Continuous update of a textbox to show motor feedback.

I'm trying to find an efficient way to have a continuously running textbox, showing feedback from an encoder. I tried to do it using threading that reads in the feedback and outputs to the textbox but Visual Studio won't allow me to access the textbox that is under a different thread. I'm new to C# and in the past I would send a Windows Message to the main task and let the main task handle the updating. I would appreciate some guidance with this.

Thanks
George