How to show progress when use richtextbox loadfile or other methods

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C#

.NET

there's a richtextbox compenent,I want to show the progress of the load file.
i tried other method such as:
1. File.ReadAllText
2. StreamReader.ReadToEnd
3. StreamReader.ReadLine
4. richtextbox.LoadFile
but it cann't show progress.
And, obviously, I test the loading time, one line of reading the file data is the slowest method, I have to take into account the speed of loading.
how to achieve my purpuse,tks for answer.

remark:it's c# winform application

What I have tried:

create a thread,and calculate richtextbox.text.leng/FileInfo(filepathe).Length.
but i cann't show progress