C#•10mo ago

✅ Nice way to get an item from a list only if there is one, otherwise return null?

Is there something like SingleOrDefault() that returns default instead of exception?

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list.Where(x => x == 'a').SingleOrDefault();
//Exception: Sequence contains more than one element

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list.Where(x => x == 'a').SingleOrDefault();
//Exception: Sequence contains more than one element

This throws an exception if more than 1, however I want 'default' if more than 1

47 Replies

Angius•10mo ago

Just

var oneOrNone = list is [var el] ? el : null;

var oneOrNone = list is [var el] ? el : null;

MODiX•10mo ago

Angius

REPL Result: Failure

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list is [var el] ? el : null;
oneOrNone

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list is [var el] ? el : null;
oneOrNone

Exception: CompilationErrorException

- Type of conditional expression cannot be determined because there is no implicit conversion between 'char' and '<null>'

- Type of conditional expression cannot be determined because there is no implicit conversion between 'char' and '<null>'

Compile: 378.813ms | Execution: 0.000ms | React with ❌ to remove this embed.

Angius•10mo ago

Well, with a cast

var oneOrNone = list is [var el] ? el : (char?)null;

var oneOrNone = list is [var el] ? el : (char?)null;

might work, or

var oneOrNone = (char?)(list is [var el] ? el : null;)

var oneOrNone = (char?)(list is [var el] ? el : null;)

maybe

cowOP•10mo ago

how does that ensure there is only 1?

Angius•10mo ago

is [var el] Pattern matching

cowOP•10mo ago

hmm ok not familar with that syntax, i'll look it up

Angius•10mo ago

Matches a list with only one element

mtreit•10mo ago

I would just check the count but I'm old fashioned.

cowOP•10mo ago

yeah just wondering if theres a "modern" way to do it or also was considering copying the SingleOrDefault source and modify to return Default instead of throw exception

MODiX•10mo ago

Angius

REPL Result: Success

var lists = new[]{
    new[]{ 1, 2, 3, 4 },
    new[]{ 1 },
    new int[]{ }
};
foreach (var list in lists)
{
    var match = list is [var el] ? el : (int?)null;
    Console.WriteLine($"List: [{string.Join(',', list)}], Match: {(match is null ? "null" : match.ToString())}");
}

var lists = new[]{
    new[]{ 1, 2, 3, 4 },
    new[]{ 1 },
    new int[]{ }
};
foreach (var list in lists)
{
    var match = list is [var el] ? el : (int?)null;
    Console.WriteLine($"List: [{string.Join(',', list)}], Match: {(match is null ? "null" : match.ToString())}");
}

Console Output

List: [1,2,3,4], Match: null
List: [1], Match: 1
List: [], Match: null

List: [1,2,3,4], Match: null
List: [1], Match: 1
List: [], Match: null

Compile: 493.418ms | Execution: 108.732ms | React with ❌ to remove this embed.

Angius•10mo ago

You can, of course, do it the old fashioned way

var match = list.Count == 1 ? list[0] : (int?)null;

var match = list.Count == 1 ? list[0] : (int?)null;

cowOP•10mo ago

no that's not what im trying to acheive

Angius•10mo ago

"Return if there's only one element, null otherwise" That's what it does Ah, it's about the default? If so, then instead of (int?)null or (char?)null use default(int) or default(char) or whatever else the type is

cowOP•10mo ago

no see the example i gave just like SingleOrDefault() behaviour, but return null/default instead of throw exception if there are more than 1

Angius•10mo ago

So, in other words,

Angius•10mo ago

If there is one and only one element, return that element If there is zero or more than one elements, return default

cowOP•10mo ago

matching elements, not list.count

Angius•10mo ago

? Which of my examples returns a count?

br4kejet•10mo ago

Do you mean like list.Where(x => x == 'a').FirstOrDefault()? Can't remember if FirstOrDefault has a predicate overload or not

