I'm trying to convert a list of objects to a dictionary using the following code:
var MyDictionary = MyList.Distinct().ToDictionary(i => i.ObjectId, i => i);
I know that a dictionary should not contain duplicate elements, hence the .Distinct(). Yet I still get the following Exception whenever there's a duplicate element:
An item with the same key has already been added.
MyList is a list of MyObject that looks like this:
public class MyObject{
public string ObjectId { get; set; }
public string FName { get; set; }
public string LName { get; set; }
}
Is there a better way to create a dictionary from a list of objects ? or am I doing something wrong?
If you want to compare on the ObjectId, you'll need to pass in a custom comparer to .Distinct(). You can do so like this:
class MyObjectComparer : IEqualityComparer<MyObject>
{
public bool Equals(MyObject x, MyObject y)
{
return x.ObjectId == y.ObjectId;
}
public int GetHashCode(MyObject obj)
{
return obj.ObjectId.GetHashCode();
}
}
var MyDictionary = MyList
.Distinct(new MyObjectComparer())
.ToDictionary(i => i.ObjectId, i => i);
You could use Group by and then select first from the List as below:
var MyDictionary = MyList.GroupBy(i => i.ObjectId, i => i).ToDictionary(i => i.Key, i => i.First());
Distinct works using the objects built in Equals and GetHashCode methods by default but your dictionary works only over the id. You need to pass in a IEqualityComparer in to distinct that does the comparison on Id to test if items are equal or make MyObject implment Equals and GetHashCode and have that compare on the Id.
Related
I have a List and MyClass is:
public class MyClass
{
public bool Selected { get; set; }
public Guid NoticeID { get; set; }
public Guid TypeID { get; set; }
}
My question is, how do i convert this list into a Dictionary<Guid, List<Guid>>, where the dictionary key is the GUID from the TypeID property, and the value is a list of all the NoticeID values corresponding to that TypeID. I have tried like so:
list.GroupBy(p => p.TypeID).ToDictionary(p => p.Key, p => p.ToList())
but this returns a Dictionary <Guid, List<MyClass>>, and I want a Dictionary<Guid, List<Guid>>.
Well, when you group you can specify the value you want for each element of the group:
var dictionary = list.GroupBy(p => p.TypeID, p => p.NoticeID)
.ToDictionary(p => p.Key, p => p.ToList());
However, I would strongly consider using a lookup instead of a dictionary:
var lookup = list.ToLookup(p => p.TypeID, p => p.NoticeID);
Lookups are much cleaner in general:
They're immutable, whereas your approach ends up with lists which can be modified
They express in the type system exactly what you're trying to express (one key to multiple values)
They make looking keys up easier by returning an empty sequence of values for missing keys, rather than throwing an exception
Is there a way to convert Dictionary<string, obj> to collection of objects such that each single object in the collection includes the key as another property
Here is the class def for obj
class someclass
{
string property1;
string property2;
}
After conversion, I am expecting each object in the collection to be like
obj.property1
obj.property2
obj.Key
I have been struggling with this since along time and I seek some help. any ideas?
thanks in advance.
Something like
var myCollection = from de in myDictionary
select new
{
de.Value.property1,
de.Value.property2,
de.Key
}.ToList(); // or .ToArray()
should do the trick.
That will return a List of a new anonymous type with the properties you requested.
You could also(in addition to the anonymous type apporach) use a List<Tuple<string, string, string>>:
var list= dictionary
.Select(kv => Tuple.Create(kv.Value.property1, kv.Value.property2, kv.Key))
.ToList();
foreach(var item in list)
{
Console.WriteLine("property1:{0 property2:{1} key:{2}"
, item.Item1
, item.Item2
, item.Item3);
}
The advantage over an anonymous type is that you can return the Tuple easily from a method.
