port1.ReadExisting() Error

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C#

programming

I need to read RFID tag and get the value through the comport to the my C# form , here i assign string called s to get readings from port1.ReadExisting();

like this --- > s += port1.ReadExisting(); and textbox1.text = s;

In this case it will give a error called " Cross-thread operation not valid: Control 'textBox1' accessed from a thread other than the thread it was created on."

please give a suggestion to this matter