Angius•10mo ago

It does, yeah

br4kejet•10mo ago

Ah then i'd just put that lambda in FirstOrDefault and remove the Where

cowOP•10mo ago

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list.Where(x => x == 'a').NonThrowSingleOrDefault(); // should return default because multiple 'a' exist
var oneOrNone = list.Where(x => x == 'b').NonThrowSingleOrDefault(); // should return 'b'
var oneOrNone = list.Where(x => x == 'z').NonThrowSingleOrDefault(); // should return default because no 'z' exist

var list = new List<char> { 'a', 'b', 'c', 'a' };

var oneOrNone = list.Where(x => x == 'a').NonThrowSingleOrDefault(); // should return default because multiple 'a' exist
var oneOrNone = list.Where(x => x == 'b').NonThrowSingleOrDefault(); // should return 'b'
var oneOrNone = list.Where(x => x == 'z').NonThrowSingleOrDefault(); // should return default because no 'z' exist

FirstOrDefault will return 'a' , not default

Angius•10mo ago

So you want default if there's a single element, and otherwise null?

cowOP•10mo ago

correct

Angius•10mo ago

Good luck using it with reference types, then, since the default for them is null

br4kejet•10mo ago

Make an extension method that counts how many items match the predicate, then if it's 1, return that element Otherwise return default

Angius•10mo ago

But sure

var match = list.Count == 1 ? default(T) : (T?)null;

var match = list.Count == 1 ? default(T) : (T?)null;

T is whatever you want it to be

br4kejet•10mo ago

list.Count(x => x == 'a') == 1 ? list.First(x => x == 'a') : default You could optimise it more but still

cowOP•10mo ago

yep ok thanks carrot I'll do that. I thought perhaps there was a more succint way with modern c# that i wasnt aware of

Angius•10mo ago

Write an extension method

cowOP•10mo ago

public static TSource NonThrowSingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
    if (source == null) throw Error.ArgumentNull("source");
    if (predicate == null) throw Error.ArgumentNull("predicate");
    TSource result = default(TSource);
    long count = 0;
    foreach (TSource element in source) {
        if (predicate(element)) {
            result = element;
            checked { count++; }
        }
    }
    switch (count) {
        case 0: return default(TSource);
        case 1: return result;
    }
    
    return default(TSource);
    //throw Error.MoreThanOneMatch();
}

public static TSource NonThrowSingleOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {
    if (source == null) throw Error.ArgumentNull("source");
    if (predicate == null) throw Error.ArgumentNull("predicate");
    TSource result = default(TSource);
    long count = 0;
    foreach (TSource element in source) {
        if (predicate(element)) {
            result = element;
            checked { count++; }
        }
    }
    switch (count) {
        case 0: return default(TSource);
        case 1: return result;
    }
    
    return default(TSource);
    //throw Error.MoreThanOneMatch();
}

oop https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,9cd0418fc9042fbe

Angius•10mo ago

Something like that, yeah

cowOP•10mo ago

from there, just replace throw with return default

Angius•10mo ago

I probably wouldn't be throwing on null arguments Since we have NRTs and all that But yeah I'd add where TSource : struct constraint maybe Unless you do expect this method to always return null with reference types

cowOP•10mo ago

oh just realised that was .net 4.8 source code, i'll see what the modern source is

Angius•10mo ago

https://source.dot.net/#System.Linq/System/Linq/Single.cs,9cd0418fc9042fbe

cowOP•10mo ago

thank you can you F12 into it on Visual Studio? is there an option for that? to see c# source

cowOP•10mo ago

cowOP•10mo ago

i can only get as far as that the function definition/comments not the actual code

Angius•10mo ago

Not sure if VS comes with a decompiler by default Might need something like, dunno, DotPeek?

mtreit•10mo ago

VS can decompile

mtreit•10mo ago

When I F12 I get this:

mtreit•10mo ago

Oh that's from SourceLink though Not actually decompiled. But it will decompile as well.

mtreit•10mo ago

You need these options enabled:

cowOP•10mo ago

awesome 👍👍👍 thank you

Unknown User•10mo ago

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MODiX•10mo ago

Use the /close command to mark a forum thread as answered

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✅ Nice way to get an item from a list only if there is one, otherwise return null?