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I have a list which has 25000 data and I am iterating in all data and in each iteration, I am searching data in another data table. my whole routine is taking a long time to finish.

private void UpdateCommentFirst(string strCommentPath, string TickerName)

 int counter = 0;
 bool QcViewAllFileExist = false;
 bool QcCommentFileExist = false;
 bool AllowUpdate = false;
 string savepath = Convert.ToString(ConfigurationManager.AppSettings["OutputPath"]).Trim() + TickerName + "\" + TickerName + "_QC-ViewwAll.xml";
 DataSet QCCommentstmp = new DataSet();
 DataSet QCViewAlltmp = new DataSet();

 if (File.Exists(strCommentPath))
 
 QCCommentstmp.ReadXml(strCommentPath);
 QcCommentFileExist = true;
 

 if (File.Exists(savepath))
 
 QCViewAlltmp.ReadXml(savepath);
 QcViewAllFileExist = true;
 

 if (QcCommentFileExist && QcViewAllFileExist)
 
 if (QCCommentstmp.Tables.Count > 0)
 
 if (!QCCommentstmp.Tables[0].Columns.Contains("IgnoreData"))
 
 AllowUpdate = true;
 
 

 if (AllowUpdate)
 

 List<clsCommentPopup> QCCommentlist = QCCommentstmp.Tables[0].AsEnumerable()
 .Select(row => new clsCommentPopup
 
 //BrokerFor,Formula,LineItem,Section,PeriodCollection
 bolFollowUP = (row.Field<string>("FollowUP")) == null ? false : Convert.ToBoolean((row.Field<string>("FollowUP"))),
 bolThisPeriod = (row.Field<string>("ThisPeriod")) == null ? false : Convert.ToBoolean((row.Field<string>("ThisPeriod"))),
 Formula = (row.Field<string>("Formula")) == null ? string.Empty : (row.Field<string>("Formula")),
 ModelValue = (row.Field<string>("ModelValue")) == null ? string.Empty : (row.Field<string>("ModelValue")),
 ExternalComment = (row.Field<string>("ExternalComment")) == null ? string.Empty : (row.Field<string>("ExternalComment")),
 InternalComment = (row.Field<string>("InternalComment")) == null ? string.Empty : (row.Field<string>("InternalComment")),
 strEndPeriod = (row.Field<string>("EndPeriod")) == null ? string.Empty : (row.Field<string>("EndPeriod")),
 strStartPeriod = (row.Field<string>("StartPeriod")) == null ? string.Empty : (row.Field<string>("StartPeriod")),
 PeriodType = (row.Field<string>("PeriodType")) == null ? string.Empty : (row.Field<string>("PeriodType")),
 SectionFor = (row.Field<string>("Section")) == null ? string.Empty : (row.Field<string>("Section")),
 LiFor = (row.Field<string>("LineItem")) == null ? string.Empty : (row.Field<string>("LineItem")),
 QcPeriodFor = (row.Field<string>("QcPeriod")) == null ? string.Empty : (row.Field<string>("QcPeriod")),
 BrokerFor = (row.Field<string>("BrokerFor")) == null ? string.Empty : (row.Field<string>("BrokerFor")),
 PeriodCollection = (row.Field<string>("PeriodCollection")) == null ? string.Empty : (row.Field<string>("PeriodCollection")),
 boolIgnoreValue = (row.Field<string>("IgnoreValue")) == null ? false : Convert.ToBoolean((row.Field<string>("IgnoreValue"))),
 IgnoreData = (!QCCommentstmp.Tables[0].Columns.Contains("IgnoreData") ? string.Empty : (row.Field<string>("IgnoreData") == null ? string.Empty : row.Field<string>("IgnoreData")))
 ).ToList();


 if (QCCommentlist != null)
 
 foreach (var comment in QCCommentlist)
 
 string section = comment.SectionFor;
 string li = comment.LiFor;
 string broker = comment.BrokerFor;
 string period = comment.PeriodCollection;
 string strQCPeriodValue = "";

 if (comment.boolIgnoreValue && period.Trim() != "")
 
 var QcViewColumnName = QCViewAlltmp.Tables[0].Columns.Cast<DataColumn>().AsParallel()
 .Where(x => x.ColumnName.Contains(period))
 .Select(x => new x.ColumnName ).FirstOrDefault();

 if (QcViewColumnName != null)
 
 period = QcViewColumnName.ColumnName;

 if (period.Trim() != "")
 
 var datarow = QCViewAlltmp.Tables[0].AsEnumerable().AsParallel()
 .Where(row => row.Field<string>("GroupKey").Split('~')[0].ToUpper() == section.ToUpper()
 && row.Field<string>("GroupKey").Split('~')[1].ToUpper() == li.ToUpper()
 && row.Field<string>("Section ").ToUpper() == broker.ToUpper());

 if (datarow != null && datarow.Count() > 0)
 
 strQCPeriodValue = (datarow.FirstOrDefault()[period] != null ? datarow.FirstOrDefault()[period].ToString() : string.Empty);
 if (strQCPeriodValue.Trim() != string.Empty)
 
 comment.IgnoreData = strQCPeriodValue;
 counter++;
 
 
 
 
 
 
 

 SerializeQcComment(QCCommentlist);
 toolTip1.Hide(this);
 
 

Loading data from XML file into a dataset.

If the ignoredata field is there in QCCommentstmp dataset table QCCommentstmp.Tables[0].Columns.Contains("IgnoreData") then deserialize QCCommentstmp table data to list List<clsCommentPopup> QCCommentlist.

Iterate in QCCommentlist's data using a for loop and finding data in QCViewAlltmp.Tables[0] data table for each iteration.

When QCCommentlist has 25000 data then I am iterating in all 25000 data and finding data in another data table. If data is found then I am updating data in the list. This process is getting very slow and the code is taking a long time to finish all the iteration and searching data in the data table.

Please review my code and tell me how to restructure my code, as a result, there will be the improvement in code execution speed.

If my approach is wrong then guide me with the right approach and also give me the relevant code which I can use in my above code, as a result, my routine will take minimum time to finish if I iterate in more than 25000 data. I'm looking for suggestions and better code to achieve the same task.