I have an IEnumerable of objects which I need to turn into a filtered dictionary. The key of the dictionary must be the object's name and the value must be one of its properties. I do so using LINQ's ToString method:
// dataPoints is of type IEnumerable
var filteredDictionary = dataPoints.Where(d => d.TypeName == typeNameOfInterest).ToDictionary(d => d.Name, d => d.Identifier)
I eventually turn the dictionary into a string as follows:
string.Join(Environment.NewLine, filteredDictionary.Select(d => $"{d.Key} : {d.Value};").ToArray()))
This works and the order of the elements in the string is the same as the order in the IEnumerable. I think that is what the documentation means by stating the following about the returned dictionary
The values within each group are in the same order as in source
Now, I need to get the first element from the dictionary. By first I mean the first one listed in the original IEnumerable. How would I need to do this? Can I rely on LINQ's first method to do so and be sure that it is the correct one? Would I need to construct a SortedDictionary before getting the first element?
I'm using the .NET Framework 4.7.2 and I never modify the content of the dictionary that I receive from ToDictionary.
Related
Help understand please why that query returns same values but if i use anonymous type it return correct result
//same values
var deviceMessageList1 = _deviceMessageRepository.Fromdevicemessages
.Where(m => m.DeviceId == deviceId).Take(1000).ToList();
//different values
var deviceMessageList2 = _deviceMessageRepository.Fromdevicemessages
.Where(m => m.DeviceId == deviceId).Take(1000).Select(x=> new { Message = x.Message }).ToList();
[]
I've made a few assumptions:
I assume your entity class is called Fromdevicemessage
I assume that Message is a String property
.Select() in LINQ does not influence which items are retrieved. It only influences how the retrieved items are presented.
If you know SQL, you will see that SELECT in SQL works exactly the same (which is why the LINQ method is intentionally called Select, as it works the same, functionally speaking)
It helps if you understand the intention of every method, I'll comment it in:
var deviceMessageList1 =_deviceMessageRepository.Fromdevicemessages
.Where(m => m.DeviceId == deviceId) //Only return the items which have their DevicedId set to [deviceId]
.Take(1000) //Give me the first 1000 items
.ToList() //Make a list from these 1000 items
Notice that you have only specified which rows you want to retrieve. You have not specified how you want the output to be formatted.
By default, you will receive objects of the corresponding entity type Fromdevicemessage.
The list you see at the end is therefore a List<Fromdevicemessage>.
var deviceMessageList1 =_deviceMessageRepository.Fromdevicemessages
.Where(m => m.DeviceId == deviceId) //Only return the items which have their DevicedId set to [deviceId]
.Take(1000) //Give me the first 1000 items
.Select(x => x.Message) //For each retrieved item, instead of the item itself, only give me its Message (= string)
.ToList() //Make a list from these 1000 strings
Notice what the Select statement adds. It basically tells you that you do not want the full Fromdevicemessage object, but only want the Message property. You're basically telling the compiler the following:
For each Fromdevicemessage object that is currently retrieved, render me its Message property.
You were originally working with a collection of Fromdevicemessage objects. But the .Select() statement converted that collection into a collection of String objects.
Simplified, the Select() method converts every object in the source collection, based on your mapping (e.g. x => x.Message), and returns you the list of mapped values.
The list you see at the end is therefore a List<String>.
I cheated a little bit...
In case you hadn't noticed, I changed your Select statement.
Your version:
.Select( x => new { Message = x.Message } )
My version:
.Select( x => x.Message )
Both are valid code, but they work a bit differently.
Your version converts the retrieved items into anonymous objects. The resulting list will therefore be a list of anonymous objects.
My version converts the retrieved items into String objects. The resulting list will therefore be a list of strings.
There are cases where creating an anonymous type is useful. Most commonly, it is useful if you want to return multiple values (e.g. both the Message and Recipient properties).
However, your example only retrieves a single property. In this case, there is no benefit to using anonymous types. Using anonymous types when you want to retrieve a single property makes the code more complex for no good reason or benefit.
You are making yourself responsible for later unwrapping that anonymous type and reading its Message property.
It's easier to simply handle the strings directly, rather than wrapping them in an anonymous type. That means you have one less wrapped layer to worry about.
Update after your comment
Taking a closer look at the image you linked; is your question specifically why the debug values (when you hover over the results like you do in the picture) are different?
What you see in the little popup (before expanding) is essentially the .ToString() output of the variable you're inspecting.
