if you have a select LINQ query that should only return one result, do you have to have a foreach loop to get the result?
Or is there a better way?
// Will return a default value if no object is found in the DB
db.Table.SingleOrDefault(x => x.something == someParameter);
or
// Will throw an exception if records are not found
db.Table.Single(x => x.something == someParameter);
Thanks to Mehrdad for the comment...both lines have been updated.
If it's possible that your query could result in more than one record being returned, then (as mentioned in comments) Single() and SingleOrDefault() are the wrong methods to call. You would keep the same syntax, but call First() and FirstOrDefault() respectively.
var myLinqObj = db.MyObjects.Take(1).SingleOrDefault();
You can use either First or Single.
First returns the first row, whether there are multiple rows or just the one.
Single expects only one row to be returned, and throws an exception if there are multiple rows.
Single is therefore potentially a better choice if you expect to only have one row, so that you'll see the problem immediately and can troubleshoot it.
You can just use .First() or .FirstOrDefault() like so:
Foo foo = query.Select(a => a.Name == "foo").FirstOrDefault();
From all the have have said my addition is the use the Value property after you have a single element if you are using LINQ-to-XML.
And the Select new { *cols* } if it a list or array or table.
Example.
... select new {c.Name, c.Value};
This tip is to enable you get the values.
Related
I have a database table called Customers. I run the following sql to get information about the first and second customer:
select FirstCustomerName, SecondCustomerName,
* from Customers where FirstCustomerName = SecondCustomerName
When I run this in sql it gives me what I want, so that one is ok. The problem is when I want to do the same thing in Linq in C#.
To achive the same thing with Linq I have tried this(still "doesn't work"):
InfoAboutBothCustomers = c.customers.FirstCustomerName == c.customers.SecondCustomerName.ToString()
Note: InfoAboutBothCustomers is an int in my ViewModel
So my question basically is how do the same thing in LINQ?
I am not sure what value you want in InfoAboutBothCustomers. Your SQL statement returns two values and you are saying that you want an int. c.customers.FirstCustomerName == c.customers.SecondCustomerName.ToString() will return a boolean to say if they are equal or not.
If you want the id or ids that match you criteria, try something like:
var ids = from cust in customers
where cust.FirstCustomerName == cust.SecondCustomerName
select cust.Id;
Alternatively you can use what is mentioned in the other answers, which is cleaner, but just be aware that FirstOrDefault will return the row of data. You can then specify the column that you want by doing something like this FirstOrDefault().Id;
Without sample it is difficult to provide any solution. But you can try this
InfoAboutBothCustomers = c.customers.Where(x=>x.FirstCustomerName == x.SecondCustomerName).FirstOrDefault()
In case of error /issue please share the sample.
Use .Where operation
InfoAboutBothCustomers = c.customers.Where(c => c.FirstCustomerName == c.SecondCustomerName).FirstOrDefault();
I have a DataTable called currencyAmounts and using lambda, my expression was working fine until the return had more than one possibility. I am searching the DataTable for a match to my "value" variable. How can I change the following expression to just select the first row returned:
DataRow resultRow = currencyAmounts.AsEnumerable().Single(r => ((decimal)r["OriginalAmount"]) == value);
I am getting the following error when this is run:
Sequence contains more than one matching element
The Error message explains it all actually. Single() throw exception when there are more than one matching element within the condition.(or there is no element). You should use First() or FirstOrDefault()
DataRow resultRow = currencyAmounts.AsEnumerable().FirstOrDefault(r => ((decimal)r["OriginalAmount"]) == value);
you need First instead of Single
You can change the "Single()" with a "First()".
That's because single (from the microsoft page) "Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence."
Single will throw an error if there are more or less than 1 element in the collection.
If you want the first, and are sure there is always at least one result, use
.Single();
If you are not sure there is always at least one result, use
.SingleOrDefault();
Stream stream = SystemStreams.Where(st => st.getName().Equals(s.OBJECT_NAME))
.Single();
in the above query in C# even if my condition of where clause is met my program reads all the records. due to which its becoming really slow. Can anyone help?
Just use FirstOrDefault() instead:
Stream stream = SystemStreams.Where(st => st.getName().Equals(s.OBJECT_NAME))
.FirstOrDefault();
or even shorter:
Stream stream = SystemStreams.FirstOrDefault(st => st.getName().Equals(s.OBJECT_NAME))
Note that FirstOrDefault() will return null if no match is found, otherwise the first item that matches.
Edit in response to comments:
In the SQL providers I have looked at Single() is translated to
SELECT TOP 2 bar,baz from foo where <some condition>
this means if there is only one match you still have to compare all records to try and find that second match - this is necessary because Single() must throw an exception if there is no match at all or more than one match.
