C# StringBuilder Examples

Improve string append performance with StringBuilder. Save allocations when building strings.

StringBuilder. Like the pyramids, a string can be built one piece at a time. For strings, each append causes a string copy. With StringBuilder we eliminate this copy.

Type notes. Unlike a string, a StringBuilder can be changed. With it, an algorithm that modifies characters in a loop runs fast. Many string copies are avoided.

First example. This program uses StringBuilder to build up a buffer of characters. We call Append() to add more data to our StringBuilder.
Part 1: We create a StringBuilder class instance (an object) by using the new-operator.
Part 2: We call Append(). This method can be called directly on its own result, in the same statement.
Info: Append(), and other methods like AppendFormat, return the same StringBuilder.
C# program that uses StringBuilder using System; using System.Text; class Program { static void Main() { // Part 1: create new StringBuilder and loop. StringBuilder builder = new StringBuilder(); for (int i = 0; i < 3; i++) { // Part 2: append to StringBuilder. builder.Append(i).Append(" "); } Console.WriteLine(builder); } } Output 0 1 2

Methods. Here we use other essential methods on StringBuilder. Methods like Append(), and ToString(), are used in most C# programs that create StringBuilders.
Part 1: We create a StringBuilder. It begins its existence empty, with no buffered characters.
Part 2: We call Append and AppendLine. Arguments are converted to strings with ToString. AppendLine appends a newline.
StringBuilder Append
Part 3: We call ToString. This returns the buffer. We will usually want ToString—it will return the contents as a string.
C# program that uses various StringBuilder methods using System; using System.Text; class Program { static void Main() { // Part 1: declare a new StringBuilder. StringBuilder builder = new StringBuilder(); // Part 2: call Append and AppendLine. builder.Append("The list starts here:"); builder.AppendLine(); builder.Append("1 cat").AppendLine(); // Part 3: call ToString and display. string innerString = builder.ToString(); Console.WriteLine(innerString); } } Output The list starts here: 1 cat

Replace. This method replaces all instances of one string with another. A string is required, but we do not need to use a string literal. The example exchanges "an" with "the."
Caution: The Replace method will replace all instances of the specified value. To replace one instance, we will need a custom method.
C# program that uses Replace using System; using System.Text; class Program { static void Main() { StringBuilder builder = new StringBuilder( "This is an example string that is an example."); builder.Replace("an", "the"); // Replaces 'an' with 'the'. Console.WriteLine(builder.ToString()); Console.ReadLine(); } } Output This is the example string that is the example.

AppendFormat. With this method, we add text to a StringBuilder based on a pattern. We can use substitution markers to fill fields in this pattern.AppendFormat
Tip: Many versions of AppendFormat in the .NET Framework (such as Console.WriteLine) are implemented with StringBuilder.
However: It is usually faster to call Append repeatedly with all the required parts. But the syntax of AppendFormat may be clearer.
C# program that uses AppendFormat using System; using System.Text; class Program { static void Main() { var builder = new StringBuilder(); // Append a format string directly. builder.AppendFormat("Hello {0}. It is {1}.", "Ankit", "Thursday"); Console.WriteLine(builder); } } Output Hello Ankit. It is Thursday.

Loops. Often we use StringBuilders in loops. If many appends are needed, sometimes StringBuilder is helpful in other contexts. Here is an example of StringBuilder in a foreach-loop.Foreach
Note: Many looping constructs, including for, while and foreach, are effective when used with StringBuilder.
C# program that uses foreach using System; using System.Text; class Program { static void Main() { string[] items = { "Cat", "Dog", "Celebrity" }; StringBuilder builder2 = new StringBuilder( "These items are required:").AppendLine(); foreach (string item in items) { builder2.Append(item).AppendLine(); } Console.WriteLine(builder2.ToString()); Console.ReadLine(); } } Output These items are required: Cat Dog Celebrity

