Reputation: 67283
If I have two lists of type string (or any other type), what is a quick way of joining the two lists?
The order should stay the same. Duplicates should be removed (though every item in both links are unique). I didn't find much on this when googling and didn't want to implement any .NET interfaces for speed of delivery.
Upvotes: 469
Views: 714549
Reputation: 888
In C#12 we can use this collection expression:
List<MyType> newList = [.. listA, .. listB] // Merge the 2 list, keeping the order the same.
newList = newList.Distinct().ToList(); // Remove doubles.
Upvotes: 4
Reputation: 1986
In C# 12 you can simply do this:
List<string> mergedList = [..firstList, ..secondList];
Upvotes: 3
Reputation: 137198
You could try:
List<string> a = new List<string>();
List<string> b = new List<string>();
a.AddRange(b);
This preserves the order of the lists, but it doesn't remove any duplicates (which Union
would do).
This does change list a
. If you wanted to preserve the original lists then you should use Concat
(as pointed out in the other answers):
var newList = a.Concat(b);
This returns an IEnumerable
as long as a
is not null.
Upvotes: 770
Reputation: 8009
One way, I haven't seen mentioned that can be a bit more robust, particularly if you wanted to alter each element in some way (e.g. you wanted to .Trim()
all of the elements.
List<string> a = new List<string>();
List<string> b = new List<string>();
// ...
b.ForEach(x=>a.Add(x.Trim()));
Upvotes: 2
Reputation: 11
The two options I use are:
list1.AddRange(list2);
or
list1.Concat(list2);
However I noticed as I used that when using the AddRange
method with a recursive function, that calls itself very often I got an SystemOutOfMemoryException because the maximum number of dimensions was reached.
(Message Google Translated)
The array dimensions exceeded the supported range.
Using Concat
solved that issue.
Upvotes: 1
Reputation: 112
I just wanted to test how Union
works with the default comparer on overlapping collections of reference type objects.
My object is:
class MyInt
{
public int val;
public override string ToString()
{
return val.ToString();
}
}
My test code is:
MyInt[] myInts1 = new MyInt[10];
MyInt[] myInts2 = new MyInt[10];
int overlapFrom = 4;
Console.WriteLine("overlapFrom: {0}", overlapFrom);
Action<IEnumerable<MyInt>, string> printMyInts = (myInts, myIntsName) => Console.WriteLine("{2} ({0}): {1}", myInts.Count(), string.Join(" ", myInts), myIntsName);
for (int i = 0; i < myInts1.Length; i++)
myInts1[i] = new MyInt { val = i };
printMyInts(myInts1, nameof(myInts1));
int j = 0;
for (; j + overlapFrom < myInts1.Length; j++)
myInts2[j] = myInts1[j + overlapFrom];
for (; j < myInts2.Length; j++)
myInts2[j] = new MyInt { val = j + overlapFrom };
printMyInts(myInts2, nameof(myInts2));
IEnumerable<MyInt> myUnion = myInts1.Union(myInts2);
printMyInts(myUnion, nameof(myUnion));
for (int i = 0; i < myInts2.Length; i++)
myInts2[i].val += 10;
printMyInts(myInts2, nameof(myInts2));
printMyInts(myUnion, nameof(myUnion));
for (int i = 0; i < myInts1.Length; i++)
myInts1[i].val = i;
printMyInts(myInts1, nameof(myInts1));
printMyInts(myUnion, nameof(myUnion));
The output is:
overlapFrom: 4
myInts1 (10): 0 1 2 3 4 5 6 7 8 9
myInts2 (10): 4 5 6 7 8 9 10 11 12 13
myUnion (14): 0 1 2 3 4 5 6 7 8 9 10 11 12 13
myInts2 (10): 14 15 16 17 18 19 20 21 22 23
myUnion (14): 0 1 2 3 14 15 16 17 18 19 20 21 22 23
myInts1 (10): 0 1 2 3 4 5 6 7 8 9
myUnion (14): 0 1 2 3 4 5 6 7 8 9 20 21 22 23
So, everything works fine.
