a,b and c node in canvase
if a connection b, b connection c
i want find who connection b??
how to know which’s text value
node is ClassDiagramNode

Basically you need to use the enumerators that IGoNode provides.
GoNode b = …;
String msg = "Sources are: ";
foreach (IGoNode n in b.Sources) {
GoNode src = (GoNode)n;
msg += src.Text + " ";
msg += "\nDestinations are: ";
foreach (IGoNode n in b.Destinations) {
GoNode dst = (GoNode)n;
msg += dst.Text + " ";
Of course you’ll probably want to use “ClassDiagramNode” instead of “GoNode” in the above code example.
There is discussion and more examples about graph traversal in the User Guide, in the section “Traversing a Diagram”.