Sequence contains no elements ошибка - Решение и исправление самых разных ошибок на TopOshibok.ru

I’m currently using a single query in two places to get a row from a database.

BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == ID
                 select p).Single();

The query is fine when retrieving the row to put data in to the text boxes, but it returns an error «Sequence contains no elements» when used to retrieve the row in order to edit it and put it back in to the database. I can’t understand why it might find an appropriate row in one instance but not another.

(Using ASP.NET MVC and LINQ)

David Basarab

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asked Aug 24, 2009 at 19:19

From «Fixing LINQ Error: Sequence contains no elements»:

When you get the LINQ error «Sequence contains no elements», this is usually because you are using the First() or Single() command rather than FirstOrDefault() and SingleOrDefault().

This can also be caused by the following commands:

FirstAsync()
SingleAsync()
Last()
LastAsync()
Max()
Min()
Average()
Aggregate()

V0d01ey

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answered Dec 5, 2011 at 13:43

Tony KiernanTony Kiernan

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Please use

.FirstOrDefault()

because if in the first row of the result there is no info this instruction goes to the default info.

answered Jun 21, 2016 at 22:23

In addition to everything else that has been said, you can call DefaultIfEmpty() before you call Single(). This will ensure that your sequence contains something and thereby averts the InvalidOperationException «Sequence contains no elements». For example:

BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == ID
                 select p).DefaultIfEmpty().Single();

Stephen Rauch♦

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answered Sep 24, 2018 at 1:41

bryc3monk3ybryc3monk3y

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Well, what is ID here? In particular, is it a local variable? There are some scope / capture issues, which mean that it may be desirable to use a second variable copy, just for the query:

var id = ID;
BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == id
                 select p).Single();

Also; if this is LINQ-to-SQL, then in the current version you get a slightly better behaviour if you use the form:

var id = ID;
BlogPost post = dc.BlogPosts.Single(p => p.BlogPostID == id);

JoshJordan

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answered Aug 24, 2009 at 19:23

Marc GravellMarc Gravell

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This will solve the problem,

var blogPosts = (from p in dc.BlogPosts
             where p.BlogPostID == ID
             select p);
if(blogPosts.Any())
{
  var post = blogPosts.Single();
}

answered Jan 16, 2013 at 0:31

Diganta KumarDiganta Kumar

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I had a similar situation on a function that calculates the average.

Example:

ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Average();

Case Solved:

ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Count == 0 ? 0 : lstMediaValues.Average();

answered Apr 25, 2019 at 11:13

Reason for error:

The query from p in dc.BlogPosts where p.BlogPostID == ID select p returns a sequence.
Single() tries to retrieve an element from the sequence returned in step1.
As per the exception — The sequence returned in step1 contains no elements.
Single() tries to retrieve an element from the sequence returned in step1 which contains no elements.
Since Single() is not able to fetch a single element from the sequence returned in step1, it throws an error.

Fix:

Make sure the query (from p in dc.BlogPosts where p.BlogPostID == ID select p)

returns a sequence with at least one element.

Ali

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answered Feb 2, 2016 at 12:41

Siddarth KantedSiddarth Kanted

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Источник

I’m currently using a single query in two places to get a row from a database.

BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == ID
                 select p).Single();

(Using ASP.NET MVC and LINQ)

David Basarab

72.2k42 gold badges129 silver badges156 bronze badges

asked Aug 24, 2009 at 19:19

From «Fixing LINQ Error: Sequence contains no elements»:

When you get the LINQ error «Sequence contains no elements», this is usually because you are using the First() or Single() command rather than FirstOrDefault() and SingleOrDefault().

This can also be caused by the following commands:

FirstAsync()
SingleAsync()
Last()
LastAsync()
Max()
Min()
Average()
Aggregate()

V0d01ey

471 silver badge6 bronze badges

answered Dec 5, 2011 at 13:43

Tony KiernanTony Kiernan

5,1672 gold badges14 silver badges10 bronze badges

Please use

.FirstOrDefault()

because if in the first row of the result there is no info this instruction goes to the default info.

answered Jun 21, 2016 at 22:23

BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == ID
                 select p).DefaultIfEmpty().Single();

Stephen Rauch♦

47.9k31 gold badges107 silver badges136 bronze badges

answered Sep 24, 2018 at 1:41

bryc3monk3ybryc3monk3y

4348 silver badges12 bronze badges

Well, what is ID here? In particular, is it a local variable? There are some scope / capture issues, which mean that it may be desirable to use a second variable copy, just for the query:

var id = ID;
BlogPost post = (from p in dc.BlogPosts
                 where p.BlogPostID == id
                 select p).Single();

Also; if this is LINQ-to-SQL, then in the current version you get a slightly better behaviour if you use the form:

var id = ID;
BlogPost post = dc.BlogPosts.Single(p => p.BlogPostID == id);

JoshJordan

12.7k10 gold badges53 silver badges63 bronze badges

answered Aug 24, 2009 at 19:23

Marc GravellMarc Gravell

1.0m267 gold badges2571 silver badges2905 bronze badges

This will solve the problem,

var blogPosts = (from p in dc.BlogPosts
             where p.BlogPostID == ID
             select p);
if(blogPosts.Any())
{
  var post = blogPosts.Single();
}

answered Jan 16, 2013 at 0:31

Diganta KumarDiganta Kumar

3,6373 gold badges27 silver badges29 bronze badges

I had a similar situation on a function that calculates the average.