FirstOrDefault() on the other hand gets translated to
SELECT TOP 1 bar,baz from foo where <some condition>
This means it can stop after the first match and return null or the record accordingly.
use SystemStreams.FirstOrDefault(query) instead of .Where
Its probably because your function getName() cannot be converted from an expression to a SQL statement, the whole record must be fetched and the where clause is evaluated locally (not on the database).
I have a rather ugly service job that runs through a legacy database and compares it to our production database:
if (vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null) {
var oldDbContractItem = vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).First();
// check to see if there were changes
if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
{
oldDbContractItem.Update(vendorContractItem);
}
}
I will get an error on var oldDbContratItem, "Sequence contains no elements", yet I just did a != null check. This must be simple, what's going on?
If I could teach people just one thing about LINQ it's this: the value of a query expression is an object that represents the query, not the results of the query. Fundamentally that's your problem; you're treating the query as its results. A query isn't a result any more than a restaurant is a club sandwich. A restaurant is a device which produces club sandwiches; a query is a device that produces results.
This is the test against null you are doing:
vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number) != null
That will always be true; That will always return at least an empty sequence... never a null.
You might be meaning to test that its length is greater than 0?
There's an easier way, though, IMO. Call FirstOrDefault() instead of First() and leave out the pre-test completely. Then instead, test if the result of FirstOrDefault() is null.
var oldDbContractItem = vendorContract.Item
.Where(x => x.ItemNumber == contractItem.Item_Number).FirstOrDefault();
if(oldDbContractItem != null) //would be null if there are no items
{
// check to see if there were changes
if (oldDbContractItem.DateStamp != vendorContractItem.Date_Stamp)
{
oldDbContractItem.Update(vendorContractItem);
}
}
}
Because your query returned a container, it just happened to be empty, the null check is on the return not what the return contains.
try this instead...
if (vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).Any())
{
.....
}
Don't run the query twice. It's inefficient and may introduce a race condition into your code. Also, your logic is much better supported by either using IEnumerator<T> directly, or with a foreach loop.
Use either:
var result = vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number).GetEnumerator();
if (result.MoveNext) {
var oldDbContractItem = result.Current;
// ...
}
or
foreach (var oldDbContractItem in vendorContract.Item.Where(x => x.ItemNumber == contractItem.Item_Number)) {
// ...
break;
}
A sequence with no elements is still an object.
Where can return a non-null value, but still resolve to a sequence containing no elements. In your failing statement you are using something else, which is Where().First(). That will be null if the sequence from Where is indeed empty.
There is difference between null and an empty sequence (e.g. something similar to new List<T>()). What you want to do is to use Any() to check whether the sequence contains any elements, or rather replace First() with FirstOrDefault(), which returns null if the sequence contains no element. (If you use that, make sure that null is not a valid value that could be returned by First()).
After using a fix from the accepted answer, you can further debug the LINQ situation (if need be) by using the following steps. LINQ is an exciting technology but takes a while to wraps ones' head around it - a bit of a paradigm shift I would say. Eric's answer hit the nail on the head because he likely helped build the stuff!
Debugging Steps for LINQ
To deal with the issue of no results from the second statement change .First() to .FirstOrDefault() If nothing is found it will return the default value of the data type - if the data type is a class it will return a null value. Your second statement should then work with a null check too and without error.
Then you can debug your LINQ statement to find out why it's doing what it does.
if using LINQ to SQL, Intellisense in Visual Studio 2010 will show you the SQL generated when you hover over a query variable (not the result of a query). If you need the visualizer for VS 2008 it's here
Similarily if you're using LINQ to Entity Framework, you can get the generated SQL using the visualizer plugin here.
I always take the generated SQL from these tools, paste it directly into a query window and run it there. It will show you the empty set you're getting back and if that's a problem you can further debug it in this manner of visualizing the generated statements.
public string GetNameBySSN(string SSN)
{
var results = from c in Clients
where c.ClientSSN.Equals(IRRCNumber)
select c.FirstName;
//return results.ToString();
}
I want to return a single column value with Linq and not sure if there is way to do a quick easy return. I commented section of code that I know does not return the value and if enumerate through collection I can get the value but wondering if a better way was around.
I do understand that Linq is Set based and that it has no upfront means know that the
result is set with a single member.
results.FirstOrDefault();
This way will not throw an exception, FYI. Where as #Mehrdad will.
return results.First().ToString();
Other possible variants:
.First() returns the first item and throws an exception if no item exists. It ignores all other items.
.FirstOrDefault() returns the first item if exists and returns the default value of the result type if nothing exists. It ignores all other items.
.Single() returns the only item if the collection contains exactly one item and throws an exception otherwise.
.SingleOrDefault() is like .Single() but returns the default value of the result type instead of throwing an exception.