diff --git a/CodeGenerator/BlendExt.cs b/CodeGenerator/BlendExt.cs
index 1fdb7ca..05da10a 100644
--- a/CodeGenerator/BlendExt.cs
+++ b/CodeGenerator/BlendExt.cs
@@ -1,9 +1,22 @@
+using System;
+using System.CodeDom;
+using System.Linq;
+using System.Runtime.InteropServices.ComTypes;
+using Kaitai;
+
 namespace CodeGenerator;
 
-public class BlendExt {
+public static class BlendExt {
     //TODO: Add an bool IsPrincipal extension method to kaitai structs lib
     //A principal (or root) block is defined by having a two digit code and by the fact that its dna_index is always valid.
     //If we have a pointer to a principal block, we can ignore the type of the pointer and use the block type.
     //Subsidiary blocks are defined by having the code "DATA", which is ommited here. Their dna_index is not
     // always correct and is only used when whichever field points to them has an "invalid" type (like void*).
+    public static bool IsPrincipal(this BlendFile.FileBlock block) {
+        string[] dataTypes = ["REND", "TEST", "GLOB", "DATA", "DNA1"];
+        bool isPrincipal = !dataTypes.Contains(block.Code);
+        if (isPrincipal && block.Count > 1)
+            throw new("Corrupt File detected. Principal block has more than one data type");
+        return isPrincipal;
+    }
 }
\ No newline at end of file