How to Set a Character at an Index in a String in C#

how do I set a character at an index in a string in c#?

If it is of type string then you can't do that because strings are immutable - they cannot be changed once they are set.

To achieve what you desire, you can use a StringBuilder

StringBuilder someString = new StringBuilder("someString");

someString[4] = 'g';

Update

Why use a string, instead of a StringBuilder? For lots of reasons. Here are some I can think of:

Accessing the value of a string is faster.
strings can be interned (this doesn't always happen), so that if you create a string with the same value then no extra memory is used.
strings are immutable, so they work better in hash based collections and they are inherently thread safe.

Replacing a char at a given index in string?

Use a StringBuilder:

StringBuilder sb = new StringBuilder(theString);
sb[index] = newChar;
theString = sb.ToString();

How can I get a character in a string by index?

string s = "hello";
char c = s[1];
// now c == 'e'

See also Substring, to return more than one character.

Getting character at a particular index in a string c#

string myString = "HelloWorld";
char myChar = myString[index];

What is the default way to change a character at a certain index in a C# string?

String is imutable. You can use StringBuilder to mutate a string:

var s = "abc";
var sb = new StringBuilder(s);
sb[1] = 'd';
s = sb.ToString();

Assign new value to a string index

Every time you go through this for loop, it will evaluate true if the string contains @, #, ^, * and + anywhere in the string. So if your string is missing any of these symbols, your if statement will evaluate to false and nothing will happen.

Luckily you can simplify this pretty easily. One way you can do this is to convert your string to a char[] array and break your logic into multiple if-else statements, or a single switch statement, for example:

public static string Decipher (string code)
{
    char[] codeArray = code.ToCharArray(); // convert your code string to a char[] array

    for (int i = 0; i < codeArray.Length; i++)
    {
        switch (codeArray[i]) // Get the char at position i in the array
        {
            case '@': // if the character at i is '@'
                codeArray[i] = 'a'; // Change the character at i to 'a'
                break; // break out of the switch statement - we don't need to evaluate anything else
            case '#': // if the character at i is '#'
                codeArray[i] = 'e'; // Change the character at i to 'e'
                break; // break out of the switch statement - we don't need to evaluate anything else
            // repeat for everything else you need to replace!
        }  
    }
    return new String(codeArray); // Once you're all done, create a string from your deciphered array and return it     
}

Replace character at specific index in Liststring, but indexer is read only

You cannot modify C# strings directly, because they are immutable. You can convert strings to char[], modify it, then make a string again, and write it to file:

File.WriteAllLines(
    @"c:\newfile.txt"
,   File.ReadAllLines(@"C:\...").Select((s, index) => {
        if (index % 2 = 0) {
            return s; // Even strings do not change
        }
        var chars = s.ToCharArray();
        chars[483] = '0';
        return new string(chars);
    })
);

How to change 1 char in the string?

I found a solution in unsafe context:

    string str = "gg"; char c = 'H'; int index = 1;
    fixed (char* arr = str) arr[index] = 'H';
    Console.WriteLine(str);

It's so simple.
And in safe context:

    string str = "gg"; char c = 'H'; int index = 1;
    GCHandle handle = GCHandle.Alloc(str, GCHandleType.Pinned);
    IntPtr arrAddress = handle.AddrOfPinnedObject();
    Marshal.WriteInt16(arrAddress + index * sizeof(char), c);
    handle.Free();
    Console.WriteLine(str);

Replace single chars in string at position x

String is immutable class, so use StringBuilder instead:

  ...
      StringBuilder temp = new StringBuilder(lblWord.Text);
      temp[i] = letter; // <- It is possible here
      lblWord.Text = temp.ToString();  
  ...

Insert value into a string at a certain position?

You can't modify strings; they're immutable. You can do this instead:

txtBox.Text = txtBox.Text.Substring(0, i) + "TEXT" + txtBox.Text.Substring(i);