What it came down to is even with the right using statements etc you can’t hook into the extension methods.. is there something I’m missing?

First tried:

<umbraco:Macro  runat="server" language="cshtml">
@using System.Linq
@using umbraco.MacroEngines
@inherits umbraco.MacroEngines.DynamicNodeContext
  
  @{
  
  var collectionItems = @Model.Descendants("individualCollection");
  var externalCollectionItems = Model.Descendants("externalCollection");
  
  
  
  if(@collectionItems != null && @externalCollectionItems != null)
  {
    var allCollectionItems = collectionItems.Cast<DynamicNode>().Union(externalCollectionItems.Cast<DynamicNode>()); 
  
    if(@allCollectionItems.Any())
    {
    <ul>
      @foreach (var item in @allCollectionItems)
      {
        <li><a href="@item.Url">@item.Name</a></li>
      }
    </ul>
    }
  }
  }
</umbraco:Macro>

And got:

Error loading Razor Script 
'umbraco.MacroEngines.DynamicNodeList' does not contain a definition for 'Cast

Then tried:

<umbraco:Macro  runat="server" language="cshtml">
@using System.Linq
@using umbraco.MacroEngines
@inherits umbraco.MacroEngines.DynamicNodeContext
  
  @{
  
  var collectionItems = @Model.Descendants("individualCollection");
  var externalCollectionItems = Model.Descendants("externalCollection");
  
  
  
  if(@collectionItems != null && @externalCollectionItems != null)
  {
    var allCollectionItems = collectionItems.ToArray().Cast<DynamicNode>().Union(externalCollectionItems.ToArray().Cast<DynamicNode>()); 
  
    if(@allCollectionItems.Any())
    {
    <ul>
      @foreach (var item in @allCollectionItems)
      {
        <li><a href="@item.Url">@item.Name</a></li>
      }
    </ul>
    }
  }
  }
</umbraco:Macro>

And got:

Error loading Razor Script 
'System.Array' does not contain a definition for 'Cast

Then we re-thought it... GOT THE LOGIC RIGHT as well as reduced it down and got:

<umbraco:Macro  runat="server" language="cshtml">
@using System.Linq
@using umbraco.MacroEngines
@inherits umbraco.MacroEngines.DynamicNodeContext
  @{
  
  var collectionItems = @Model.Descendants("individualCollection");
  var externalCollectionItems = @Model.Descendants("externalCollection");
  var allCollectionItems = new List<DynamicNode>();
  
  if (@collectionItems != null)
  {
    allCollectionItems.AddRange(Enumerable.Cast<DynamicNode>(@collectionItems));
  }
  
  if (@externalCollectionItems != null)
  {
    allCollectionItems.AddRange(Enumerable.Cast<DynamicNode>(@externalCollectionItems));
  }
  
  if(@allCollectionItems.Count > 0)
  { 
    <ul>
      @foreach (var item in @allCollectionItems)
      {
        <li><a href="@item.Url">@item.Name</a></li>
      }
    </ul>
  }
  }
</umbraco:Macro>

And that works…

Basically we couldn’t get extension methods to work!

Any ideas why they don't work?