When you write C# winforms application with multithreading you may encounter the most common issue:
An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll
Additional information: Cross-thread operation not valid: Control 'textBox1' accessed from a thread other than the thread it was created on.
You need to make calls through the main thread it was created on using invoke method:
Refer to the link for example: http://www.softwareandfinance.com/CSharp/CS_Cross-thread_Operation.html