Change file extension in C #

I do not know why this, but a simple solution is you do like this:

//aqui vai o caminho que você leu no OpenFileDialog
string caminho = "arquivo.txt";

//e aqui você cria uma variável que irá receber o novo caminho.
string novoCaminho = caminho.Remove(caminho.Length -3, 3) + "cnf";

Basically what the above line does is remove the txt from the file and add cnf ;

Only you will have to save the file for it to have this extension.

Or you can use a native function of C# :

string caminhoPath = Path.ChangeExtension(caminho, ".cnf");

See more about Path.ChangeExtension () .

See Running