Linq Order by a specific number first then show all rest in order

If i have a list of numbers:

1,2,3,4,5,6,7,8

and I want to order by a specific number and then show the rest.
For example if i pick ‘3’ the list should be:

3,1,2,4,5,6,7,8

Looking for linq and c#.
Thank you

Answers:

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Method 1

You can use a comparison in OrderBy or ThenBy to perform a conditional sorting.

list.OrderByDescending(i => i == 3).ThenBy(i => i);

I use OrderByDescending because i want matching results first(true is “higher” than false).

Method 2

A couple of answers already sort the last few numbers (which may be correct since you’re only showing an already sorted list). If you want the “unselected” numbers to be displayed in their original, not necessarily sorted order instead of sorted, you can instead do;

int num = 3;
var result = list.Where(x => x == num).Concat(list.Where(x => x != num));

As @DuaneTheriot points out, IEnumerable’s extension method OrderBy does a stable sort and won’t change the order of elements that have an equal key. In other words;

var result = list.OrderBy(x => x != 3);

works just as well to sort 3 first and keep the order of all other elements.

Method 3

Maybe something like this:

List<int> ls=new List<int>{1,2,3,4,5,6,7,8};
int nbr=3;
var result= ls.OrderBy (l =>(l==nbr?int.MinValue:l));

Method 4

public static IEnumerable<T> TakeAndOrder<T>(this IEnumerable<T> items, Func<T, bool> f)
{       
    foreach ( var item in items.Where(f))
        yield return item;
    foreach (var item in items.Where(i=>!f(i)).OrderBy(i=>i))
        yield return item;
}


var items = new [] {1, 4, 2, 5, 3};
items.TakeAndOrder(i=> i == 4);

Method 5

Using @joachim-isaksson idea I came up with this extension method:

public static IOrderedEnumerable<TSource> OrderByWithGivenValueFirst<TSource, TKey>(
    this IEnumerable<TSource> source, 
    Func<TSource, TKey> keySelector,
    TKey value
    ) 
    => source.OrderBy(x => !keySelector(x).Equals(value));

Test:

[TestFixture]
public class when_ordering_by_with_given_value_first
{
    [Test]
    public void given_value_is_first_in_the_collection()
    {
        var languages = new TestRecord[] {new("cs-CZ"), new("en-US"), new("de-DE"), new("sk-SK")};

        languages.OrderByWithGivenValueFirst(x => x.Language, "en-US")
            .ShouldBe(new TestRecord[] {new("en-US"), new("cs-CZ"), new("de-DE"), new("sk-SK")});

    }

    private record TestRecord(string Language);
}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0