Sequence contains no elements, ERROR

Question

Sequence contains no elements, ERROR

rob m 261

Not sure how to interpret this
db.tblSWATScores.Single(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal").Value = precipTotal;135297-waprecipitationcontroller.txt
it is an old Entity Framework application that I am trying to reconstruct and get running.

Server Error in '/' Application.
Sequence contains no elements
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: Sequence contains no elements

> Source Error:
>
>
> Line 69: //robm
> Line 70:
> Line 71: db.tblSWATScores.Single(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal").Value = precipTotal;
> Line 72: if (precipTotal != null)
> Line 73: {
>
> Source File: D:\code\SwatCORS\merrittr-swat-0bfb4e658cdd\SWAT_legacy\SWAT-Source Code\SWAT\SWAT\Controllers\WAPrecipitationController.cs Line: 71
>
> Stack Trace:

Yijing Sun-MSFT 7,096 Reputation points

2021-09-28T02:18:39.313+00:00

Hi @rob m ,
According to your description, I suggest you could check if the tblSWATScores database is null. And then single() will return the only element of a sequence. So,you need to check if your sequence has elements that meet the conditions. Your problem is the value return null.

Best regards,
Yijing Sun

2 answers

Your answer

Yijing Sun-MSFT 7,096 Reputation points

2021-09-28T02:18:39.313+00:00

Hi @rob m ,
According to your description, I suggest you could check if the tblSWATScores database is null. And then single() will return the only element of a sequence. So,you need to check if your sequence has elements that meet the conditions. Your problem is the value return null.

Best regards,
Yijing Sun

Answer 1

Viorel 122.5K

Probably the data are incomplete or invalid, because this error means that Single cannot find any row.

To skip this step, try something like this:

var s = db.tblSWATScores.SingleOrDefault(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal");
if( s != null) s.Value = precipTotal;

Check other Single calls too.

But also clarify why it does not return the items.

Rob merritt 6 Reputation points

2021-10-03T05:11:45.62+00:00
Hey How would I add a new row and insert this data the else below doesnr seem to work ...table schema below that

var s = db.tblSWATScores.SingleOrDefault(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal"); if (s != null) { s.Value = precipTotal; } else { db.tblSWATScores.Add(tblSWATScore) }

SELECT TOP (1000) [ID]
,[SurveyID]
,[VariableID]
,[Value]
,[VarName]
,[Description]
FROM [swat].[dbo].[tblSWATScores]

Answer 2

rob m 261

OK I will try that
I dont undestand what this line is doing , is it updating a table based on the predicates inside the ()
db.tblSWATScores.Single(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal").Value = precipTotal;

Viorel 122.5K Reputation points

2021-09-28T06:32:09.707+00:00

It finds a unique row by SurveyID and VarName, and then update the Value of found row. It expects that such row always exists.

If the legacy code does not work with current data, then maybe the data are not good.
Yijing Sun-MSFT 7,096 Reputation points

2021-09-29T06:57:54.527+00:00

Hi @rob m ,

I dont undestand what this line is doing , is it updating a table based on the predicates inside the ()
db.tblSWATScores.Single(e => e.SurveyID == tblswatwaprecipitation.SurveyID && e.VarName == "precipTotal").Value = precipTotal;

This line means it will get the only value which the SurveyID is same with tblswatwaprecipitation.SurveyID and the VarName is same with "precipTotal" in the tblSWATScores database.
I suggest you must check if you have the data which meet the conditions in the tblSWATScores.

Best regards,
Yijing Sun
rob m 261 Reputation points

2021-10-03T23:55:31.09+00:00

does this make sense the e object is empty
Yijing Sun-MSFT 7,096 Reputation points

2021-10-04T08:36:18.143+00:00

Hi @rob m
,The e variable is only visible within the LINQ expression itself. So while the debugger is on the assignment statement ,you won't see it. You can set a breakpoint on e.SurveyID then you will see the e value when it is hit.

Best regards,
Yijing Sun

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Sequence contains no elements, ERROR

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