forget to close and dispose the file.