I like to think of myself as pretty good with LINQ, but every now and then I try to accomplish something that I just can't get to work. I'd like to convert a SPListItemCollection to a dictionary so I can use the key to look up a value without the need for a LINQ query each time:
var formsConfigItems = new Dictionary<string, string>();
foreach (SPListItem item in list.GetItems(query))
formsConfigItems.Add(item.Title, (item["Value"] == null ? string.Empty : item["Value"].ToString()));
This works, but I was hoping to do it in a cleaner fashion, using LINQ. (not a big deal but I like to use LINQ over for-loops whenever possible, although it's the same thing behind the scenes.
I tried to do something like this:
var formsConfigItems = (from SPListItem i in list.GetItems(query)
select new { i.Title, i["Value"].ToString() }).ToDictionary<string, string>(k=>k.Key, k=>k.Value);
But that doesn't seem to work. If I try to use a lambda expression on list.GetItems(query), I'm not given the option to use .Where or any LINQ commands (which is weird because it is an SPListCollection)
Thanks in advance.
Try:
var formsConfigItems = list.GetItems(query)
.Cast<SPListItem>()
.ToDictionary(item => item.Title,
item => Convert.ToString(item["Value"]));
To answer your queries:
If I try to use a lambda expression on list.GetItems(query), I'm not
given the option to use .Where or any linq commands (which is weird
because it is an SPListCollection)
That's because SPListCollection is an "old-school" collection that implements IEnumerable but not IEnumerable<T>, so C# / LINQ (at compile-time anyway) can't tell what type of items it contains. The Cast<SPListItem>() call helps work around this issue - it turns an IEnumerable into an IEnumerable<T>, allowing the type-algebra to work out at compile-time . Your for loop doesn't have this issue since you explicitly specify the type of the loop variable - the compiler inserts a cast on your behalf for each item in the sequence.
I tried to do something like this (query expression). But that
doesn't seem to work.
That's because you are not constructing the anonymous type instance correctly (property names can't be inferred for arbitrary expressions) and your lambda expression isn't quite right either (the property names you use don't match the property names of the anonymous type). Try this instead:
var formsConfigItems = (from SPListItem i in list.GetItems(query)
select new
{
i.Title,
Value = Convert.ToString(i["Value"])
}).ToDictionary(a => a.Title, a => a.Value);
Ani's got the better solution IMO, but one other thing you're missing: Your LINQ statement is creating a collection of anonymous items, but you're not giving names to the properties in that anonymous class.
The k=>k.Key expression doesn't work, because it doesn't know what Key is - you've only defined Title (since you didn't give it a name, it borrowed the one from the object). The Value one can't be automatically figured out, so it would throw a compiler error.
To do it this way, you'd need to specifically declare the names:
new { Key = i.Title, Value = i["Value"].ToString() }
Related
I'm new to C# and LINQ, so I'm unsure what the best practice for this would be:
I have an IEnumerable<Something>, where Something has properties A, B and C. I want to group my list of Somethings by the property A: However, I want the list of values for each group to be mapped to a different object, TargetSomething. I have a helper class, MyHelper, which has a static function converting any Something into a TargetSomething.
Therefore, I have source value / structure as follows:
IEnumerable<Something>
And target value / structure as follows:
IEnumerable<IGrouping<A, TargetSomething>>
Although, really, instead of an IGrouping I'd prefer to have a Dictionary or something, basically the equivalent of a Map in other languages.
So far, to group by keys, I used the following code:
from smth in smths group smth by smth.A into grp select grp;
Which seems pretty verbose for a simple groupBy operation, but what can I do.
Now my question is: How can I add the mapping of Something to TargetSomething using MyHelper.map(mySomething) in this LINQ query?
Also, as far as I'm aware, the IEnumerable resulting from this query is lazy so that not the entire content has to be in memory all the time - Is that a misconception? If not, will I lose this feature by adding a map somewhere?
Thank you
You can use ToLookup to return a Lookup which is a kind of multi-valued Dictionary.
A Lookup<TKey, TElement> implements IEnumerable<IGrouping<TKey,TElement>> so this should work fine for your use case.
var lookup = smths.ToLookup(smth => smth.A, HelperFunction);
The HelperFunction should have the return type TargetSomething and a single parameter of Something.
You could also use a lambda for that:
var lookup = smths.ToLookup(smth => smth.A, smth => new TargetSomething(...));
I am trying to achieve a simple task, and I don't find a way to do it. I am trying to convert a LINQ result to a list of objects. I am following the syntax posted in this accepted SO answer.. Here is my code:
var LinqHallazgos = (from RA_Hallazgos in context.RA_Hallazgos
select new
{
HallazgoId = RA_Hallazgos.HallazgoId,
Hallazgo = RA_Hallazgos.Hallazgo
});
List<RA_Hallazgos> Hallazgos = LinqHallazgos.ToList<RA_Hallazgos>();
And this is the error that I get:
It says that "LinqHallazgos", which is an iqueryable, doesn't contain a definition for ToList. But I see the "ToList" method in intellisense. It also says that it expect an IEnumerable but this answer seems to use similar syntax.
What I am doing wrong and how in the world I get the LINQ result as a list of a particular object??
