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Join multiple lists of objects in c#

I have three lists that contain objects with following structure:
List1
- Status
- ValueA
List2
- Status
- ValueB
List3
- Status
- ValueC
I want to joint the lists by status to get a final list that contains object with following structure:
- Status
- ValueA
- ValueB
- ValueC
Not every list has all the status. So a simple (left) join won't do it. Any ideas how to achieve the desired result? I tried with
var result = from first in list1
join second in list2 on first.Status equals second.Status into tmp1
from second in tmp1.DefaultIfEmpty()
join third in list3 on first.Status equals third.Status into tmp2
from third in tmp2.DefaultIfEmpty()
select new { ... };
But result is missing a status. Here is a full MRE:
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
List<C> third = new List<C>() { new C("BAZ", 5) };
var result = from f in first
join s in second on f.Status equals s.Status into tmp1
from s in tmp1.DefaultIfEmpty()
join t in third on f.Status equals t.Status into tmp2
from t in tmp2.DefaultIfEmpty()
select new
{
Status = f.Status,
ValueA = f.ValueA,
ValueB = s.ValueB,
ValueC = t.ValueC,
};
}
}
public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);

Unfortunately it is unclear, what should happen, if a status occurs multiple times within one list, cause your aggregate can only hold one value per status.
One possibility to solve this issue would be:
using System;
using System.Linq;
using System.Collections.Generic;
public class Program
{
public static void Main()
{
List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
List<C> third = new List<C>() { new C("BAZ", 5) };
var allStates = first.Select(a => a.Status)
.Concat(second.Select(b => b.Status))
.Concat(third.Select(c => c.Status))
.Distinct();
var result = allStates
.Select(Status => new
{
Status,
ValueA = first.FirstOrDefault(a => a.Status == Status),
ValueB = second.FirstOrDefault(b => b.Status == Status),
ValueC = third.FirstOrDefault(c => c.Status == Status),
});
foreach (var item in result)
{
Console.WriteLine(item);
}
}
}
public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);
Depending on the amount of items that have to be aggregated and the premise that each status occurs only once or never it could make sense to convert your lists to a Dictionary<string, A>, Dictionary<string, B>, etc. to improve the lookup and do something like this in the aggregate:
ValueA = dictFirst.ContainsKey(Status) ? dictFirst[Status] : null
For further improvements (this line makes the lookup twice) you could also factor out a method like this
private static T GetValueOrDefault<T>(IReadOnlyDictionary<string, T> dict, string status)
{
dict.TryGetValue(status, out T value);
return value;
}
And within the .Select() method call it with
ValueA = GetValueOrDefault(firstDict, Status);
Creating the dictionary for the list could be done with:
var firstDict = first.ToDictionary(a => a.Status);

With assumption that status names are unique per list here is a solution
in a single query with help of switch expressions (available since C# 8.0):
using System;
using System.Linq;
using System.Collections.Generic;
List<A> first = new List<A>() { new A("FOO", 1), new A("BAR", 2) };
List<B> second = new List<B>() { new B("FOO", 6), new B("BAR", 3) };
List<C> third = new List<C>() { new C("BAZ", 5) };
var result = first
// concat lists together
.Cast<object>()
.Concat(second)
.Concat(third)
// group on Status value with help of switch expression
.GroupBy(el => el switch {
A a => a.Status,
B b => b.Status,
C c => c.Status,
},
// project groups with anonymous type
(Status, group) => new {
Status,
ValueA = group.OfType<A>().Select(a => a.ValueA).Cast<int?>().FirstOrDefault(),
ValueB = group.OfType<B>().Select(b => b.ValueB).Cast<int?>().FirstOrDefault(),
ValueC = group.OfType<C>().Select(c => c.ValueC).Cast<int?>().FirstOrDefault()
});
public record A(string Status, int ValueA);
public record B(string Status, int ValueB);
public record C(string Status, int ValueC);

