How to modify string contents in C#

Article
09/15/2021

This article demonstrates several techniques to produce a string by modifying an existing string. All the techniques demonstrated return the result of the modifications as a new string object. To demonstrate that the original and modified strings are distinct instances, the examples store the result in a new variable. You can examine the original string and the new, modified string when you run each example.

Note

The C# examples in this article run in the Try.NET inline code runner and playground. Select the Run button to run an example in an interactive window. Once you execute the code, you can modify it and run the modified code by selecting Run again. The modified code either runs in the interactive window or, if compilation fails, the interactive window displays all C# compiler error messages.

There are several techniques demonstrated in this article. You can replace existing text. You can search for patterns and replace matching text with other text. You can treat a string as a sequence of characters. You can also use convenience methods that remove white space. Choose the techniques that most closely match your scenario.

Replace text

The following code creates a new string by replacing existing text with a substitute.

string source = "The mountains are behind the clouds today.";

// Replace one substring with another with String.Replace.
// Only exact matches are supported.
var replacement = source.Replace("mountains", "peaks");
Console.WriteLine($"The source string is <{source}>");
Console.WriteLine($"The updated string is <{replacement}>");

The preceding code demonstrates this immutable property of strings. You can see in the preceding example that the original string, source, is not modified. The String.Replace method creates a new string containing the modifications.

The Replace method can replace either strings or single characters. In both cases, every occurrence of the sought text is replaced. The following example replaces all ' ' characters with '_':

string source = "The mountains are behind the clouds today.";

// Replace all occurrences of one char with another.
var replacement = source.Replace(' ', '_');
Console.WriteLine(source);
Console.WriteLine(replacement);

The source string is unchanged, and a new string is returned with the replacement.

Trim white space

You can use the String.Trim, String.TrimStart, and String.TrimEnd methods to remove any leading or trailing white space. The following code shows an example of each. The source string does not change; these methods return a new string with the modified contents.

// Remove trailing and leading white space.
string source = "    I'm wider than I need to be.      ";
// Store the results in a new string variable.
var trimmedResult = source.Trim();
var trimLeading = source.TrimStart();
var trimTrailing = source.TrimEnd();
Console.WriteLine($"<{source}>");
Console.WriteLine($"<{trimmedResult}>");
Console.WriteLine($"<{trimLeading}>");
Console.WriteLine($"<{trimTrailing}>");

Remove text

You can remove text from a string using the String.Remove method. This method removes a number of characters starting at a specific index. The following example shows how to use String.IndexOf followed by Remove to remove text from a string:

string source = "Many mountains are behind many clouds today.";
// Remove a substring from the middle of the string.
string toRemove = "many ";
string result = string.Empty;
int i = source.IndexOf(toRemove);
if (i >= 0)
{
    result= source.Remove(i, toRemove.Length);
}
Console.WriteLine(source);
Console.WriteLine(result);

Replace matching patterns

You can use regular expressions to replace text matching patterns with new text, possibly defined by a pattern. The following example uses the System.Text.RegularExpressions.Regex class to find a pattern in a source string and replace it with proper capitalization. The Regex.Replace(String, String, MatchEvaluator, RegexOptions) method takes a function that provides the logic of the replacement as one of its arguments. In this example, that function, LocalReplaceMatchCase is a local function declared inside the sample method. LocalReplaceMatchCase uses the System.Text.StringBuilder class to build the replacement string with proper capitalization.

Regular expressions are most useful for searching and replacing text that follows a pattern, rather than known text. For more information, see How to search strings. The search pattern, "the\s" searches for the word "the" followed by a white-space character. That part of the pattern ensures that it doesn't match "there" in the source string. For more information on regular expression language elements, see Regular Expression Language - Quick Reference.

string source = "The mountains are still there behind the clouds today.";

// Use Regex.Replace for more flexibility.
// Replace "the" or "The" with "many" or "Many".
// using System.Text.RegularExpressions
string replaceWith = "many ";
source = System.Text.RegularExpressions.Regex.Replace(source, "the\\s", LocalReplaceMatchCase,
    System.Text.RegularExpressions.RegexOptions.IgnoreCase);
Console.WriteLine(source);

string LocalReplaceMatchCase(System.Text.RegularExpressions.Match matchExpression)
{
    // Test whether the match is capitalized
    if (Char.IsUpper(matchExpression.Value[0]))
    {
        // Capitalize the replacement string
        System.Text.StringBuilder replacementBuilder = new System.Text.StringBuilder(replaceWith);
        replacementBuilder[0] = Char.ToUpper(replacementBuilder[0]);
        return replacementBuilder.ToString();
    }
    else
    {
        return replaceWith;
    }
}

The StringBuilder.ToString method returns an immutable string with the contents in the StringBuilder object.

Modifying individual characters

You can produce a character array from a string, modify the contents of the array, and then create a new string from the modified contents of the array.

The following example shows how to replace a set of characters in a string. First, it uses the String.ToCharArray() method to create an array of characters. It uses the IndexOf method to find the starting index of the word "fox." The next three characters are replaced with a different word. Finally, a new string is constructed from the updated character array.

string phrase = "The quick brown fox jumps over the fence";
Console.WriteLine(phrase);

char[] phraseAsChars = phrase.ToCharArray();
int animalIndex = phrase.IndexOf("fox");
if (animalIndex != -1)
{
    phraseAsChars[animalIndex++] = 'c';
    phraseAsChars[animalIndex++] = 'a';
    phraseAsChars[animalIndex] = 't';
}

string updatedPhrase = new string(phraseAsChars);
Console.WriteLine(updatedPhrase);

Programmatically build up string content

Since strings are immutable, the previous examples all create temporary strings or character arrays. In high-performance scenarios, it may be desirable to avoid these heap allocations. .NET Core provides a String.Create method that allows you to programmatically fill in the character content of a string via a callback while avoiding the intermediate temporary string allocations.

// constructing a string from a char array, prefix it with some additional characters
char[] chars = { 'a', 'b', 'c', 'd', '\0' };
int length = chars.Length + 2;
string result = string.Create(length, chars, (Span<char> strContent, char[] charArray) =>
{
    strContent[0] = '0';
    strContent[1] = '1';
    for (int i = 0; i < charArray.Length; i++)
    {
        strContent[i + 2] = charArray[i];
    }
});

Console.WriteLine(result);

You could modify a string in a fixed block with unsafe code, but it is strongly discouraged to modify the string content after a string is created. Doing so will break things in unpredictable ways. For example, if someone interns a string that has the same content as yours, they'll get your copy and won't expect that you are modifying their string.

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