如何在PDFSharp中遍历Pdf对象树

marihanzo在PDFSharp论坛上发表的这篇文章对我们有用：

http://forum.pdfsharp.net/viewtopic.php?f=2&t=527&p=1603

我们遇到的唯一问题是处理字段中包含的字段。这是一份代码副本，以防论坛帖子丢失。

PDFParser.cs

public class PDFParser
{
    /// BT = Beginning of a text object operator
    /// ET = End of a text object operator
    /// Td move to the start of next line
    ///  5 Ts = superscript
    /// -5 Ts = subscript
    #region Fields
    #region _numberOfCharsToKeep
    /// <summary>
    /// The number of characters to keep, when extracting text.
    /// </summary>
    private static int _numberOfCharsToKeep = 15;
    #endregion
    #endregion

    #region ExtractTextFromPDFBytes
    /// <summary>
    /// This method processes an uncompressed Adobe (text) object
    /// and extracts text.
    /// </summary>
    /// <param name="input">uncompressed</param>
    /// <returns></returns>
    public string ExtractTextFromPDFBytes(byte[] input)
    {
        if (input == null || input.Length == 0) return "";
        try
        {
            string resultString = "";
            // Flag showing if we are we currently inside a text object
            bool inTextObject = false;
            // Flag showing if the next character is literal
            // e.g. '''' to get a ''' character or ''(' to get '('
            bool nextLiteral = false;
            // () Bracket nesting level. Text appears inside ()
            int bracketDepth = 0;
            // Keep previous chars to get extract numbers etc.:
            char[] previousCharacters = new char[_numberOfCharsToKeep];
            for (int j = 0; j < _numberOfCharsToKeep; j++) previousCharacters[j] = ' ';

            for (int i = 0; i < input.Length; i++)
            {
                char c = (char)input[i];
                if (inTextObject)
                {
                    // Position the text
                    if (bracketDepth == 0)
                    {
                        if (CheckToken(new string[] { "TD", "Td" }, previousCharacters))
                        {
                            resultString += "'n'r";
                        }
                        else
                        {
                            if (CheckToken(new string[] { "'", "T*", "'"" }, previousCharacters))
                            {
                                resultString += "'n";
                            }
                            else
                            {
                                if (CheckToken(new string[] { "Tj" }, previousCharacters))
                                {
                                    resultString += " ";
                                }
                            }
                        }
                    }
                    // End of a text object, also go to a new line.
                    if (bracketDepth == 0 &&
                        CheckToken(new string[] { "ET" }, previousCharacters))
                    {
                        inTextObject = false;
                        resultString += " ";
                    }
                    else
                    {
                        // Start outputting text
                        if ((c == '(') && (bracketDepth == 0) && (!nextLiteral))
                        {
                            bracketDepth = 1;
                        }
                        else
                        {
                            // Stop outputting text
                            if ((c == ')') && (bracketDepth == 1) && (!nextLiteral))
                            {
                                bracketDepth = 0;
                            }
                            else
                            {
                                // Just a normal text character:
                                if (bracketDepth == 1)
                                {
                                    // Only print out next character no matter what.
                                    // Do not interpret.
                                    if (c == '''' && !nextLiteral)
                                    {
                                        nextLiteral = true;
                                    }
                                    else
                                    {
                                        if (((c >= ' ') && (c <= '~')) ||
                                            ((c >= 128) && (c < 255)))
                                        {
                                            resultString += c.ToString();
                                        }
                                        nextLiteral = false;
                                    }
                                }
                            }
                        }
                    }
                }
                // Store the recent characters for
                // when we have to go back for a checking
                for (int j = 0; j < _numberOfCharsToKeep - 1; j++)
                {
                    previousCharacters[j] = previousCharacters[j + 1];
                }
                previousCharacters[_numberOfCharsToKeep - 1] = c;
                // Start of a text object
                if (!inTextObject && CheckToken(new string[] { "BT" }, previousCharacters))
                {
                    inTextObject = true;
                }
            }
            return resultString;
        }
        catch
        {
            return "";
        }
    }
    #endregion
    #region CheckToken
    /// <summary>
    /// Check if a certain 2 character token just came along (e.g. BT)
    /// </summary>
    /// <param name="search">the searched token</param>
    /// <param name="recent">the recent character array</param>
    /// <returns></returns>
    private bool CheckToken(string[] tokens, char[] recent)
    {
        foreach (string token in tokens)
        {
            if (token.Length > 1)
            {
                if ((recent[_numberOfCharsToKeep - 3] == token[0]) &&
                    (recent[_numberOfCharsToKeep - 2] == token[1]) &&
                    ((recent[_numberOfCharsToKeep - 1] == ' ') ||
                    (recent[_numberOfCharsToKeep - 1] == 0x0d) ||
                    (recent[_numberOfCharsToKeep - 1] == 0x0a)) &&
                    ((recent[_numberOfCharsToKeep - 4] == ' ') ||
                    (recent[_numberOfCharsToKeep - 4] == 0x0d) ||
                    (recent[_numberOfCharsToKeep - 4] == 0x0a))
                    )
                {
                    return true;
                }
            }
            else
            {
                return false;
            }
        }
        return false;
    }
    #endregion
}

呼叫代码：

   public override String ExtractText()
    {
        String outputText = "";
        try
        {
            PdfDocument inputDocument = PdfReader.Open(this._sDirectory + this._sFileName, PdfDocumentOpenMode.ReadOnly);
            foreach (PdfPage page in inputDocument.Pages)
            {
                for (int index = 0; index < page.Contents.Elements.Count; index++)
                {
                    PdfDictionary.PdfStream stream = page.Contents.Elements.GetDictionary(index).Stream;
                    outputText += new PDFParser().ExtractTextFromPDFBytes(stream.Value);
                }
            }
        }
        catch (Exception e)
        {
            PDF_ParseException oEx = new PDF_ParseException(this, e);
            oEx.Log();
            oEx.ToPdf(this._sDirectoryException);
        }
        return outputText;
    }

读取并分析整个集合，并构建自己的内存树。然后走那棵树。