Question

Is there any way to read file with custom extension to textBox? Is there any convertation to some supported formats?
I need read file with custom extension, like text.blahblahblah.

Answer 1

Is there any way to read file with custom extension to textBox?

Sure, just load the file text to the TextBox like:

textBox1.Text = File.ReadAllText(@"Your input file path");

You can let the user choose the file using OpenFileDialog and specify a format accepted in the dialog filter.

If you want to also read rtf text then I think you need a RichTextBox instead of TextBox here so you can read rtf text probably using richTextBox1.Rtf string.