SOOrderEntry Table Action button

If the adapter is coming from an action of a particular graph, you can cast adapter.View.Graph as the graph type:

var orderGraph = adapter.View.Graph as SOOrderEntry;

Once you have a graph, you can use it the way you normally would use Base:

SOOrder order = orderGraph.Document.Current;
var lines = orderGraph.Transactions.Select();
SOLine line = orderGraph.Transactions.Current;

For example:

public virtual IEnumerable Test(PXAdapter adapter)
{
    var orderGraph = adapter.View.Graph as SOOrderEntry;
    SOOrder order = orderGraph.Document.Current;
    if (order is null) return adapter.Get();

    var lines = orderGraph.Transactions.Select();
    SOLine line = orderGraph.Transactions.Current;

    // do stuff

    return adapter.Get();
}