This can't using left join.First you must get all keies,then using all keies left join other lists:
var keys = first.Select(item => item.Status).ToList();
keys.AddRange(second.Select(item => item.Status));
keys.AddRange(third.Select(item => item.Status));
keys = keys.Distinct().ToList();
var result = (from k in keys JOIN
f in first on k equals f.Status into tmp0
from f in tmp0.DefaultIfEmpty()
join s in second on k equals s.Status into tmp1
from s in tmp1.DefaultIfEmpty()
join t in third on k equals t.Status into tmp2
from t in tmp2.DefaultIfEmpty()
select new {
Status = k,
ValueA = f?.ValueA,
ValueB = s?.ValueB,
ValueC = t?.ValueC,
}
).ToList();

How to combine two different GroupedStreams in Rx.NET?

This question is similar, but it does not apply to my case, since the user needed the merge observable streams from the same IGroupedObservable, while I want to combine streams from different groups.
I have the following structures and streams:
type A = {
Id: int
Value: int
}
type B = {
Id: int
Value: int
}
//subjects to test input, just any source of As and Bs
let subjectA: Subject<A> = Subject.broadcast
let subjectB: Subject<B> = Subject.broadcast
//grouped streams
let groupedA: IObservable<<IGroupedObservable<int, A>> = Observable.groupBy (fun a -> a.Id) subjectA
let groupedB: IObservable<<IGroupedObservable<int, B>> = Observable.groupBy (fun b -> b.Id) subjectB
I want to somehow merge the internal observables of A and B when groupedA.Key = groupedB.Key, and get an observable of (A, B) pairs where A.Id = B.Id
The signature I want is something like
IObservable<IGroupedObservable<int, A>> -> IObservable<IGroupedObservable<int, B>> -> IObservable<IGroupedObservable<int, (A, B)>> where for all (A, B), A.Id = B.Id
I tried a bunch of combineLatest, groupJoin, filters and maps variations, but with no success.
I'm using F# with Rx.Net and FSharp.Control.Reactive, but if you know the answer in C# (or any language, really) please post it

Here is a custom operator GroupJoin that you could use. It is based on the Select, Merge, GroupBy and Where operators:
/// <summary>
/// Groups and joins the elements of two observable sequences, based on common keys.
/// </summary>
public static IObservable<(TKey Key, IObservable<TLeft> Left, IObservable<TRight> Right)>
GroupJoin<TLeft, TRight, TKey>(
this IObservable<TLeft> left,
IObservable<TRight> right,
Func<TLeft, TKey> leftKeySelector,
Func<TRight, TKey> rightKeySelector,
IEqualityComparer<TKey> keyComparer = null)
{
// Arguments validation omitted
keyComparer ??= EqualityComparer<TKey>.Default;
return left
.Select(x => (x, (TRight)default, Type: 1, Key: leftKeySelector(x)))
.Merge(right.Select(x => ((TLeft)default, x, Type: 2, Key: rightKeySelector(x))))
.GroupBy(e => e.Key, keyComparer)
.Select(g => (
g.Key,
g.Where(e => e.Type == 1).Select(e => e.Item1),
g.Where(e => e.Type == 2).Select(e => e.Item2)
));
}
Usage example:
var subjectA = new Subject<A>();
var subjectB = new Subject<B>();
IObservable<IGroupedObservable<int, (A, B)>> query = subjectA
.GroupJoin(subjectB, a => a.Id, b => b.Id)
.SelectMany(g => g.Left.Zip(g.Right, (a, b) => (g.Key, a, b)))
.GroupBy(e => e.Key, e => (e.a, e.b));