Example:

ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Average();

Case Solved:

ws.Cells[lastRow, startingmonths].Value = lstMediaValues.Count == 0 ? 0 : lstMediaValues.Average();

answered Apr 25, 2019 at 11:13

Reason for error:

The query from p in dc.BlogPosts where p.BlogPostID == ID select p returns a sequence.
Single() tries to retrieve an element from the sequence returned in step1.
As per the exception — The sequence returned in step1 contains no elements.
Single() tries to retrieve an element from the sequence returned in step1 which contains no elements.
Since Single() is not able to fetch a single element from the sequence returned in step1, it throws an error.

Fix:

Make sure the query (from p in dc.BlogPosts where p.BlogPostID == ID select p)

returns a sequence with at least one element.

Ali

3,3835 gold badges42 silver badges54 bronze badges

answered Feb 2, 2016 at 12:41

Siddarth KantedSiddarth Kanted

5,7381 gold badge29 silver badges20 bronze badges

Источник

Remove From My Forums

Question
Hi NG,

I’m using the following statemtn to get a record from my database

Code Snippet

var productItem = productDtcx.Product.Single(p => p.ProductNumber == «TP00002»);

In case, when this record is not existing in my database I’m receiving the following exception:

Code Snippet

System.InvalidOperationException was unhandled
Message=»Sequence contains no elements»
Source=»System.Core»
StackTrace:
       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
       at System.Data.Linq.SqlClient.SqlProvider.SqlQueryResults`1.PostProcess(Expression query, IEnumerable`1 results)
       at System.Data.Linq.SqlClient.SqlProvider.SqlQueryResults`1.GetEnumerator()
       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
       at System.Data.Linq.Table`1.System.Linq.IQueryable.Execute[S](Expression expression)
       at System.Linq.Queryable.Single[TSource](IQueryable`1 source, Expression`1 predicate)
       at Sample0040.Program.Main(String[] args) in C:\Documents and Settings\oaytekin\Desktop\LINQSamples\Sample0040\Program.cs:line 19
       at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
InnerException:

I don’t want to catch this exception with a try/catch block. Is there any another method/way to check, if a record is existing or not?

I want to delete a record. To get and delete the record I’m using the following code

Code Snippet

string connString = «Data Source=.;Initial Catalog=AdventureWorks;Integrated Security=True»;

ProductDataContext productDtcx = new ProductDataContext(connString);

var productItem = productDtcx.Product.Single(p => p.ProductNumber == «TP00002»);

productDtcx.Product.Remove(productItem);

Thx for your help,

Ozgur Aytekin

Answers

Use SingleOrDefault. It returns null if the row isn’t found.

Anders

Источник

Problem

When you call .First() on an empty IEnumerable, you get the following exception:

System.InvalidOperationException: Sequence contains no elements

Solution

Option 1 – Use .FirstOrDefault() instead of .First()

When the IEnumerable is empty, .FirstOrDefault() returns the default value for the type.

IEnumerable<IMessage> data = GetData();
var first = data.FirstOrDefault();
Console.WriteLine($"First = {first == default(IMessage)}");
Code language: C# (cs)

For reference types this returns null. For value types this returns 0 or that type’s equivalent of 0.

Option 2 – Use .Any() to check if the IEnumerable is empty

Instead of dealing with the default value (which is either null or 0), you can use .Any() to check if the IEnumerable is empty.

IEnumerable<IMessage> data = GetData();

if(data.Any())
{
	var first = data.First();
	Console.WriteLine($"First = {first}");
}
else
{
	Console.WriteLine("There's no data");
}
Code language: C# (cs)

Option 3 – Get your own default object from .FirstOrDefault()

Create an extension method that returns a default object you specify.

static class IEnumerableExtension
{
	public static T FirstOrDefault<T>(this IEnumerable<T> list, T defaultObj)
	{
		if (!list.Any())
			return defaultObj;
		else
			return list.First();
	}
}
Code language: C# (cs)

With reference types you can use the Null Object pattern (an object that does nothing). The benefit of this you don’t need to deal with null checking. In my case, I’m setting the default to EmptyMessage.

IEnumerable<IMessage> data = GetData();
var first = data.FirstOrDefault(new EmptyMessage());
Code language: C# (cs)

With value types you can specify a value that has special meaning in your system.

IEnumerable<int> intData = GetIntData();
var firstInt = intData.FirstOrDefault(-1);
Code language: C# (cs)

Источник

If you are using LINQ or Lambda expression and getting error: InvalidOperationException “Sequence contains no elements”. It means you are trying to retrieve an element from an empty sequence(may be returned by LINQ query). There are some basic rules to handle this in LINQ or lambda expression.

1. If you are using Single in LINQ, change your habit to use SingleOrDefault.
2. If you are using First or Last in LINQ, use FirstOrDefault or LastOrDefault.
3. If you are using ElementAt, use ElementAtOrDefault.
4. If you are getting this error on Aggregate Functions like Average, Sum…etc.

To understand this, let us take an example. See following code.

var items = new int[] {1, 2, 3, 4, 5};
Double avg = items.Average();

You will get avg = 3.0

Now, On adding where condition as in following, you will get error InvalidOperationException “Sequence contains no elements”.

Double avg = items.Where(x=>x > 10).Average();

because there is no item greater than 10. So what is the solution? Do you need to add one if condition every time to check whether items are available or not?
You need not to change code too much, Just use DefaultIfEmpty function before Aggregate Function.

 Double avg = items.Where(x=>x > 10).DefaultIfEmpty().Average();

Now, avg = 0.0

Hope, It helps.

Источник

Question

Answers

Problem

Solution

Option 1 – Use .FirstOrDefault() instead of .First()

Option 2 – Use .Any() to check if the IEnumerable is empty

Option 3 – Get your own default object from .FirstOrDefault()