The first example consists of a List<Fromdevicemessage>. Since Fromdevicemessage is a custom class you've built (and I assume you did not override its .ToString() method, the default ouput will be the name of the class, not its contents.
That's just how it works. If you override the .ToString() method in your Fromdevicemessage class, then you can change what it looks like.
public override string ToString()
{
return $"Message : {this.Message}";
}
In the second example, you are dealing with a List of anonymous types. Anonymous types have a custom .ToString() method, which already shows you the content of the object rather than its classname (since anonymous objects have no classname, at least as far as a developer can see).
Both the queries are same except in second query you are using anonymous type.
first query will return original object with all the properties in that object and second query will just return message as you are using anonymous type.
To better understand this is like
//first query
select Top 1000 * from Fromdevicemessages
//Second query
select Top 1000 Message from Fromdevicemessages
I need an array or list that contains names that the user will add to. I also need to perform a check that won't allow a name to be added if that name already exists. I know this should be fairly simple but I have not been able to find any solution.
I think you may just be after:
if (list.Contains(name))
Or:
if (array.Contains(name))
It's as simple as that! Both List<T> and T[] implement the IList<T> interface with its Contains method. This is assuming you're happy with an exact match, of course. If you need something more complicated, I'd probably use Any from LINQ. For example:
if (array.Any(x => x.StartsWith(name))
Use a HashSet<string>; HashSet's are collections of unique elements. Their .Add method returns true if the element was added, and false otherwise.
If you cannot use such a data structure, then simply call .Contains on the list and pass in the element to be added - if it returns true, then do not add it. Otherwise, you're good to go and can append it to the list.
This will work if the list is instantiated:
//List<string> list
list.contains(specifiedstring)
Otherwise if it is a complex object with a string field - List then I would use:
//For a List<ClassA> list
var exists = list.Any(x => !String.IsNullOrWhitespace(x.StringAField) &&
x.StringAField.Equals(string));
This gives you the bool value to check in an if statement or other condition.
I ran across an unexpected problem with the following code.
List<string> items = new List<string>();
items = items.OrderBy(item => item);
This code generates the error:
Cannot implicitly convert type 'System.Linq.IOrderedEnumerable' to 'System.Collections.Generic.List'. An explicit conversion exists (are you missing a cast?)
It appears I can change items to be of type IEnumerable<string> and the error goes away. But I need to be able to add items to the list, which IEnumerable doesn't support.
Can someone help me understand this error, and what the easiest fix is? Is it safe to simply cast the result?
Why not just sort the list in place using the Sort() instance method; then you can add items to it later if you like:
List<string> items = GetSomeItems();
items.Sort();
Or, use an ordered collection like a binary search tree. SortedSet<T> might fit the bill, depending on your needs.
The solution suggested by the others:
items = items.OrderBy(item => item).ToList();
... creates another list with the original items in a new order. This is only useful if you need to preserve the original ordering for some other purpose; it's rather more wasteful of memory than sorting the list in place.
As far as understanding the error, it's simple: List<T> isn't a subtype of IOrderedEnumerable<T>, so there's no implicit reference conversion between the two. The explicit cast that the compiler suggests will satisfy the compiler, but it will fail at run time because the object returned by OrderBy<T> does not inherit from List<T>.
EDIT
An example of List<T>.Sort(Comparison<T>), assuming the type MyType has a Key property of some type type T where T : IComparable<T>:
List<MyType> items = GetSomeItems();
items.Sort((a, b) => a.Key.CompareTo(b.Key));
You need to convert the IEnumerable to a List. Try this:
items = items.OrderBy(item => item).ToList();
You need to use LINQ's ToList() method
items = items.OrderBy(item => item).ToList();
You can't cast directly from IEnumerable<> to List<>
Try this
items = items.OrderBy(item => item).ToList();
For sorting a list of strings you do not need Linq in the first place - just use Sort():
List<string> items = new List<string>();
//add items here
items.Sort();
OrderBy() is an extension method of IEnumerable - and not List.
When the compiler encounters the OrderBy() extension method, it casts the range variable to an IOrderedEnumerable where it can perform the required sorting via CreateOrderedEnumerable method using IComparer et al. Once sorted, the compiler spits out the variable as IEnumerable - usually.
Suggestion: use the var keyword to type 'items' in the LinQ clause.
Certainly the options offered above using the Sort() and ToList() methods will work - however, using them involves greedy operators and you lose the advantage of lazy loading.
