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C# string.Join Examples

Use the string.Join method. Join combines an array of strings into one string with a separator.
Join. The string.Join method combines many strings into one. It receives two arguments: an array (or IEnumerable) and a separator string.IEnumerableStrings
When we call Join, it places the separator between every element of the collection in the returned string. The separator is not added to the start or end of the result.
First example. We combine string elements from an array into a new, single string with dividing characters. This example will produce the output with separating commas.

Step 1: The first line in Main creates a string array with 3 elements. It specified 3 string literals.

Array

Step 2: We call join with a lowercase "string." This is a static method. We need no string instance to call it.

Static

Step 3: We use an uppercase "String" which means the same thing. You do not need to specify a string instance.

Tip: You can specify 4 arguments on string.Join. The last two are the startIndex and the count.

Overload
C# program that joins strings using System; class Program { static void Main() { // Step 1: create string array. string[] arr = { "one", "two", "three" }; // Step 2: call lowercase "string" method. Console.WriteLine(string.Join(",", arr)); // Step 3: use uppercase "String" method. Console.WriteLine(String.Join(",", arr)); } } Output one,two,three one,two,three
HTML. We use string.Join to generate HTML. Often with HTML you need a separating tag or element. Join helps because it doesn't insert the separating tag at the end.

Next: The strings are concatenated with Join into four lines of markup in HTML, separated by the BR tag.

C# program that joins HTML strings using System; class Program { static void Main() { // Problem: combine these words into lines in HTML string[] dinosaurs = new string[] { "Aeolosaurus", "Deinonychus", "Jaxartosaurus", "Segnosaurus" }; // Solution: join with break tag. string html = string.Join("<br/>\r\n", dinosaurs); Console.WriteLine(html); } } Output Aeolosaurus<br/> Deinonychus<br/> Jaxartosaurus<br/> Segnosaurus
StringBuilder. We can replace code that appends strings in loops with a single call to string.Join. The string.Join method is often faster in addition to being simpler.

Part A: We call a method that combines strings with Join. A delimiter is not added onto the end.

Part B: This version of the method combines strings with StringBuilder and its Append method. A delimiter is added to the end.

Note: The end delimiter is added to the StringBuilder, but it is later removed. We call TrimEnd to remove the end delimiter.

StringBuilder ToStringTrimEnd, TrimStart
C# program that combines strings with Join using System; using System.Text; class Program { static void Main() { string[] animals = { "bird", "cat", "dog", "frog" }; // Part A: use method that calls string.Join. Console.WriteLine(CombineA(animals)); // Part B: use StringBuilder method. Console.WriteLine(CombineB(animals)); } static string CombineA(string[] arr) { return string.Join(",", arr); } static string CombineB(string[] arr) { StringBuilder builder = new StringBuilder(); foreach (string s in arr) { builder.Append(s).Append(","); } return builder.ToString().TrimEnd(new char[] { ',' }); } } Output bird,cat,dog,frog bird,cat,dog,frog
Exceptions. String.Join can throw 3 different exceptions. The first 2 exceptions (ArgumentNullException, ArgumentOutOfRangeException) are often possible.ArgumentExceptionOutOfMemoryException

Tip: This code shows what happens when you call string.Join with null parameters. It will throw an ArgumentNullException.

Exception
C# program that throws exception on Join using System; class Program { static void Main() { try { string bug = string.Join(null, null); // Null arguments are bad } catch (Exception ex) { Console.WriteLine(ex); } } } Output System.ArgumentNullException: Value cannot be null. Parameter name: value
List. It is possible to join a List generic. This example includes the System.Collections.Generic namespace. Here a List is instantiated with three string literals in it.ListString Literal

Next: We call the string.Join<string> method. The first argument indicates the separator. The second is a reference to the List.

Returns: The method returns a joined string containing the separator. It works the same way as the array version.

Tip: This method eliminates copies. It is preferable to use this version on your List if you do not have an array of your strings handy.

C# program that joins List of strings using System; using System.Collections.Generic; class Program { static void Main() { // Create a List of three strings. var list = new List<string>() { "cat", "dog", "rat" }; // Join the strings from the List. string joined = string.Join<string>("*", list); // Display. Console.WriteLine(joined); } } Output cat*dog*rat
A benchmark. We test the general performance of string.Join. This method appears to have excellent performance. We see that string.Join performs well—often better than loops.Benchmark

Version 1: We concatenate all the strings and delimiters together with the string.Join method.

Version 2: This version of the code concatenates the strings with StringBuilder, and leaves the trailing delimiter.

StringBuilderForeach

Result: The string.Join method finishes its task in less time. We should prefer string.Join when possible.

C# program that benchmarks string.Join, StringBuilder using System; using System.Diagnostics; class Program { static string CombineA(string[] arr) { return string.Join(",", arr); } static string CombineB(string[] arr) { var builder = new System.Text.StringBuilder(); foreach (string s in arr) { builder.Append(s).Append(","); } return builder.ToString(); // Has ending comma. } const int _max = 1000000; static void Main() { string[] arr = { "one", "two", "three", "four", "five" }; var s1 = Stopwatch.StartNew(); // Version 1: use string.Join. for (int i = 0; i < _max; i++) { if (CombineA(arr).Length == 0) { return; } } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use StringBuilder. for (int i = 0; i < _max; i++) { if (CombineB(arr).Length == 0) { return; } } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); } } Output 89.58 ns string.Join 141.05 ns StringBuilder, Append, ToString
A summary. Join is an important operation on the string type. It simplifies certain common operations on string arrays. It is helpful when StringBuilder is not needed.
We created comma-separated values and generated HTML source. And we found string.Join has excellent performance for common usages.
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