I've inserted a few StaffRole files into a combobox using the below;
for (int i=0; i < staffRoles.Count; i++)
{
user_Role_Combo.Items.Add(staffRoles[i]);
}
I'm trying to search the index of a specific element within the combo box so it displays the correct element when loaded, I've got this, but it just returns -1 everytime;
StaffRole sr = new StaffRole("",roleID);
int comboBoxID = user_Role_Combo.Items.IndexOf(sr);
I'm doing this correct way no?!
In order for your new StaffRole instance to be 'found' in the combobox you need to describe why two StaffRole instances should be considered equivalent.
So you need to override Equals and GetHashCode. Technically, you need only Equals, but these two methods need to be overriden together.
One way to deal with it is to base object equality on roleId equality, like this:
public override int GetHashCode() {
return roleId.GetHashCode();
}
public override bool Equals(object obj) {
if (obj == this) return true;
var other = obj as StaffRole;
if (other == null) return false;
return roleId == other.roleId;
}
I'm doing this correct way no?!
No. By default IndexOf will check if the same reference exists in the items list. Since it's a new StaffRole that you just instanciated, it doesn't exist in the list.
I think what you want to do is compare by ID. To do this, you could override Equals and GetHashCode in the StaffRole class. In your custom Equals method, you would compare two objects by role ID. After doing this, IndexOf will work as you expect it to, by comparing using IDs instead of references.
Perhaps you could use either
FindString(String)
FindStringExact(String)
Both methods will return the index of the element in the list that matches the value of the string parameter that the method receives.
Combobox documentation here.
I didn't want to replace the equal / hashcode mehtods as I need them to be different for different instances.
So, I used some Linq to find the proper element inside the collection:
this.comboBox_group.SelectedIndex =
this.comboBox_group.Items.IndexOf
(comboBox_group.Items.Cast<Group>().Where(x => x.Id == SelectedId).First());
Related
I have two DataTables. I applied the Except operator as follows,
and got either unfiltered or undesired results.
resultDataTable = dtA.AsEnumerable().Except(dtB.AsEnumerable()).CopyToDataTable();
Could anyone please kindly explain to me why Except(dtB.AsEnumerable()) is not the way to put it?
Note:
Both DataTables are plain simple with just one column.
dtA contains a dozen rows of strings.
dtB contains thousands of rows of strings.
I also tried the same syntax with another use case the set operator, Intersect. This does not work either.
resultDataTable2 =dtA.AsEnumerable().Intersect(dtB.AsEnumerable()).CopyToDataTable();
Except will use default comparer ie. it will compare references.
I think you are expecting to filter result and comparison is based on members.
I will recommend you to implement your own IEqualityComparer to compare two objects based on member.
e.g.
resultDataTable = dtA.AsEnumerable().Except(dtB.AsEnumerable(), new TestComparer()).CopyToDataTable();
class TestComparer : IEqualityComparer<MyTestClass>
{
public bool Equals(MyTestClass b1, MyTestClass b2)
{
if (b2 == null && b1 == null)
return true;
else if (b1 == null || b2 == null)
return false;
else if(b1.Prop1 == b2.Prop1 && b1.Prop2 == b2.Prop2) // ToDo add more check based on class
return true;
else
return false;
}
public int GetHashCode(MyTestClass)
{
int hCode = MyTestClass.Height ^ MyTestClass.Length ^ ....; // Add more based on class properties
return hCode.GetHashCode();
}
}
Doc
https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.except?view=net-6.0#system-linq-enumerable-except-1(system-collections-generic-ienumerable((-0))-system-collections-generic-ienumerable((-0))-system-collections-generic-iequalitycomparer((-0)))
Could anyone please kindly explain to me why Except(dtB.AsEnumerable()) is not the way to put it?
When you do a.Except(b) the contents of b are loaded into a hash set. A hash set is a device that doesn't accept duplicates, so it returns false when you try to add something that is already there inside it.