Argument. StringBuilder can be passed as an argument. This is a nice optimization. It avoids converting back and forth to strings.
Tip: Eliminating allocations of strings (and StringBuilders) is an effective way to improve program performance.
Caution: Usually it is best to use descriptive names, not "A1" or "b." Not all examples can be perfect.
C# program that uses StringBuilder argument using System; using System.Text; class Program { static string[] _items = new string[] { "cat", "dog", "giraffe" }; /// <summary> /// Append to the StringBuilder param, void method. /// </summary> static void A2(StringBuilder b) { foreach (string item in _items) { b.AppendLine(item); } } static void Main() { StringBuilder b = new StringBuilder(); A2(b); } }

Indexer. It is possible to use the indexer to access or change certain characters. This syntax is the same as the syntax for accessing characters in a string instance.
Next: The example tests and changes characters in a StringBuilder. It uses the indexer.
C# program that uses indexer using System; using System.Text; class Program { static void Main() { StringBuilder builder = new StringBuilder(); builder.Append("cat"); // Write second letter. Console.WriteLine(builder[1]); // Change first letter. builder[0] = 'r'; Console.WriteLine(builder.ToString()); } } Output a rat

Remove. This method removes a range of characters by index from the internal buffer. As with other StringBuilder methods, this just rearranges the internal buffer.
Here: We remove characters starting at index 4. We remove three characters past that index.
C# program that uses Remove using System; using System.Text; class Program { static void Main() { StringBuilder builder = new StringBuilder("Dot Net Perls"); builder.Remove(4, 3); Console.WriteLine(builder); } } Output Dot Perls

Append substring. We can append a substring directly from another string. No Substring call is needed. We use the Append method to do this.
IndexOf: We try to find the index of the space in the string. The char after the space is the start of the substring we want.
Append: We pass the string, the start index, and then the computed length (which continues until the end of the string).
Tip: Appending a substring directly is faster than calling Substring() to create an intermediate string first.
C# program that append Substring directly using System; using System.Text; class Program { static void Main() { var builder = new StringBuilder(); string value = "bird frog"; // Get the index of the char after the space. int afterSpace = value.IndexOf(' ') + 1; // Append a substring, computing the length of the target range. builder.Append(value, afterSpace, value.Length - afterSpace); Console.WriteLine("APPEND SUBSTRING: {0}", builder); } } Output APPEND SUBSTRING: frog

ToString. This method is optimized. It will not copy data in certain situations. These optimizations are hard to duplicate in custom code.
No arguments: We can call ToString with no arguments. This converts the entire StringBuilder into a string.
Range: We can pass a start index, and a length, to the ToString method. The second argument is the count of chars, not the end index.
Info: The ToString method has some advanced optimizations to reduce copying. It should be used when a string is required.
C# program that uses ToString using System; using System.Text; class Program { static void Main() { var builder = new StringBuilder("abcdef"); // Use ToString with no arguments. string result = builder.ToString(); Console.WriteLine("TOSTRING: {0}", result); // Use a start and length. string resultRange = builder.ToString(3, 3); Console.WriteLine("TOSTRING RANGE: {0}", resultRange); } } Output TOSTRING: abcdef TOSTRING RANGE: def

AppendJoin. In .NET Core in 2020, we can invoke AppendJoin on the StringBuilder. This eliminates a loop: we can append many elements (joined together) in a single statement.
Part A: We create a StringBuilder and a string array. We call AppendJoin on the StringBuilder—this is like calling string.Join and Append.
Part B: We can use AppendJoin with an int array. The ints are appended with inner separators.
Int, uint
C# program that uses AppendJoin using System; using System.Text; class Program { static void Main() { // Part A: use AppendJoin with string array. var builder = new StringBuilder(); string[] elements = { "bird", "frog", "dog" }; builder.AppendJoin(",", elements); Console.WriteLine(builder); // Part B: use AppendJoin with int array. builder.Clear(); int[] values = { 10, 20, 30 }; builder.AppendJoin(".", values); Console.WriteLine(builder); } } Output bird,frog,dog 10.20.30

