.NET Memory does not free even after exiting a function

Question

I have C # code that reads a huge file , and after some manipulations sets its link to null and exits the function, but the memory is not freed.

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml(xmlString);
XmlService.ConvertExcelToXML(xmlDoc);
int sdfid = 320;
XmlService.CompareXML(xmlDoc, ref sdfid, pkid);
xmlDoc.RemoveAll();
xmlDoc = null;

xmlDoc- A very large string, usually around 50 MB. When I exit the function, this memory is constantly occupied, and I have to restart my service a couple of times a day, otherwise its memory usage will reach 1 GB.

I tried using GC.Collect but did not use.

Thanks in advance.

Edit

Here is the class declaration for XmlService. It has no variables. All methods are static.

 public class XmlService

ConvertExcelToXML Function Code

public static bool ConvertExcelToXML(XmlDocument xmlDoc) {
        XmlNamespaceManager nm = new XmlNamespaceManager(xmlDoc.NameTable);
        nm.AddNamespace("z", "urn:schemas-microsoft-com:office:spreadsheet");
        nm.AddNamespace("o", "urn:schemas-microsoft-com:office:office");
        nm.AddNamespace("x", "urn:schemas-microsoft-com:office:excel");
        nm.AddNamespace("ss", "urn:schemas-microsoft-com:office:spreadsheet");
        nm.AddNamespace("html", "http://www.w3.org/TR/REC-html40");
        XmlNodeList rows = xmlDoc.DocumentElement.SelectNodes("//z:Worksheet/z:Table/z:Row", nm);
        if (rows != null && rows.Count > 0)
        {
            XmlNode nodeNames = rows[0];
            XmlNode nodeValues = rows[1];

            XmlNode destRootNode = xmlDoc.CreateNode(XmlNodeType.Element, "ParentNode", null);
            XmlNode fieldNode = null;
            XmlNode dataNode = null;
                for (int i = 0; i < nodeNames.ChildNodes.Count; i++)
                {
                    if (nodeNames.ChildNodes[i].HasChildNodes)
                    {
                        string nodeName = nodeNames.ChildNodes[i].ChildNodes[0].InnerXml;
                        //string nodeValue = nodeValues.ChildNodes[i].ChildNodes[0].InnerXml;
                        string nodeValue = "DataField" + i.ToString();

                        fieldNode = xmlDoc.CreateNode(XmlNodeType.Element, "Field", null);
                        dataNode = xmlDoc.CreateNode(XmlNodeType.Element, "Data", null);
                        dataNode.InnerXml = nodeName;
                        fieldNode.AppendChild(dataNode);
                        destRootNode.AppendChild(fieldNode);

                        fieldNode = xmlDoc.CreateNode(XmlNodeType.Element, "Field", null);
                        dataNode = xmlDoc.CreateNode(XmlNodeType.Element, "Data", null);
                        dataNode.InnerXml = nodeValue;
                        fieldNode.AppendChild(dataNode);
                        destRootNode.AppendChild(fieldNode);
                    }
                }
            xmlDoc.LoadXml("<ParentNode>" + destRootNode.InnerXml + "</ParentNode>");
            return true;
            } 
            return false;

}

and code for CompareXML

        public static void CompareXML(XmlDocument filexmlDoc, ref int maxSDFID, string PKID)
    {
        FieldsListBO tmpFieldListBO = null;

        ResponseDTO responseDTO = DbService.getConnection();
        DbConnection con = (DbConnection)responseDTO.ReturnedObjects[Constants.CONNECTION_OBJECT];
        DbProviderFactory factory = (DbProviderFactory)responseDTO.ReturnedObjects[Constants.FACTORY_OBJECT];
        DbCommand cmd = factory.CreateCommand();

        cmd.CommandText = "select * from tree_store";
        cmd.Connection = con;
        con.Open();

        DbDataReader dr = cmd.ExecuteReader();
        dr.Read();
        String pXmlizedString = (String)dr["TransactionTree"];
        dr.Dispose();
        cmd.Dispose();
        con.Dispose();
        XmlSerializer xs = new XmlSerializer(typeof(FieldsListBO));
        MemoryStream memoryStream = new MemoryStream(StringToUTF8ByteArray(pXmlizedString));
        XmlTextWriter xmlTextWriter = new XmlTextWriter(memoryStream, Encoding.ASCII);
        tmpFieldListBO = (FieldsListBO)xs.Deserialize(memoryStream);
        memoryStream.Dispose();
        xmlTextWriter.Close();
        if (tmpFieldListBO.FieldsList.Count < 1)
        {
            maxSDFID = 0;
            return;
        }

        FieldsListBO fieldListBO = new FieldsListBO();

        for (int i = 0; i < tmpFieldListBO.FieldsList.Count; i++)
        {
            if (tmpFieldListBO.FieldsList[i]._pkid.Equals(PKID))
            {
                fieldListBO.FieldsList.Add(tmpFieldListBO.FieldsList[i]);
            }
        }

        GetMaxSDFID(filexmlDoc, ref maxSDFID, fieldListBO);
    }

filexmlDoc passed to GetMaxSDFID just transforms node to node update / delation not performed

Answer 1

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, - , . GC.Collect , . , .

(, , XmlService.ConvertExcelToXML ), , , , . ProcDump, .

WinDbg. !dumpheap –stat, , 1 .

Answer 2

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, , , null, XmlService , RemoveAll.

, XmlDocument , . xmlString , - , , LOH . , .

XML ? (, )?