You are making this too complex. It seems all you want to do is materialize the DbSet<RA_Hallazgos> to memory (retrieve the content from the database). If that is the case just use the ToList() linq extension on the DbSet and you are done. Note that it will retrieve everything as you did not specify a filter (ie. any limiting predicates like Where / Take / Skip / etc).
List<RA_Hallazgos> Hallazgos = context.RA_Hallazgos.ToList();
Just call .ToList(). You have an anonymous type there (the new {}) so you can't specify the type explicitly even if you wanted to.
Otherwise, if RA_Hallazgos is an actual type, use new RA_Hallazgos{} to avoid creating an anonymous type.
Try
select new RA_Hallazgos
{
HallazgoId = RA_Hallazgos.HallazgoId,
Hallazgo = RA_Hallazgos.Hallazgo
}
List<RA_Hallazgos> Hallazgos = LinqHallazgos.ToList()();
The title is pretty unclear. But I couldn't find the proper words. Generally Linq works in the below syntax
MyList.Where().Select(x => {MyFunction(x);})
It is good in ordinary conditions but in some situation like in my case. I am creating a tree structure using dictionary. In this if I want to add a set
Set.Foreach(x => {(MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).Add(x);});
But it would be nice if I can do like below
(MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).Add(MySet.Foreach(x => {return x;}));
Is there any way possible to dothis ?
You can do it, if object stored in Dict has AddRange method which accepts IEnumerable<T>. But you should ski[ ForEach and just pass MySet:
MyDict[logEvent.level][logEvent.event][logEvent.subevent][logEvent.filePath]).AddRange(MySet);
How do I remove an object directly from an IGrouping IGrouping<DateTime, VMAppointment>?
The only way I know of currently is to generate a new IGrouping without the concering element, but I don't like this way because it causes some trouble within my application.
Any ideas?
No, there's no way to mutate an IGrouping<,>, at least in general - and even if you knew the concrete type, I don't believe any of the implementations exposed by the .NET framework allow the group to be mutated.
Presumably the grouping is the result of some query - so if possible, change the original query to exclude the values you aren't interested in.
I know this is old question, but hopefully this helps someone else. A workaround for this is to cast the group to a list, then use the values from the list instead of the group.
var groups = someList.GroupBy(x => x...);
foreach (var group in groups)
{
var groupList = group.ToList();
...
groupList.Remove(someItem);
//Process the other code from groupList.
}
You could cast using Select and use TakeWhile if you have a testable condition (such as null as in the example) on a property in your group:
var removedItemsList = group.Select(x => x.TakeWhile(t => t.someProperty != null));
This will return an IEnumerable<IEnumerable<YourGroup>>.
Think this is a very basic question, but it's my first LINQ query and I'm completely stuck:
I have a dictionary with string key and list value (see definition below) and want to pull out elements of a list of a particular type having selected the list by the dictionary key.
IDictionary<string, IList<MyBaseType>> dataItemMap;
Where MySubType extends MyBaseType.
My dodgy query is:
string identCode = "foo";
IEnumerable<MySubType> query =
from entry in dataItemMap
where entry.Key == identCode
select entry.Value.OfType<MySubType>();
And the error message (from LinqPad):
Cannot implicitly convert type
'System.Collections.Generic.IEnumerable<System.Collections.Generic.IEnumerable<MySubType>>'
to 'System.Collections.Generic.IEnumerable<MySubType>'.
An explicit conversion exists (are you missing a cast?)
The problem is clearly in the entry.Value.OfType<> but how can I specify the lists elements? I'm looking for something like entry.Value.Items.OfType<> ?
thanks.
I think you want something like this:
IEnumberable<MySubType> query = dataItemMap[identCode].OfType<MySubType>();
This will get the list with the given key, and then filter it to return only MySubType elements.
EDIT: I've been focusing on why the existing solution didn't work (and the general problem of "I've got a list of values for each element, and I want to flatten it") rather than taking a step back. As Andy's answer shows, you should almost certainly use the fact that it's a dictionary - turning it from an O(n) operation to O(1) :)
Two caveats:
Your current code will always perform an ordinal, culture-insensitive comparison with identCode and the dictionary keys; using the dictionary lookup will use whatever comparer it was constructed with.
Your current code will return an empty sequence if identCode isn't found in the dictionary; the dictionary indexer will throw an exception. You can use TryGetValue if you want to avoid that.
Note that if you know that all the elements in the last you're picking are actually of the right type, it would probably be better to use Cast than OfType:
var query = dataItemMap[identCode].Cast<MySubType>();
I generally prefer Cast to OfType when both would work, as it means that if my assumptions about the data in the sequence prove incorrect, I find out about it with an exception rather than silently missing data.
Note that Cast will also return null elements, whereas OfType won't.
No, the problem isn't in using OfType<> - it's that you've ended up with a sequence of sequences, but you're trying to assign that to a single sequence.
Either change the return type, or use another from clause to flatten the results:
IEnumerable<MySubType> query = from entry in dataItemMap
where entry.Key == identCode
from value in entry.Value.OfType<MySubType>()
select value;
I'd be tempted to use the extension methods directly:
var query = dataItemMap.Where(e => e.Key == identCode)
.SelectMany(e => e.Value.OfType<MySubType>());