Trim. StringBuilder has no Trim, TrimStart or TrimEnd methods. But we can implement similar methods. Here we add a TrimEnd method that removes a final character.
TrimEnd: This custom method tests the last character of a StringBuilder for a matching char. It then reduces Length by 1 to erase it.
Caution: There are issues here. Only one char will be removed—we could use a while-loop to remove multiple matching chars.
C# program that implements Trim on StringBuilder using System; using System.Text; class Program { static void TrimEnd(StringBuilder builder, char letter) { // ... If last char matches argument, reduce length by 1. if (builder[builder.Length - 1] == letter) { builder.Length -= 1; } } static void Main() { StringBuilder builder = new StringBuilder(); builder.Append("This has an end period."); Console.WriteLine(builder); TrimEnd(builder, '.'); Console.WriteLine(builder); } } Output This has an end period. This has an end period

Benchmark, string concat. Sometimes we make a StringBuilder mistake that reduces speed. We use the plus operator on strings within a StringBuilder. This is bad.
Version 1: This concats 2 strings and then passes that string to the StringBuilder Append() method.
Version 2: This version appends each string individually, avoiding a temporary string creation.
Result: It is much faster to call Append() individually and never to create any temporary strings with the plus operator.
C# program that improves StringBuilder performance using System; using System.Diagnostics; using System.Text; class Program { const int _max = 1000000; static void Main() { var builder1 = new StringBuilder(); var builder2 = new StringBuilder(); var tempString = 100.ToString(); // Version 1: concat strings then Append. var s1 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { builder1.Append(tempString + ","); } s1.Stop(); // Version 2: append individual strings. var s2 = Stopwatch.StartNew(); for (int i = 0; i < _max; i++) { builder2.Append(tempString).Append(","); } 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 38.92 ns Concat strings, then Append 15.42 ns Append twice

Benchmark, append substring. With the appropriate StringBuilder Append overload, we append a part of another string. This eliminates extra string copies.
Version 1: Here we take a substring of the input string and then append that to the StringBuilder.
Version 2: We call Append with 3 arguments. This is equivalent to the Substring call but much faster.
Result: The StringBuilder Append version that avoids a separate Substring call is faster.
C# program that uses StringBuilder overload using System; using System.Diagnostics; using System.Text; class Program { static void Method1(string input, StringBuilder buffer) { buffer.Clear(); string temp = input.Substring(3, 2); buffer.Append(temp); } static void Method2(string input, StringBuilder buffer) { buffer.Clear(); buffer.Append(input, 3, 2); } static void Main() { const int m = 100000000; var builder = new StringBuilder(); var s1 = Stopwatch.StartNew(); // Version 1: take Substring. for (int i = 0; i < m; i++) { Method1("perls", builder); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: append range with Append. for (int i = 0; i < m; i++) { Method2("perls", builder); } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / m).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / m).ToString("0.00 ns")); } } Output 33.47 ns Append string 25.14 ns Append range of string

Equals. This method compares the contents of 2 StringBuilders. It avoids lots of error-prone code that might otherwise be needed. It returns true or false.Equals
Caution: The Equals method will return false if the capacities of the objects are different, even if their data is identical.

Clear. To clear a StringBuilder, it is sometimes best to allocate a new one. Other times, we can assign the Length property to zero or use the Clear method.Clear

Exceptions. We get an ArgumentOutOfRangeException if we put too much data in a StringBuilder. The maximum number of characters is equal to Int32.MaxValue.ArgumentException
Note: The Int32.MaxValue constant is equal to 2,147,483,647. This is the max length of a StringBuilder.

Memory. In garbage collection, there is memory pressure. As more temporary objects are created, GC runs more often. StringBuilder creates fewer temporary objects than string appends.StringBuilder Memory

Performance, char arrays. To use char arrays, code must be more precise. We must know a maximum or absolute size of the output string. This can enhance performance.Char Array

A review. StringBuilder is mainly a performance optimization. Here we test the performance and memory usage of it. In reviewing, we learn when it is superior to strings.

© 2007-2020 Sam Allen. Send bug reports to info@dotnetperls.com.