How to join a list into a string in C#

List<string> langs = ["C#", "Go", "Python"];

Console.WriteLine(string.Join(", ", langs));
Console.WriteLine(string.Join(" | ", langs));
Console.WriteLine(string.Join("", langs));

// A char separator works too (.NET Core 2.0+)
Console.WriteLine(string.Join(';', langs));

// Any IEnumerable<string>: here a sorted set
var fruit = new SortedSet<string> { "pear", "apple", "fig" };
Console.WriteLine(string.Join(", ", fruit));

// ...and a string[], or the values written straight into the call
string[] parts = ["2026", "10", "04"];
Console.WriteLine(string.Join("-", parts));
Console.WriteLine(string.Join("/", "usr", "local", "bin"));

// Join part of an array: start index 1, take 2 elements
Console.WriteLine(string.Join(", ", parts, 1, 2));

// Edge cases: an empty list and a single element
Console.WriteLine("[" + string.Join(", ", new List<string>()) + "]");
Console.WriteLine(string.Join(", ", new List<string> { "solo" }));

List<int> nums = [3, 1, 4, 1, 5];

// string.Join<T> calls ToString() on each element: no conversion step
Console.WriteLine(string.Join(", ", nums));

// Brackets around the whole result are plain concatenation
Console.WriteLine("[" + string.Join(", ", nums) + "]");

// Filter, sort and format with LINQ, then join
Console.WriteLine(string.Join(" ", nums.Where(n => n > 1).Order().Select(n => $"#{n}")));

// A format string per element
double[] prices = [9.5, 12, 0.25];
Console.WriteLine(string.Join(", ", prices.Select(p => p.ToString("F2"))));

// Records print their properties; pick one with Select
List<Point> points = [new(1, 2), new(3, 4)];
Console.WriteLine(string.Join("; ", points));
Console.WriteLine(string.Join(", ", points.Select(p => p.X)));

// A class without a ToString override prints its type name
List<Box> boxes = [new(), new()];
Console.WriteLine(string.Join(", ", boxes));

record Point(int X, int Y);
class Box { }

string?[] names = ["Ada", null, "Grace", null];

// string.Join writes a null element as an empty string
Console.WriteLine(string.Join(", ", names));

// Drop the nulls...
Console.WriteLine(string.Join(", ", names.Where(n => n is not null)));

// ...or replace each one with a default
Console.WriteLine(string.Join(", ", names.Select(n => n ?? "?")));

// Empty and blank strings are not null: filter them too if they matter
List<string?> tags = ["csharp", "", "  ", null, "linq"];
Console.WriteLine(string.Join(",", tags));
Console.WriteLine(string.Join(",", tags.Where(t => !string.IsNullOrWhiteSpace(t))));

// A null separator is treated as an empty string
Console.WriteLine(string.Join(null, ["a", "b", "c"]));

// A null list is an error
List<string>? missing = null;
try
{
    Console.WriteLine(string.Join(", ", missing!));
}
catch (ArgumentNullException e)
{
    Console.WriteLine($"{e.GetType().Name}: {e.Message}");
}

using System.Text;

List<string> langs = ["C#", "Go", "Python"];

// No separator: string.Concat
Console.WriteLine(string.Concat(langs));
Console.WriteLine(string.Concat(langs.Select(l => l.ToLower())));

// LINQ's Aggregate can join, but it builds a new string at every step
Console.WriteLine(langs.Aggregate((acc, next) => acc + ", " + next));

// ...and it throws on an empty sequence
try
{
    Console.WriteLine(new List<string>().Aggregate((acc, next) => acc + ", " + next));
}
catch (InvalidOperationException e)
{
    Console.WriteLine($"{e.GetType().Name}: {e.Message}");
}

// The seeded overload doesn't throw, but needs the separator trimmed
string seeded = langs.Aggregate("", (acc, next) => acc + next + ", ");
Console.WriteLine("[" + seeded + "]");
Console.WriteLine("[" + seeded.TrimEnd(',', ' ') + "]");

// Aggregate with a StringBuilder seed avoids the copying
string built = langs
    .Aggregate(new StringBuilder(), (sb, next) => sb.Append(sb.Length > 0 ? ", " : "").Append(next))
    .ToString();
Console.WriteLine(built);
Console.WriteLine(built == string.Join(", ", langs));

using System.Text;

List<string> langs = ["C#", "Go", "Python"];

// The bug: a separator after every item leaves one at the end
var buggy = new StringBuilder();
foreach (var lang in langs)
{
    buggy.Append(lang).Append(", ");
}
Console.WriteLine("[" + buggy + "]");

// Fix 1: a separator variable that starts empty
var sb = new StringBuilder();
var sep = "";
foreach (var lang in langs)
{
    sb.Append(sep).Append(lang);
    sep = ", ";
}
Console.WriteLine("[" + sb + "]");

// Fix 2: trim the tail afterwards, guarding against an empty list
if (buggy.Length > 0)
{
    buggy.Length -= 2;
}
Console.WriteLine("[" + buggy + "]");

// Fix 3: AppendJoin (.NET Core 2.0+) joins straight into the builder
var report = new StringBuilder("Languages: ");
report.AppendJoin(", ", langs).Append('.');
Console.WriteLine(report);

Console.WriteLine(sb.ToString() == string.Join(", ", langs));

Console.WriteLine(Humanize(["C#"], oxford: false));
Console.WriteLine(Humanize(["C#", "Go"], oxford: false));
Console.WriteLine(Humanize(["C#", "Go", "Python"], oxford: false));
Console.WriteLine(Humanize(["C#", "Go", "Python"], oxford: true));

// One item per line
Console.WriteLine(string.Join(Environment.NewLine, ["first", "second"]));

// Dictionary entries as key=value pairs, sorted by key
var query = new Dictionary<string, int> { ["port"] = 8080, ["host"] = 1, ["debug"] = 0 };
Console.WriteLine(string.Join("&", query.OrderBy(kv => kv.Key).Select(kv => $"{kv.Key}={kv.Value}")));

// Repeat a string or a char
Console.WriteLine(string.Concat(Enumerable.Repeat("ab", 3)));
Console.WriteLine(new string('-', 20));

// Repeat with a separator: placeholders for a SQL IN (...) clause
Console.WriteLine(string.Join(", ", Enumerable.Repeat("?", 3)));

// "a, b and c" (or "a, b, and c" with the Oxford comma)
static string Humanize(IReadOnlyList<string> items, bool oxford)
{
    if (items.Count <= 2)
    {
        return string.Join(" and ", items);
    }
    string head = string.Join(", ", items.Take(items.Count - 1));
    return head + (oxford ? ", and " : " and ") + items[^1];
}