I'm not clear if this is what you want. So it may be helpful to clarify first with runner code. Assuming the following runner code:
var aSubject = new Subject<A>();
var bSubject = new Subject<B>();
var groupedA = aSubject.GroupBy(a => a.Id);
var groupedB = bSubject.GroupBy(b => b.Id);
//Initiate solution
solution.Merge()
.Subscribe(t => Console.WriteLine($"(Id = {t.a.Id}, AValue = {t.a.Value}, BValue = {t.b.Value} )"));
aSubject.OnNext(new A() { Id = 1, Value = 1 });
aSubject.OnNext(new A() { Id = 1, Value = 2 });
bSubject.OnNext(new B() { Id = 1, Value = 10 });
bSubject.OnNext(new B() { Id = 1, Value = 20 });
bSubject.OnNext(new B() { Id = 1, Value = 30 });
Do you want to see the following output:
(Id = 1, AValue = 1, BValue = 10)
(Id = 1, AValue = 2, BValue = 10)
(Id = 1, AValue = 1, BValue = 20)
(Id = 1, AValue = 2, BValue = 20)
(Id = 1, AValue = 1, BValue = 30)
(Id = 1, AValue = 2, BValue = 30)
If that's the case, you can get to solution as follows:
var solution = groupedA.Merge()
.Join(groupedB.Merge(),
_ => Observable.Never<Unit>(),
_ => Observable.Never<Unit>(),
(a, b) => (a, b)
)
.Where(t => t.a.Id == t.b.Id)
.GroupBy(g => g.a.Id);
I'll caution that there are memory/performance impacts here if this is part of a long-running process. This keeps all A and B objects in memory indefinitely, waiting to see if they can be paired off. To shorten the amount of time they're kept in memory, change the Observable.Never() calls to appropriate windows for how long to keep each object in memory.

As a start, this has the signature you want:
let cartesian left right =
rxquery {
for a in left do
for b in right do
yield a, b
}
let mergeGroups left right =
rxquery {
for (leftGroup : IGroupedObservable<'key, 'a>) in left do
for (rightGroup : IGroupedObservable<'key, 'b>) in right do
if leftGroup.Key = rightGroup.Key then
let merged = cartesian leftGroup rightGroup
yield {
new IGroupedObservable<_, _> with
member __.Key = leftGroup.Key
member __.Subscribe(observer) = merged.Subscribe(observer)
}
}
However, in my testing, the groups are all empty. I don't have enough Rx experience to know why, but perhaps someone else does.

How to perform a recursive function in c# using linq

I have the following table:
Column_1 Column_2
val_1 | val_14
val_2 | val_17
val_1 | val_2
val_4 | null
val_1 | val_3
val_20 | val_4
val_17 | null
val_2 | val_20
val_14 | val_6
val_14 | null
Val_6 | null
val_3 | val_30
val_3 | val_19
I want to display Column_2 values
Eg: Select with Column_1 = val_1 will return (val_14, val_2, val_3) from Column_2.
Now, I want for each values in (val_14, val_2, val_3) to return also values from Column_2.
In summary:
val_1 => (val_14, val_2, val_3)
val_14 => (val_6, null)
val_6 => null
val_2 => (val_17, val_20)
val_17 => null
val_20 => (val_4)
val_4 => null
val_3 => (val_30, val_19)
etc...
Final output (val_14, val_2, val_3, val_6, val_17, val_20, val_4, val_30, val_19)
I have a function, with string parameter and list of all rows data
public List<string> MyFunction(string value)
{
return (from s in myListOfData where value.Contains(s.Column_1) select s).ToList();
}
This function return only the first level.
how can i do this query to display all children in linq? My attempts are unsuccessful.
Thank you