After b is loaded into the hash set, then a is looped over, also being added to the hash set. Anything that adds successfully (set.Add returns true because it is not already there) is returned. Anything that is already there (set.Add returns false because it was added by being in b, or appearing earlier in a) is not returned. You should note that this process also dedupes a, so 1,1,2,3 except 2,3 would return just a single 1. You've achieved "every unique thing in a that isn't in b" - but do check whether you wanted a to be deduped too
A hash set is a wonderful thing, that enables super fast lookups. To do this it relies on two methods that every object has: GetHashcode and Equals. GetHashcode converts an object into a probably-unique number. Equals makes absolutely sure an object A equals B. Hash set tracks all the hashcodes and objects it's seen before, so when you add something it first gets the hashcode of what youre trying to add.. If it never saw that hashcode before it adds the item. If you try add anything that has the same hashcode as something it saw already, it uses Equals to check whether or not it's the same as what it saw it already (sometimes hashcodes are the same for different data) and adds the item if Equals declares it to be different. This whole operation is very fast, much faster than searching object by object through all the objects it saw before.
By default any class in C# gets its implementation of GetHashcode and Equals from object. Object's versions of these methods essentially return the memory address for GetHashcode, and compare the memory addresses for Equals
This works fine for stuff that really is at the same memory address:
var p = new Person(){Name="John"};
var q = p; //same mem address as p
But it doesn't work for objects that have the same data but live at different memory addresses:
var p = new Person(){Name="John"};
var q = new Person(){Name="John"}; //not same mem address as p
If you define two people as being equal if they have the same name, and you want C# to consider them equal in the same way, you have to instruct C# to compare the names, not the memory addresses.
A DataRow is like Person above: just because it has the same data as another DataRow, doesn't mean it's the same row in C#'s opinion. Further, because a single DataRow cannot belong to two datatables, it's certain that the "John" in row 1 of dtA, is a different object to the "John" in row 1 of dtB..
By defaul Equals returns false for these two data rows, so Except will never consider them equal and remove dtA's John because of the presence of John in dtB..
..unless you provide an alternative comparison strategy that overrides C#s default opinion of equality. That might look like:
provide a comparer, like Kalpesh's answer, typos on the class name aside),
override Equals/GetHashcode for the datarows so they work off column data, not memory addresses,
or use some other thing that does already have Equals and GetHashcode that work off data rather than memory addresses
As these are just datatables of a single column full of strings they're notionally not much more than an array of string. If we make them into an array of strings, when we do a.Except(b) we will be comparing strings. By default C#s opinion of whether one string equals another is based on the data content of the string rather than the memory address it lives at1, so you can either use string arrays/lists to start with or convert your dtA/B to a string array:
var arrA = dtA.Rows.Cast<DataRow>().Select(r => r[0] as string).ToArray();
var arrB = dtB.Rows.Cast<DataRow>().Select(r => r[0] as string).ToArray();
var result = arra.Except(arrB);
Techncially we don't even need to call ToArray()..
