Question

i wrote a code for replacing lowercase characters to *.
but it does not work.
where is the problem?

   private void CharacterReplacement()
    {
        Console.WriteLine("Enter a string to replacement : ");
        string TargetString = Console.ReadLine();
        string MainString = TargetString;
        for (int i = 0; i < TargetString.Length; i++)
        {
            if (char.IsLower(TargetString[i]))
            {
                TargetString.Replace(TargetString[i], '*');
            }
        }
        Console.WriteLine("The string {0} has converted to {1}", MainString, TargetString);

    }

Answer 1

Replace() returns a new string, so you need to re-assign it to TargetString:

TargetString =  TargetString.Replace(TargetString[i], '*');

Another way to express your intend would be with Linq – not sure which I like better, this avoids all the temporary strings but has other overhead:

TargetString = new string(TargetString.Select(c => char.IsLower(c) ? '*' : c)
                                     .ToArray());