Edit: A third option(my favorite) is simply to create instances of a class that you've declared somewhere. That's the ideal way. I don't know why i thought that you want a class "on the fly".
class someOtherClass
{
public string property1{ get; set; };
public string property2{ get; set; };
public string Key{ get; set; };
}
List<someOtherClass> objects = dictionary
.Select(kv => new someOtherClass(){
property1 = kv.Value.property1,
property2 = kv.Value.property2,
Key = kv.Key
})
.ToList();
You may use anonymous type if you don't want to store the result like this:
In case you just wana use it as datasource for example.
var res = myDictionary.Select(pair => new { pair.Key, pair.Value.Property1, pair.Value.Property2 });
The other answers are good, so this is just a supplement.
You could use arrays of Length three:
var arrays = myDictionary
.Select(kv => new[] { kv.Value.property1, kv.Value.property2, kv.Key, });
Or you could write a new class
class SomeclassAndKey
{
public string property1;
public string property1;
public string Key;
}
and then say
var someclassAndKeys = myDictionary
.Select(kv => new SomeclassAndKey { property1 = kv.Value.property1, property2 = kv.Value.property2, Key = kv.Key, });
In each case you could append .ToList() if you wanted not to defer enumeration and get a full List<> out.
I have a Dictionary and I want to sort it by Person, could anyone write me how to do it? Yeah, I tried to find it in the internet, but without success :((
Class subject has 3 parametres - name (string), family_name (string), age (int).
And I want to sort it by for example family_name, if same then by age and finally by name (if also age is same) - could anyone write me the code how to do it?? Thanks a lot for your help ;-)
There are SortedDictionary and SortedList you can use; you'd want to make an IComparer for Person as well. See this for a discussion of their differences: http://msdn.microsoft.com/en-us/library/5z658b67(VS.80).aspx
The Dictionary class can't be sorted due to the way it stores values, but you can easily get a sorted collection of the values that you have in your dictionary using LINQ.
var sorted = dictionary.Values.OrderBy(s => s.name)
.ThenBy(s => s.family_name)
.ThenBy(s => s.age);
Dictionary is structure without order, so you cannot sort it - you can sort ordered structures like list
You can use SortedList or SortedDictionary, both of which sort by the key.
Since the key is a class you created, you can pass an IEqualityComparer into the constructor to control how it gets sorted.
Define a Person and a Comparer class :
public class Person
{
public string Name { get; set; }
public string FamilyName { get; set; }
public int Age { get; set; }
}
public class PersonComparer : IComparer<KeyValuePair<string, Person>>
{
public int Compare(KeyValuePair<string, Person> x, KeyValuePair<string, Person> y)
{
var result = x.Value.FamilyName.CompareTo(y.Value.FamilyName);
if(result != 0)
return result;
return x.Value.Age.CompareTo(y.Value.Age);
}
}
and use these classes lie this :
var myDictionary = new Dictionary<string, Person>();
//fill your dictionary
var list = myDictionary.ToList();
list.Sort(new PersonComparer());
this is a simple and working solution.
I have a List<MyClass> someList.
class MyClass
{
public int Prop1...
public int Prop2...
public int Prop3...
}
I would like to know how to get a new distinct List<MyClass> distinctList from List<MyClass> someList, but only comparing it to Prop2.
You can emulate the effect of DistinctBy using GroupBy and then just using the first entry in each group. Might be a bit slower that the other implementations though.
someList.GroupBy(elem=>elem.Prop2).Select(group=>group.First());
Unfortunately there's no really easy built-in support for this in the framework - but you can use the DistinctBy implementation I have in MoreLINQ.
You'd use:
var distinctList = someList.DistinctBy(x => x.Prop2).ToList();
(You can take just the DistinctBy implementation. If you'd rather use a Microsoft implementation, I believe there's something similar in the System.Interactive assembly of Reactive Extensions.)
you need to use .Distinct(..); extension method.
