System.ComponentModel.MaskedTextProvider.SetChar C# (CSharp) Method

        private void SetChar(char input, int position, CharDescriptor charDescriptor)
        {
            Debug.Assert(position >= 0 && position < _testString.Length, "Position out of range.");
            Debug.Assert(charDescriptor != null, "Null character descriptor.");

            // Get the character info from the char descriptor table.
            CharDescriptor charDex = _stringDescriptor[position];

            // If input is space or prompt and is to be escaped, we are actually resetting the position if assigned,
            // this doesn't affect literal positions.
            if (TestEscapeChar(input, position, charDescriptor))
            {
                ResetChar(position);
                return;
            }

            Debug.Assert(!IsLiteralPosition(charDex), "Setting char in literal position.");

            if (Char.IsLetter(input))
            {
                if (Char.IsUpper(input))
                {
                    if (charDescriptor.CaseConversion == CaseConversion.ToLower)
                    {
                        input = _culture.TextInfo.ToLower(input);
                    }
                }
                else // Char.IsLower( input )
                {
                    if (charDescriptor.CaseConversion == CaseConversion.ToUpper)
                    {
                        input = _culture.TextInfo.ToUpper(input);
                    }
                }
            }

            _testString[position] = input;

            if (!charDescriptor.IsAssigned) // if position not counted for already (replace case) we do it (add case).
            {
                charDescriptor.IsAssigned = true;
                _assignedCharCount++;

                if (charDescriptor.CharType == CharType.EditRequired)
                {
                    _requiredCharCount++;
                }
            }

            Debug.Assert(_assignedCharCount <= EditPositionCount, "Invalid count of assigned chars.");
        }