Desired order of records is a bit-tricky to get - looks like at first you want plain 1-st level and then traverse tree in down-left direction. It's a bit tricky.
If order is not important you can:
public List<string> MyFunction(string value)
{
return myListOfData
.Where(x => value.Contains(x.Column_1) && x.Column2 != null)
.Select(x => x.Column2)
.Aggregate(new List<string>(), (t, x) => {
t.Add(x);
t.AddRange(MyFunction(x));
return t; })
.ToList();
}
However, this results in lots of intermediate List creation. So better have enumerable:
public IEnumerable<string> MyFunction(string value)
{
foreach (var record in myListOfData.Where(x => value.Contains(x.Column_1) && x.Column2 != null)
{
yield return record.Column_2;
foreach (var child in MyFunction(record.Column_2))
yield return child;
}
}
And then take ToList() of this IEnumerable.
Still, if order is important you need two functions:
public List<string> MyFunction(string value)
{
.Where(x => value.Contains(x.Column_1) && x.Column2 != null)
.Select(x => new Tuple<string, IEnumerable<string>>(x.Column2, Traverse(x.Column2))
.Aggregate(new List<string>(), (t, x) => {
t.Add(x.Item1);
t.AddRange(x.Item2);
return t; })
.ToList();
}
public IEnumerable<string> Traverse(string value)
{
foreach (var record in myListOfData.Where(x => value.Contains(x.Column_1) && x.Column2 != null)
{
yield return record.Column_2;
foreach (var child in MyFunction(record.Column_2))
yield return child;
}
}

Assuming this is all in memory and nothing to do with an ORM.
You could use recursion. However, queues and stacks are safer and easier to debug.
Given some weird ill-defined class
public class Data
{
public int? Col1 { get; set; }
public int? Col2 { get; set; }
public Data(int? col1, int? col2)
{
Col1 = col1;
Col2 = col2;
}
}
You could use an iterator method and a Queue
public static IEnumerable<int> GetRecusive(List<Data> source,int val)
{
var q = new Queue<int>();
q.Enqueue(val);
while (q.Any())
{
var current = q.Dequeue();
var potential = source.Where(x => x.Col1 == current && x.Col2 != null);
foreach (var item in potential)
{
yield return item.Col2.Value;
q.Enqueue(item.Col2.Value);
}
}
}
Usage
// some ill-defined test data
var list = new List<Data>()
{
new Data(1, 14),
new Data(2, 17),
new Data(1, 2),
new Data(4, null),
new Data(1, 3),
new Data(20, 4),
new Data(17, null),
new Data(2, 20),
new Data(14, 6),
new Data(14, null),
new Data(6, null),
new Data(3, 30),
new Data(3, 19),
};
var results = GetRecusive(list,1);
// compose as a comma separated list
Console.WriteLine(string.Join(", ",results));
Output
14, 2, 3, 6, 17, 20, 30, 19, 4
Full Demo Here
If you like, you can turn it into an extension method to give you a LINQ Chain Method feel
public static IEnumerable<int> GetRecusive(this List<Data> source, int val)
Important Note : If you have a circular references then kiss your app goodbye. This will be the same for recursion or queues. If you need to protect against this, then I suggest using a HashSet of visited ids

How to remove duplicate pairs in a List

I got a List with pairs of integers. How do I remove pairs if they're duplicates? Distinct wont work cause the pair could be (2, 1) instead of (1, 2).
My list looks like this:
1, 2
2, 3
3, 1
3, 2
2, 4
4, 3
... I don't need (2, 3) and (3, 2)
I made a public struct FaceLine with public int A and B, then var faceline = new List<FaceLine>();.
I'm new to C# and lost.

You could use a custom IEqualityComparer<FaceLine>:
public class UnorderedFacelineComparer : IEqualityComparer<FaceLine>
{
public bool Equals(FaceLine x, FaceLine y)
{
int x1 = Math.Min(x.A, x.B);
int x2 = Math.Max(x.A, x.B);
int y1 = Math.Min(y.A, y.B);
int y2 = Math.Max(y.A, y.B);
return x1 == y1 && x2 == y2;
}
public int GetHashCode(FaceLine obj)
{
return obj.A ^ obj.B;
}
}
Then the query was very simple:
var comparer = new UnorderedFacelineComparer();
List<FaceLine> nonDupList = faceLine
.GroupBy(fl => fl, comparer)
.Where(g => g.Count() == 1)
.Select(g => g.First())
.ToList();
If you wanted to keep one of the duplicates you just need to remove the Where:
List<FaceLine> nonDupList = faceLine
.GroupBy(fl => fl, comparer)
.Select(g => g.First())
.ToList();

