I want to assign class by thematic layer in ecognition. I have digitized road edges and I want to classify the roads in my 'road class' using the shapefile (polyline) of my roads. How do I do this?

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eCognition does not know which side of the polyline is the road - that is your main challenge here. While it might be painfully obvious for you to tell which side of the polyline is the road, actually specifying that based on the rules available in eCognition is not easily done.
As such, what you need to do is work towards creating a road polygon layer, which you can then use as a classifying feature in eCognition. The difficulty of creating such a road polygon layer will depend on how you made your road polyline layer, especially on how consistent you were when digitizing. If there are many gaps in your polylines, then you'll have a hard time utilizing the general polyline to polygon functions.

Another possible option, depending on the general consistency of the roads in your area would be to buffer your polylines and then create a polygon layer where two buffered polylines overlap. This new polygon will primarily exist where you have roads, but this is not a perfect solution and you will have many misclassification.

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