The problem related to comma in the Textbox is because you are using an Int64.TryParse(), so the expected value are an integer… 
When you use a value with comma, isn’t a integer value anymore, so you need to use Double.TryParse() or Decimal.TryParse().
But, even using this, I recomend you to use a MaskedTextbox (like described in the comments), so you don’t need to worry about handling if the user inputs a comma or a dot.
solved Text-box Functions