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C# StringBuilder Secrets
These C# examples use the StringBuilder class. StringBuilder improves string append performance.StringBuilder. A string can be appended to a million times.
This works but is slow. Its contents are copied each time data is appended.
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.
An intro. This program uses StringBuilder to build up a buffer of characters. We call Append to add more data to our StringBuilder.
Append: The Append method can be called directly on its own result, in the same statement.
Because: Append(), and other methods like AppendFormat, return the same StringBuilder.
Based on:
.NET 4.5
C# program that uses StringBuilder
using System;
using System.Text;
class Program
{
static void Main()
{
StringBuilder builder = new StringBuilder();
// Append to StringBuilder.
for (int i = 0; i < 10; i++)
{
builder.Append(i).Append(" ");
}
Console.WriteLine(builder);
}
}
Output
0 1 2 3 4 5 6 7 8 9
Methods. Next, we look at other essential methods on StringBuilder. The methods shown here allow us to use it effectively in many programs. We append strings and lines.
Note: This example shows no loop, and is not ideal as a program. It is for demonstration purposes.
Append: Adds the contents of its argument to the buffer in the StringBuilder. Arguments are converted to strings with ToString.
AppendLine: This does the same thing as Append, except with a newline on the end. We use a terse syntax form.
ToString: This returns the buffer. We will almost always want ToString—it will return the contents as a string.
C# program that uses StringBuilder methods
using System;
using System.Text;
using System.Diagnostics;
class Program
{
static void Main()
{
// 1.
// Declare a new StringBuilder.
StringBuilder builder = new StringBuilder();
// 2.
builder.Append("The list starts here:");
// 3.
builder.AppendLine();
// 4.
builder.Append("1 cat").AppendLine();
// 5.
// Get a reference to the StringBuilder's buffer content.
string innerString = builder.ToString();
// Display with Debug.
Debug.WriteLine(innerString);
}
}
Output
The list starts here:
1 cat
Memory. In garbage-collected systems, there is memory pressure. As more temporary objects are created, garbage collection runs more often. This slows a program down.
Tip: StringBuilder creates fewer temporary objects than string appends. Thus it adds less memory pressure.
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.
Immutable. This term indicates that an object's data is not changeable. To see an example of an immutable object, try to assign to a character in a string. This causes a compile-time error.
And: This happens because the string type does not define a set accessor. It cannot be modified once changed.
Arrays: Character arrays can be changed. Internally StringBuilder uses mutable char arrays for its buffer.
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.
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.
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.
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
Equals. This method compares the contents of two StringBuilders. It avoids lots of error-prone code that might otherwise be needed. It returns true or false.
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.
Also: Please see the "Cache objects" optimization tip for a way to use Clear to avoid allocations.
Argument. StringBuilder can be passed as an argument. This is an ice optimization. It avoids converting back and forth to strings.
Tip: Eliminating allocations of strings (and StringBuilders) is a highly effective way to improve program performance.
Caution: Usually it is best to use descriptive names, not "A1" or "b". Code should describe its intent.
C# program that creates many StringBuilders
using System;
using System.Text;
class Program
{
static string[] _items = new string[]
{
"cat",
"dog",
"giraffe"
};
/// <summary>
/// Append to a new StringBuilder and return it as a string.
/// </summary>
static string A1()
{
StringBuilder b = new StringBuilder();
foreach (string item in _items)
{
b.AppendLine(item);
}
return b.ToString();
}
static void Main()
{
// Called in loop.
A1();
}
}
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()
{
// Called in loop.
StringBuilder b = new StringBuilder();
A2(b);
}
}
Results
Version 1: 5039 ms
Version 2: 3073 ms
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# 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# 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
Substring. This is a neat trick with the Append method. We can append a substring directly from another string. No Substring call is needed.
ToString. This method is optimized. It will not copy data in certain situations. These optimizations are hard to duplicate in custom code.
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.
And: The method will throw an exception if the StringBuilder is empty. We can add a Length >= 1 check to fix this.
C# 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
Exceptions. We get an ArgumentOutOfRangeException if we put too much data in a StringBuilder. The maximum number of characters is equal to Int32.MaxValue.
Note: The Int32.MaxValue constant is equal to 2,147,483,647. This is the max length of a StringBuilder.
Constructor. Often we need to append a string to a StringBuilder right after allocating it. When we specify that string in the constructor, performance improves.
Tip: This optimization can be used with a capacity. Specify the initial string value along with the capacity.
Version 1
var builder = new StringBuilder(10);
builder.Append("Cat");
Version 2
var builder = new StringBuilder("Cat", 10);
Times
38.80 ns
32.47 ns
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.
Info: The StringBuilder is an optimization for building up strings. A char array is an alternative, and sometimes superior, optimization.
A mistake. Sometimes we make a StringBuilder mistake that reduces speed. We use the plus operator on strings within a StringBuilder. This is bad.
Performance. StringBuilder is mainly a performance optimization. Here we test the performance and memory usage of it. We learn when it is superior to strings.
Performance: AppendAppend CharsAppend IntegersCacheCapacityData TypesGraph
StringReader. We read and write string data with the StringReader and StringWriter types. StringReader loops over and reads lines (or blocks) from a source string.
Some comments. These secrets are widely known. But this makes them no less effective. StringBuilder can improve the performance of a program—even if we misuse it.
With optimal use, though, results are better. A program's memory use declines. Fewer garbage collections are done. Everything works better.