Say that I have LINQ query such as:
var authors = from x in authorsList
where x.firstname == "Bob"
select x;
Given that authorsList is of type List<Author>, how can I delete the Author elements from authorsList that are returned by the query into authors?
Or, put another way, how can I delete all of the firstname's equalling Bob from authorsList?
Note: This is a simplified example for the purposes of the question.
Well, it would be easier to exclude them in the first place:
authorsList = authorsList.Where(x => x.FirstName != "Bob").ToList();
However, that would just change the value of authorsList instead of removing the authors from the previous collection. Alternatively, you can use RemoveAll:
authorsList.RemoveAll(x => x.FirstName == "Bob");
If you really need to do it based on another collection, I'd use a HashSet, RemoveAll and Contains:
var setToRemove = new HashSet<Author>(authors);
authorsList.RemoveAll(x => setToRemove.Contains(x));
It'd be better to use List<T>.RemoveAll to accomplish this.
authorsList.RemoveAll((x) => x.firstname == "Bob");
If you really need to remove items then what about Except()?
You can remove based on a new list, or remove on-the-fly by nesting the Linq.
var authorsList = new List<Author>()
{
new Author{ Firstname = "Bob", Lastname = "Smith" },
new Author{ Firstname = "Fred", Lastname = "Jones" },
new Author{ Firstname = "Brian", Lastname = "Brains" },
new Author{ Firstname = "Billy", Lastname = "TheKid" }
};
var authors = authorsList.Where(a => a.Firstname == "Bob");
authorsList = authorsList.Except(authors).ToList();
authorsList = authorsList.Except(authorsList.Where(a=>a.Firstname=="Billy")).ToList();
You cannot do this with standard LINQ operators because LINQ provides query, not update support.
But you can generate a new list and replace the old one.
var authorsList = GetAuthorList();
authorsList = authorsList.Where(a => a.FirstName != "Bob").ToList();
Or you could remove all items in authors in a second pass.
var authorsList = GetAuthorList();
var authors = authorsList.Where(a => a.FirstName == "Bob").ToList();
foreach (var author in authors)
{
authorList.Remove(author);
}
I was wondering, if there is any difference between RemoveAll and Except and the pros of using HashSet, so I have done quick performance check :)
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
namespace ListRemoveTest
{
class Program
{
private static Random random = new Random( (int)DateTime.Now.Ticks );
static void Main( string[] args )
{
Console.WriteLine( "Be patient, generating data..." );
List<string> list = new List<string>();
List<string> toRemove = new List<string>();
for( int x=0; x < 1000000; x++ )
{
string randString = RandomString( random.Next( 100 ) );
list.Add( randString );
if( random.Next( 1000 ) == 0 )
toRemove.Insert( 0, randString );
}
List<string> l1 = new List<string>( list );
List<string> l2 = new List<string>( list );
List<string> l3 = new List<string>( list );
List<string> l4 = new List<string>( list );
Console.WriteLine( "Be patient, testing..." );
Stopwatch sw1 = Stopwatch.StartNew();
l1.RemoveAll( toRemove.Contains );
sw1.Stop();
Stopwatch sw2 = Stopwatch.StartNew();
l2.RemoveAll( new HashSet<string>( toRemove ).Contains );
sw2.Stop();
Stopwatch sw3 = Stopwatch.StartNew();
l3 = l3.Except( toRemove ).ToList();
sw3.Stop();
Stopwatch sw4 = Stopwatch.StartNew();
l4 = l4.Except( new HashSet<string>( toRemove ) ).ToList();
sw3.Stop();
Console.WriteLine( "L1.Len = {0}, Time taken: {1}ms", l1.Count, sw1.Elapsed.TotalMilliseconds );
Console.WriteLine( "L2.Len = {0}, Time taken: {1}ms", l1.Count, sw2.Elapsed.TotalMilliseconds );
Console.WriteLine( "L3.Len = {0}, Time taken: {1}ms", l1.Count, sw3.Elapsed.TotalMilliseconds );
Console.WriteLine( "L4.Len = {0}, Time taken: {1}ms", l1.Count, sw3.Elapsed.TotalMilliseconds );
Console.ReadKey();
}
private static string RandomString( int size )
{
StringBuilder builder = new StringBuilder();
char ch;
for( int i = 0; i < size; i++ )
{
ch = Convert.ToChar( Convert.ToInt32( Math.Floor( 26 * random.NextDouble() + 65 ) ) );
builder.Append( ch );
}
return builder.ToString();
}
}
}
Results below:
Be patient, generating data...
Be patient, testing...
L1.Len = 985263, Time taken: 13411.8648ms
L2.Len = 985263, Time taken: 76.4042ms
L3.Len = 985263, Time taken: 340.6933ms
L4.Len = 985263, Time taken: 340.6933ms
As we can see, best option in that case is to use RemoveAll(HashSet)
You can remove in two ways
var output = from x in authorsList
where x.firstname != "Bob"
select x;
or
var authors = from x in authorsList
where x.firstname == "Bob"
select x;
var output = from x in authorsList
where !authors.Contains(x)
select x;
I had same issue, if you want simple output based on your where condition , then first solution is better.
Below is the example to remove the element from the list.
List<int> items = new List<int>() { 2, 2, 3, 4, 2, 7, 3,3,3};
var result = items.Remove(2);//Remove the first ocurence of matched elements and returns boolean value
var result1 = items.RemoveAll(lst => lst == 3);// Remove all the matched elements and returns count of removed element
items.RemoveAt(3);//Removes the elements at the specified index
LINQ has its origins in functional programming, which emphasises immutability of objects, so it doesn't provide a built-in way to update the original list in-place.
Note on immutability (taken from another SO answer):
Here is the definition of immutability from Wikipedia.
In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created.