Here's a good breakdown here:
C# Sort and OrderBy comparison between running Sort() and OrderBy().
There are a few posts on the site about how to order by using lambda expressions however I cannot seem to get mine to work. I am trying to reorder a list that is already populated. Am i wrong in thinking that i can rearrange the order of this list using lambada expressions?
QuarterMileTimes.OrderByDescending(c => c.PquartermileTime);
I was wondering if it's down to PquartermileTime being a string? I also tried this expression on a date
QuarterMileTimes.orderBy(c => c.RaceDay);
Still no luck where am I going wrong?
When you call OrderByDescending, the method returns a new IEnumerable<T> - it does not reorder the collection in place.
Try doing:
QuarterMileTimes = QuarterMileTimes.OrderByDescending(c => c.PquartermileTime).ToList();
(This is if your collection is a List<T>...)
The result of OrderByDescending (and all of the other Enumerable extension methods) is an IEnumerable<T> that projects the source data in the order you're describing. It does not alter the original data in any way.
If you prefer, you can use the ToList() extension method to create a new List<T> from that result and assign it back to the original variable.
QuarterMileTimes = QuarterMileTimes.OrderByDescending(/*...*/).ToList();
(This is assuming, of course, that QuarterMileTimes is a List<T>)
The gist of the answer is no, OrderByDescending does not alter the data source in any way.
You are assigning it to a new variable aren't you?
var sortedTimes = QuarterMileTimes.OrderByDescending(c => c.PquartermileTime);
It isn't like e.g. the List.Sort method, that sorts the existing list.
The result of the method has to be assigned to a variable.
OrderByDescending returns an IOrderedEnumerable<T> i.e. a new sequence with the items in the specified order. You'll have to re-assign QuarterMileTimes to get the behaviour you expect:
QuarterMileTimes = QuarterMileTimes.OrderByDescending(c => c.PquarterMileTime).ToList();
Alternatively you can just use the returned sequence separately, which is the usual approach.
QuarterMileTimes.OrderByDescending(c => c.PquartermileTime) returns a new enumerable, ordered by PquartermileTime. It does not reorder QuarterMileTimes in place.
I have a the following dictionary:
IDictionary<int, IList<MyClass>> myDictionary
and I am wanting to get all the values in the dictionary as an IList....
Just to add a bit of a background as to how I've gotten into this situation....
I have a method that gets me a list of MyClass. I then have another method that converts that list into a dictionary where they key is the id for MyClass. Later on...and without access to that original list...I'm needing to obtain the original ungrouped list of MyClass.
When I pass myDictionary.Values.ToList() to a method that takes an IList I get a compile error that says that it can't convert from
System.Collections.Generic.List<System.Collections.Generic.IList<MyClass>>
to:
System.Collections.Generic.IList<MyClass>
Now, I can understand that its gone and added each of the groups of IList to the new list as separate elements of the list....but in this instance its not really what I'm after. I just want a list of all the values in the entire dictionary.
How then can I get what I'm after without looping through each of the key values in the dictionary and creating the list I want?
Noticed a lot of answer were quite old.
This will also work:
using System.Linq;
dict.Values.ToList();
Because of how a dictionary (or hash table) is maintained this is what you would do. Internally the implementation contains keys, buckets (for collision handling) and values. You might be able to retrieve the internal value list but you're better of with something like this:
IDictionary<int, IList<MyClass>> dict;
var flattenList = dict.SelectMany( x => x.Value );
It should do the trick ;) SelectMany flattens the result which means that every list gets concatenated into one long sequence (IEnumerable`1).
A variation on John's suggestion:
var flattenedValues = dict.Values.SelectMany(x => x);
If you need them in a list, you can of course call ToList:
var flattenedList = dict.Values.SelectMany(x => x).ToList();
dictionary.values.toList();
if You want to get Sum just do
myDictionary.values.sum();
Values gets a ICollection containing the values of your dictionary. As implied by the definition of your dictionary, it can be defined as a ICollection<IList<MyClass>> collection. So if you really want a IList<IList<MyClass>>, use spacedog's solution.
If what you really want is a flat `IList', then there is no other solution than looping through each value :
IList<MyClass> l=new List<MyClass>();
foreach (IList<MyClass> v in myDictionary.Values)
l.AddRange(v);
Note that this is so grossly inefficient that you should think again about using a dictionary for what you are trying to achieve.