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C# string.Join ExamplesCall the string.Join method to combine an array of strings into one string with a separator.
Join. The string.Join method combines many strings into one. It receives 2 arguments: an array (or IEnumerable) and a separator string.
Separator note. Join() 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.
Array The first code statement in Main creates a string array with 3 elements (each a string literal).
Array
Join The string.Join method is called on the words array, and it returns a string, which is printed to the console.
Console
C# program that invokes string.Join method
using System; class Program { static void Main() { // Create string array. string[] words = { "one", "two", "three" }; // Invoke the Join method. // ... Print the resulting string. Console.WriteLine(string.Join(",", words)); } }
one,two,three
HTML. We can use string.Join to generate HTML. Often with HTML we need a separating tag or element. Join helps because it does not insert the separating tag at the end.
Step 1 The strings are concatenated with Join into 3 lines of markup in HTML, separated by the BR tag.
Step 2 We write the resulting HTML to a file, and then can test it in a web browser to see the rendered HTML.
C# program that joins HTML strings
using System; using System.IO; class Program { static void Main() { var items = new string[] { "Line 1", "Line 2", "Final line" }; // Step 1: join with break element in HTML. string html = string.Join("<br/>\r\n", items); // Step 2: write to text file. File.WriteAllText(@"C:\programs\test.html", html); Console.WriteLine("DONE"); } }
DONE
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.
StringBuilder
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.
TrimEnd, 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[] { ',' }); } }
bird,cat,dog,frog bird,cat,dog,frog
List. It is possible to join a List generic. This example includes the System.Collections.Generic namespace. Here a List is instantiated with 3 string literals in it.
List
String Literal
Next We call the string.Join<string> method. The first argument indicates the separator, and 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 a List if we do not have a string array available.
C# program that joins List of strings
using System; using System.Collections.Generic; class Program { static void Main() { // Create a List of 3 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); } }
cat*dog*rat
Exceptions. String.Join can throw 3 different exceptions. The first 2 exceptions (ArgumentNullException, ArgumentOutOfRangeException) are often possible.
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); } } }
System.ArgumentNullException: Value cannot be null. Parameter name: value
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.
Foreach
Result With .NET 5 for Linux, 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")); } }
58.42 ns string.Join 122.52 ns StringBuilder, Append, ToString
Result note. When we call Join, we do not get an ending delimiter on the result. The separator only is placed between elements—this is desired in some programs, but not in others.
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.
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