How to replace all occurrences of a string with a specified string from rich text box in c#?

I think the problem is that you are creating a new instance of your spellChecker_frm. That's not going to work. Take a look at two of my blog posts about passing data between forms/classes. In particular, look at using Interfaces (option #3 in my first post) or using events (as is explained in the second post):

http://geek-goddess-bonnie.blogspot.com/2011/01/passing-data-between-forms.html

http://geek-goddess-bonnie.blogspot.com/2012/12/passing-data-between-forms-redux_31.html

Hope that helps ...

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~~Bonnie Berent DeWitt [C# MVP]

http://geek-goddess-bonnie.blogspot.com