index outside range c#

Question

I am trying to make an assignment of value in a certain index of a String, however, I get the following error that I do not understand the reason:

System.Argued tofrangeexception: 'The index was out of range. It should be non-negative and smaller than the collection size.*'

        string tela;
        int counter = 0;
        StringBuilder telaBuilder = new StringBuilder();


        private void btnSendWord_Click(object sender, EventArgs e)
        {
        char letra = Convert.ToChar(txtGetWord.Text);
        Boolean codigoVerificador;
        codigoVerificador =    verificador.VerificaLetra(comboPalavra[0],letra);
        if (codigoVerificador == true)
        {

        foreach(char c in comboPalavra[0].ToCharArray())
           {
                counter = counter + 1;
                if(c == letra)
                {

                    telaBuilder[counter] = Convert.ToChar( letra);
                    tela = telaBuilder.ToString();
                }
            }
       }
    }

Answer 1

You are trying to assign a letter to a position that does not exist in Stringbuilder.

To do this you must initialize Stringbuilder with a number of positions equal to the word you want to discover:

private StringBuilder telaBuilder;
private int numeroDeLetras = 10;

...
...
telaBuilder = new StringBuilder(numeroDeLetras);

On the other hand, traditionally, the game usually represents the letters not yet found with a -.

For that use

telaBuilder = new StringBuilder(new string('-', numeroDeLetras));

See working on .NET Fiddle