If you're happy using the common DistinctBy Linq extension (available via NuGet) you can do this fairly simply like so:
var result = list.DistinctBy(x => (x.A > x.B) ? (x.A, x.B) : (x.B, x.A));
Sample console app:
using System;
using System.Collections.Generic;
using MoreLinq;
namespace Demo
{
class Test
{
public Test(int a, int b)
{
A = a;
B = b;
}
public readonly int A;
public readonly int B;
public override string ToString()
{
return $"A={A}, B={B}";
}
}
class Program
{
static void Main()
{
var list = new List<Test>
{
new Test(1, 2),
new Test(2, 3),
new Test(3, 1),
new Test(3, 2),
new Test(2, 4),
new Test(4, 3)
};
var result = list.DistinctBy(x => (x.A > x.B) ? (x.A, x.B) : (x.B, x.A));
foreach (var item in result)
Console.WriteLine(item);
}
}
}

Using Linq :
List<List<int>> data = new List<List<int>>() {
new List<int>() {1, 2},
new List<int>() {2, 3},
new List<int>() {3, 1},
new List<int>() {3, 2},
new List<int>() {2, 4},
new List<int>() {4, 3}
};
List<List<int>> results =
data.Select(x => (x.First() < x.Last())
? new { first = x.First(), last = x.Last() }
: new { first = x.Last(), last = x.First() })
.GroupBy(x => x)
.Select(x => new List<int>() { x.First().first, x.First().last }).ToList();

Form a set of sets and you get the functionality for free (each smaller set contains exactly two integers).

How do I merge records using LINQ?

I'd like to merge two records using a condition for each column in the row. I'd give you a code sample but I don't know where to start.
class Foo
{
public int i {get;set;}
public int b{get;set;}
public string first{get;set;}
public string last{get;set;}
}
//...
var list = new List<Foo>() {
new Foo () { i=1, b=0, first="Vince", last="P"},
new Foo () { i=1, b=1, first="Vince", last="P"},
new Foo () { i=1, b=0, first="Bob", last="Z"},
new Foo () { i=0, b=1, first="Bob", last="Z"},
} ;
// This is how I'd like my result to look like
// Record 1 - i = 1, b = 1, first="Vince", last = "P"
// Record 2 - i = 1, b = 1, first="Bob", last = "Z"

You can group the result, then aggregate the fields from the items in the group:
var result = list.GroupBy(f => f.first).Select(
g => new Foo() {
b = g.Aggregate(0, (a, f) => a | f.b),
i = g.Aggregate(0, (a, f) => a | f.i),
first = g.Key,
last = g.First().last
}
);

You could use the Aggregate method in LINQ.
First add a method to Foo, say Merge that returns a new Foo based on your merging rules.
public Foo Merge (Foo other)
{
// Implement merge rules here ...
return new Foo {..., b=Math.Max(this.b, other,b), ...};
}
You could also, instead, create a helper method outside the Foo class that does the merging.
Now use Aggregate over your list, using the first element as the seed, merging each record with the current aggregate value as you go. Or, instead of using Aggregate (since it's a somewhat contrived use of LINQ in this case), just do:
Foo result = list.First();
foreach (var item in list.Skip(1)) result = result.Merge(item);
How are your merge rules specified?

I found a non-elegant solution that works
var result = list.GroupBy(i=>i.first);
foreach (IGrouping<string, Foo> grp in result)
{
grp.Aggregate ((f1, f2) => {
return new Foo() {
b = f1.b | f2.b,
i = f1.i | f2.i,
first = f1.first,
last = f1.last
};
});
}