C# Trim String Tips

Trim eliminates leading and trailing whitespace.

We need to remove whitespace from the beginning or ending of a string. We use the .NET Framework's Trim method to do this efficiently. This method removes any characters specified.

Trim input and output

String input:       "   This is an example string. "
Trim method result: "This is an example string."

String input:       "This is an example string.\r\n\r\n"
Trim method result: "This is an example string."

String input:       "\t, again and again."
TrimStart method:   "again and again."
Note:               You can remove commas, spaces, and tabs.

Example. This first example removes space characters from the beginning and end of a C# string. Note that when you call Trim on a string, it copies the string and returns a modified version of that copy. The original is not changed.

C# program that uses Trim

using System;

class Program
{
    static void Main()
    {
	// Input string
	string st = "  This is an example string. ";

	// Call Trim instance method.
	// This returns a new string copy.
	st = st.Trim();

	Console.WriteLine(st);
    }
}

Output
    (Spaces were removed.)

This is an example string.

The Trim method is used with no parameters, meaning it uses a default set of whitespace characters to Trim. You can provide an array parameter of chars, as we see in one of the following examples.

Char Array

Next: The Console.WriteLine statement prints the copied string that has no spaces at the beginning or end.

Console.WriteLine

Line breaks. We can remove new lines from strings with Trim(). The Trim method will remove both UNIX style line breaks and Windows style line breaks. This example shows Windows line breaks, \r\n.

It uses two Console.WriteLine method calls, each of which format the string they display. The output is surrounded by square brackets. Trim copies the input string, modifies the copy, and then returns a new copy.

Another program that uses Trim: C#

using System;

class Program
{
    static void Main()
    {
	// Input string
	string st = "This is an example string.\r\n\r\n";

	Console.WriteLine("[{0}]",
	    st);

	st = st.Trim();

	Console.WriteLine("[{0}]",
	    st);
    }
}

Output

[This is an example string.

]
[This is an example string.]

Files. We next trim each line from a file. This works well with StreamReader or other File methods. The following example demonstrates Trim on each line in the file. The file contains three lines, each ending with a space.

The code first uses File.ReadAllLines. Then it loops through the lines using foreach. Each line is trimmed. Because all the lines are in memory, the original file is not changed. Neither is the array returned by ReadAllLines.

File.ReadAllLines

C# program that uses Trim with file

using System;
using System.IO;

class Program
{
    static void Main()
    {
	foreach (string line in File.ReadAllLines("file.txt"))
	{
	    Console.WriteLine("[" + line + "]");
	    string trimmed = line.Trim();
	    Console.WriteLine("[" + trimmed + "]");
	}
	// Note:
	// In the loop you can use both strings in their
	// original forms. No string is actually changed.
	// Trim copies the string.
    }
}

Output: example file

[This file ]
[This file]
[Has some ]
[Has some]
[Whitespace you don't want ]
[Whitespace you don't want]

TrimStart, TrimEnd. This site contains a separate article that is an example of the TrimStart instance method on the string type in the C# programming language. Please consult the article for a detailed discussion of TrimStart and its usage patterns.

TrimEnd, TrimStart

Also: It is useful to call TrimEnd to remove punctuation from the ends of strings, such as the ending punctuation in a sentence.

Implementation. Looking in IL Disassembler at the MSIL code, Trim calls an internal method called TrimHelper. This contains several loops, and uses an O(N squared) algorithm. This means it loops within loops. Finally it calls InternalSubString.

IL Disassembler Tutorial

Tip: If you have to Trim many different chars, it would be faster to use a lookup table to test if the character is to be removed.

Therefore: Implementing a custom Trim that uses an array lookup instead of a nested loop would be faster. This was not benchmarked here.

Performance. As part of my analysis, I benchmarked how Trim performs on different kinds of strings. For example, I benchmarked Trim on a string that has characters to be trimmed, against a string that doesn't need changing.

My results were that Trim takes more time when it has more characters to remove from the source string. This is because it iterates through the characters until it can't trim any more. No numbers are available in this document.

Methods. There are other ways you will need to change or replace the whitespace in your C# strings. For example, you may need to change whitespace in the middle of strings, not just on the end as you can do with Trim methods.

Whitespace

Punctuation is another common character type we need to trim. This can be challenging with just the Trim method—a custom array is required. One alternative is to use looping and the char.IsPunctuation method.

Remove, Trim Punctuation

Regex. There are other ways to Trim strings, such as with regular expressions in the System.Text.RegularExpressions namespace. These approaches are documented elsewhere on this site. They tend to be more complex.

Regex TrimRegex.Match

Summary. We saw many examples and tips for the Trim method. We found potential problems with Trim. And we pointed to various alternatives. As a developer, I have used Trim in every nontrivial program I have written.

Related Links