char Array ROT13 String Constructor String Replace String ToCharArray String ToLower, ToUpper String Uppercase First Letter

Change characters

In C# programs, strings are immutable. If we want to change their characters, we must first convert them to arrays.

C# performance

A char array can be changed in memory before conversion back to a string. This is efficient. We show how to change string characters in this way.

Input and output

Consider the string "aaa 123." We may wish to make 2 separate changes in it at once—change the letter, and the whitespace.

INPUT:  aaa 123
OUTPUT: bbb-123

Example code

We use a series of if-statements to change characters in a string. You can make many mutations in a single pass through the string.

Next We make 3 changes: we lowercase characters, and we change the values of spaces and one letter.

Info The ToCharArray method will cause an allocation upon the managed heap. Then, the new string constructor will allocate another string.

Info Compared to making multiple changes with ToLower and Replace, this approach saves allocations.

Tip This will reduce memory pressure and ultimately improve runtime performance.

using System;

string input = "aaa 123";
// Change uppercase to lowercase.
// Change space to hyphen.
// Change a to b.
char[] array = input.ToCharArray();
for (int i = 0; i < array.Length; i++)
{
    char let = array[i];
    if (char.IsUpper(let))
    {
        array[i] = char.ToLower(let);
    }
    else if (let == ' ')
    {
        array[i] = '-';
    }
    else if (let == 'a')
    {
        array[i] = 'b';
    }
}
string result = new string(array);
Console.WriteLine(result);
bbb-123

One letter

If you need to change one letter at a certain position (such as the first letter), this approach to string mutation is also ideal.

Summary

To change chars in a string, you must use a lower-level representation of the character data, which you can acquire with ToCharArray. You cannot simply assign indexes in a string.

Summary, continued

This style of method introduces complexity. It is often best to wrap this logic in a helper method, and call that method.