how to open .dxf file in C#

I think DXF is not directly supported under .NET Classes and you will probably need to use a library that has the necessary components to load or convert DXF.

One approach you can try is to load the DXF as a raster image using the LEADTOOLS SDK (https://www.leadtools.com). The code to do this is as follows:

Once you have the DXF loaded as an image, you can use it to display in a viewer or save to another format.