C# / .NET - replace first character in string

In this article, we would like to show you how to replace the first character in a string in C# / .NET.

Quick solution:

string text = "ABC";
string replacement = "x";
string result = replacement + text.Substring(1);

Console.WriteLine(result);  // xBC

1. Practical examples

1.1 With + and range operator

In this example, we will take the text string without the first character and add it at the end of the replacement.

using System;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABC";
        string replacement = "x";
        string result = replacement + text.Substring(1);

        Console.WriteLine(result);  // xBC
    }
}

1.2 With string.Concat() method and range operator

This approach is equivalent to the above one. Instead of + operator we concatenate the strings using string.Concat() method.

using System;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABC";
        string replacement = "x";
        string result = string.Concat(replacement, text.Substring(1));

        Console.WriteLine(result);  // xBC
    }
}

2. Practical example using string Regex.Replace() with regex pattern

In this example, we use Regex.Replace() with "^." regex to replace the first character in the text string.

Regex explanation:

• ^ - matches the beginning of the string,
• . - matches any character except linebreaks.

using System;
using System.Text.RegularExpressions;

public class StringUtils
{
    public static void Main(string[] args)
    {
        string text = "ABC";
        string replacement = "x";
        string pattern = @"^.";
        string result = Regex.Replace(text, pattern, replacement);

        Console.WriteLine(result);  // xBC
    }
}

Note:

Regular expressions are slower than substring() method.