If you really need the result to be a datatable, make one and add all the strings to it:
var resultDt = new DataTable();
resultDt.Columns.Add("x");
foreach(var s in result)
resultDt.Rows.Add(s);
1: we'll ignore interning for now
I have a table that has combo pairs identifiers, and I use that to go through CSV files looking for matches. I'm trapping the unidentified pairs in a List, and sending them to an output box for later addition. I would like the output to only have single occurrences of unique pairs. The class is declared as follows:
public class Unmatched:IComparable<Unmatched>
{
public string first_code { get; set; }
public string second_code { get; set; }
public int CompareTo(Unmatched other)
{
if (this.first_code == other.first_code)
{
return this.second_code.CompareTo(other.second_code);
}
return other.first_code.CompareTo(this.first_code);
}
}
One note on the above code: This returns it in reverse alphabetical order, to get it in alphabetical order use this line:
return this.first_code.CompareTo(other.first_code);
Here is the code that adds it. This is directly after the comparison against the datatable elements
unmatched.Add(new Unmatched()
{ first_code = fields[clients[global_index].first_match_column]
, second_code = fields[clients[global_index].second_match_column] });
I would like to remove all pairs from the list where both first code and second code are equal, i.e.;
PTC,138A
PTC,138A
PTC,138A
MA9,5A
MA9,5A
MA9,5A
MA63,138A
MA63,138A
MA59,87BM
MA59,87BM
Should become:
PTC, 138A
MA9, 5A
MA63, 138A
MA59, 87BM
I have tried adding my own Equate and GetHashCode as outlined here:
http://www.morgantechspace.com/2014/01/Use-of-Distinct-with-Custom-Class-objects-in-C-Sharp.html
The SE links I have tried are here:
How would I distinct my list of key/value pairs
Get list of distinct values in List<T> in c#
Get a list of distinct values in List
All of them return a list that still has all the pairs. Here is the current code (Yes, I know there are two distinct lines, neither appears to be working) that outputs the list:
parser.Close();
List<Unmatched> noDupes = unmatched.Distinct().ToList();
noDupes.Sort();
noDupes.Select(x => x.first_code).Distinct();
foreach (var pair in noDupes)
{
txtUnmatchedList.AppendText(pair.first_code + "," + pair.second_code + Environment.NewLine);
}
Here is the Equate/Hash code as requested:
public bool Equals(Unmatched notmatched)
{
//Check whether the compared object is null.
if (Object.ReferenceEquals(notmatched, null)) return false;
//Check whether the compared object references the same data.
if (Object.ReferenceEquals(this, notmatched)) return true;
//Check whether the UserDetails' properties are equal.
return first_code.Equals(notmatched.first_code) && second_code.Equals(notmatched.second_code);
}
// If Equals() returns true for a pair of objects
// then GetHashCode() must return the same value for these objects.
public override int GetHashCode()
{
//Get hash code for the UserName field if it is not null.
int hashfirst_code = first_code == null ? 0 : first_code.GetHashCode();
//Get hash code for the City field.
int hashsecond_code = second_code.GetHashCode();
//Calculate the hash code for the GPOPolicy.
return hashfirst_code ^ hashsecond_code;
}
I have also looked at a couple of answers that are using queries and Tuples, which I honestly don't understand. Can someone point me to a source or answer that will explain the how (And why) of getting distinct pairs out of a custom list?
(Side question-Can you declare a class as both IComparable and IEquatable?)
The problem is you are not implementing IEquatable<Unmatched>.
public class Unmatched : IComparable<Unmatched>, IEquatable<Unmatched>
EqualityComparer<T>.Default uses the Equals(T) method only if you implement IEquatable<T>. You are not doing this, so it will instead use Object.Equals(object) which uses reference equality.
The overload of Distinct you are calling uses EqualityComparer<T>.Default to compare different elements of the sequence for equality. As the documentation states, the returned comparer uses your implementation of GetHashCode to find potentially-equal elements. It then uses the Equals(T) method to check for equality, or Object.Equals(Object) if you have not implemented IEquatable<T>.
You have an Equals(Unmatched) method, but it will not be used since you are not implementing IEquatable<Unmatched>. Instead, the default Object.Equals method is used which uses reference equality.
Note your current Equals method is not overriding Object.Equals since that takes an Object parameter, and you would need to specify the override modifier.
For an example on using Distinct see here.
You have to implement the IEqualityComparer<TSource> and not IComparable<TSource>.
The Find() method for LinkedList in C# works fine for strings, etc, but how to use it with structs, objects, etc?
Here is my code:
{
LinkedList<Item> TestLinkedList = new LinkedList<Item>();
TestLinkedList.AddFirst(new Item(3, "Head n Shoulders"));
TestLinkedList.AddAfter(TestLinkedList.First, new Item(45, "Dell"));
//Get the 2nd node in the linklist
Item c = new Item(3, "Head n Shoulders");
LinkedListNode<Item> Node2 = TestLinkedList.Find(c);
TestLinkedList.AddAfter((Node2), new Item(32, "Adidas"));
foreach (Item i in TestLinkedList)
{
i.Print();
}
Console.ReadKey();
}
Its returning NULL for the Node2. Am I making the mistake at not using the unique Hashcode etc?