Upvotes: 0
Reputation: 401
See this link
public class ProductA
{
public string Name { get; set; }
public int Code { get; set; }
}
public class ProductComparer : IEqualityComparer<ProductA>
{
public bool Equals(ProductA x, ProductA y)
{
//Check whether the objects are the same object.
if (Object.ReferenceEquals(x, y)) return true;
//Check whether the products' properties are equal.
return x != null && y != null && x.Code.Equals(y.Code) && x.Name.Equals(y.Name);
}
public int GetHashCode(ProductA obj)
{
//Get hash code for the Name field if it is not null.
int hashProductName = obj.Name == null ? 0 : obj.Name.GetHashCode();
//Get hash code for the Code field.
int hashProductCode = obj.Code.GetHashCode();
//Calculate the hash code for the product.
return hashProductName ^ hashProductCode;
}
}
ProductA[] store1 = { new ProductA { Name = "apple", Code = 9 },
new ProductA { Name = "orange", Code = 4 } };
ProductA[] store2 = { new ProductA { Name = "apple", Code = 9 },
new ProductA { Name = "lemon", Code = 12 } };
//Get the products from the both arrays //excluding duplicates.
IEnumerable<ProductA> union =
store1.Union(store2);
foreach (var product in union)
Console.WriteLine(product.Name + " " + product.Code);
/*
This code produces the following output:
apple 9
orange 4
lemon 12
*/
Upvotes: 1
Reputation: 8378
targetList = list1.Concat(list2).ToList();
It's working fine I think so. As previously said, Concat returns a new sequence and while converting the result to List, it does the job perfectly. Implicit conversions may fail sometimes when using the AddRange method.
Upvotes: 22
Reputation: 19707
var bigList = new List<int> { 1, 2, 3 }
.Concat(new List<int> { 4, 5, 6 })
.ToList(); /// yields { 1, 2, 3, 4, 5, 6 }
Upvotes: 7
Reputation: 56984
Something like this:
firstList.AddRange (secondList);
Or, you can use the 'Union' extension method that is defined in System.Linq. With 'Union', you can also specify a comparer, which can be used to specify whether an item should be unioned or not.
Like this:
List<int> one = new List<int> { 1, 2, 3, 4, 5 };
List<int> second=new List<int> { 1, 2, 5, 6 };
var result = one.Union (second, new EqComparer ());
foreach( int x in result )
{
Console.WriteLine (x);
}
Console.ReadLine ();
#region IEqualityComparer<int> Members
public class EqComparer : IEqualityComparer<int>
{
public bool Equals( int x, int y )
{
return x == y;
}
public int GetHashCode( int obj )
{
return obj.GetHashCode ();
}
}
#endregion
Upvotes: 29
Reputation: 2487
List<string> list1 = new List<string>();
list1.Add("dot");
list1.Add("net");
List<string> list2 = new List<string>();
list2.Add("pearls");
list2.Add("!");
var result = list1.Concat(list2);
Upvotes: 4
Reputation: 3506
If some item(s) exist in both lists you may use
var all = list1.Concat(list2).Concat(list3) ... Concat(listN).Distinct().ToList();
Upvotes: 14
Reputation: 3480
As long as they are of the same type, it's very simple with AddRange:
list2.AddRange(list1);
Upvotes: 7
Reputation: 755587
The way with the least space overhead is to use the Concat extension method.
var combined = list1.Concat(list2);
It creates an instance of IEnumerable<T>
which will enumerate the elements of list1 and list2 in that order.
Upvotes: 144
Reputation: 51196
The Union method might address your needs. You didn't specify whether order or duplicates was important.
Take two IEnumerables and perform a union as seen here:
int[] ints1 = { 5, 3, 9, 7, 5, 9, 3, 7 };
int[] ints2 = { 8, 3, 6, 4, 4, 9, 1, 0 };
IEnumerable<int> union = ints1.Union(ints2);
// yields { 5, 3, 9, 7, 8, 6, 4, 1, 0 }
Upvotes: 49