// Find all Wall instances in the document by using category filter
ElementCategoryFilter filter = new ElementCategoryFilter(BuiltInCategory.OST_Walls);

// Apply the filter to the elements in the active document,
// Use shortcut WhereElementIsNotElementType() to find wall instances only
FilteredElementCollector collector = new FilteredElementCollector(document);
IList<Element> walls = collector.WherePasses(filter).WhereElementIsNotElementType().ToElements();
String prompt = "The walls in the current document are:\n";
foreach (Element e in walls)
{
    prompt += e.Name + "\n";
}
TaskDialog.Show("Revit", prompt);

' Find all Wall instances in the document by using category filter
Dim filter As New ElementCategoryFilter(BuiltInCategory.OST_Walls)

' Apply the filter to the elements in the active document,
' Use shortcut WhereElementIsNotElementType() to find wall instances only
Dim collector As New FilteredElementCollector(document)
Dim walls As IList(Of Element) = collector.WherePasses(filter).WhereElementIsNotElementType().ToElements()
Dim prompt As [String] = "The walls in the current document are:" & vbLf
For Each e As Element In walls
    prompt += e.Name + vbLf
Next
TaskDialog.Show("Revit", prompt)