In order for the Find to return the correct object, your Item class needs to override the Equals method. Of course you also need to override GetHashCode as well: although Find of LinkedList does not call GetHashCode, the two methods need to be changed together:
Types that override Equals(Object) must also override GetHashCode; otherwise, hash tables might not work correctly.
I have 2 IList<T> of the same type of object ItemsDTO. I want to exclude one list from another. However this does not seem to be working for me and I was wondering why?
IList<ItemsDTO> related = itemsbl.GetRelatedItems();
IList<ItemsDTO> relating = itemsbl.GetRelatingItems().Except(related).ToList();
I'm trying to remove items in related from the relating list.
Since class is a reference type, your ItemsDTO class must override Equals and GetHashCode for that to work.
From MSDN:
Produces the set difference of two sequences by using the default
equality comparer to compare values.
The default equality comparer is going to be a reference comparison. So if those lists are populated independently of each other, they may contain the same objects from your point of view but different references.
When you use LINQ against SQL Server you have the benefit of LINQ translating your LINQ statement to a SQL query that can perform logical equality for you based on primary keys or value comparitors. With LINQ to Objects you'll need to define what logical equality means to ItemsDTO. And that means overriding Equals() as well as GetHashCode().
Except works well for value types. However, since you are using Ref types, you need to override Equals and GethashCode on your ItemsDTO in order to get this to work
I just ran into the same problem. Apparently .NET thinks the items in one list are different from the same items in the other list (even though they are actually the same). This is what I did to fix it:
Have your class inherit IEqualityComparer<T>, eg.
public class ItemsDTO: IEqualityComparer<ItemsDTO>
{
public bool Equals(ItemsDTO x, ItemsDTO y)
{
if (x == null || y == null) return false;
return ReferenceEquals(x, y) || (x.Id == y.Id); // In this example, treat the items as equal if they have the same Id
}
public int GetHashCode(ItemsDTO obj)
{
return this.Id.GetHashCode();
}
}
I have a list of items.
The problem is the returned items (which I have no control over) return the same items THREE time.
So while the actual things that should be in the list are:
A
B
C
I get
A
B
C
A
B
C
A
B
C
How can I cleanly and easily remove the duplicates? Maybe count the items, divide by three and delete anything from X to list.Count?
The quickest, simplest thing to do is to not remove the items but run a distinct query
var distinctItems = list.Distinct();
If it's a must that you have a list, you can always append .ToList() to the call. If it's a must that you continue to work with the same list, then you'd just have to iterate over it and keep track of what you already have and remove any duplicates.
Edit: "But I'm working with a class"
If you have a list of a given class, to use Distinct you need to either (a) override Equals and GetHashCode inside your class so that appropriate equality comparisons can be made. If you do not have access to the source code (or simply don't want to override these methods for whatever reason), then you can (b) provide an IEqualityComparer<YourClass> implementation as an argument to the Distinct method. This will also allow you to specify the Equals and GetHashCode implementations without having to modify the source of the actual class.
public class MyObjectComparer : IEqualityComparer<MyObject>
{
public bool Equals(MyObject a, MyObject b)
{
// code to determine equality, usually based on one or more properties
}
public int GetHashCode(MyObject a)
{
// code to generate hash code, usually based on a property
}
}
// ...
var distinctItems = myList.Distinct(new MyObjectComparer());
if you are 100% sure that you receive everything you need 3 times, then just
var newList = oldList.Take(oldList.Count / 3).ToList()
Linq has a Distinct() method which does exactly this. Or put the items in a HashSet if you want to avoid duplicated completely.
If you're using C# 3 or up:
var newList = dupList.Distinct().ToList();
If not then sort the list and do the following:
var lastItem = null;
foreach( var item in dupList )
{
if( item != lastItem )
{
newItems.Add(item);
}
lastItem = item;
}
you could simply create a new list and add items to it that are not already there.