Here's a quick sample:
public class Comparer : IEqualityComparer<Point>
{
public bool Equals(Point x, Point y)
{
return x.X == y.X;
}
public int GetHashCode(Point obj)
{
return (int)obj.X;
}
}
Do not forget about GetHashCode.
Usage:
List<Point> p = new List<Point>();
// add items
p.Distinct(new Comparer());
Override Equals(object obj) and GetHashCode() methods:
class MyClass
{
public int Prop1 { get; set; }
public int Prop2 { get; set; }
public int Prop3 { get; set; }
public override bool Equals(object obj)
{
return ((MyClass)obj).Prop2 == Prop2;
}
public override int GetHashCode()
{
return Prop2.GetHashCode();
}
}
and then just call:
List<MyClass> distinctList = someList.Distinct().ToList();
Since the introduction of value tuples, if you want a LINQ equivalent to SQL's DISTINCT
items.GroupBy(item => (item.prop1, item.prop2, ...)).Select(group => group.First())
If you would like to Distinct your list by multiple fields, You have to create an instance of IEqualityComparer interface:
public class MyComparer : IEqualityComparer<MyModel>
{
public bool Equals(MyModel x, MyModel y)
{
// compare multiple fields
return
x.Field1 == y.Field1 &&
x.Field2 == y.Field2 &&
x.Field3 == y.Field3 ;
}
public int GetHashCode(MyModel obj)
{
return
obj.Field1.GetHashCode() +
obj.Field2.GetHashCode() +
obj.Field3.GetHashCode();
}
}
Then use the comparer to distinct your list:
var distinctedList = myList.Distinct(new MyComparer()).ToList();
I know it's been a while, but I needed the simplest answer and at this time (with .NET 4.5.1) I found the following to be the most straight-forward answer I could get to:
IEnumerable<long> allIds = waitingFiles.Values.Select(wf => wf.groupId).Distinct();
My situation is that I have a ConcurrentDictionary that looks something like:
ConcurrentDictionary<long, FileModel>
The ConcurrentDictionary Values property is basically my List<FileModel>.
*FileModel has a groupId that isn't necessarily unique (though, obviously the key (long) that I use to add the FileModel object into the dictionary is unique to the FileModel).
*Named for clarity in the example.
The point is that I have a large number of FileModels (imagine 100) in the ConcurrentDictionary and within those 100 FileModels there are 5 different groupIds.
At this point I just need a list of the distinct groupId.
So, again if I just had a list of FileModel the code would look like the following:
IEnumerable <long> allIds = allFileModel.Select(fm => fm.groupId).Distinct();
I have a list of objects where the object has 2 attributes: id and name. I have to order the list by the name of the objects
How can I do this?
Have you tried using linq?
using System.Collections.Generic;
using System.Linq;
namespace Demo
{
public class Test
{
public void SortTest()
{
var myList = new List<Item> { new Item { Name = "Test", Id = 1 }, new Item { Name = "Other", Id = 1 } };
var result = myList.OrderBy(x => x.Name);
}
}
public class Item
{
public string Name { get; set; }
public int Id { get; set; }
}
}
Linq is overkill for this when there are built-in methods to do this.
If you mean an array, just use Array.Sort:
Array.Sort(items, (x, y) => x.Name.CompareTo(y.Name));
If you mean a List<T>, use the Sort function there:
items.Sort((x, y) => x.Name.CompareTo(y.Name));
If you are dealing with a List of these objects, the easiest way is to supply a Comparison to the Sort method:
var list = new List<MyObject>();
...
list.Sort((item1, item2) => item1.Name.CompareTo(item2.Name));
(Above sample doesn't do null checking on the name)
This is nice and simple because string implements IComparable<string>. An alternative would be for you to implement IComparable<MyObject> on your class, and then you could just call list.Sort()
The Sort method of the List<T> class has an overload that takes a custom comparison:
list.Sort((x,y) => x.Name.CompareTo(y.Name));