Dynamic Web TWAIN: How to get a list of all supported DPI/resolution from a selected TWAIN data source?

You can use capability negotiation to get the supported DPI/resolution list from a selected TWAIN data source.

Code Snippet

// Here DWObject is the object of Dynamic Web TWAIN. You may change it according to your code.
DWObject.SelectSource();
DWObject.OpenSource();
DWObject.Capability = EnumDWT_Cap.ICAP_XRESOLUTION;  //  capability  X-Resolution
DWObject.CapGet();
for(var index=0;index < DWObject.CapNumItems-1; index++){
  var capitem=DWObject.GetCapItems(index); //get each supported DPI value
}

Reference:

Capabilities

For more information, please refer to How to Perform Capability Negotiation.