// Identify the difference, if any, between two assembly instances
void FindAssemblyDifferences(AssemblyInstance instance1, AssemblyInstance instance2)
{
    Autodesk.Revit.DB.Document doc = instance1.Document;
        AssemblyDifference difference = AssemblyInstance.CompareAssemblyInstances(instance1, instance2);

        AssemblyDifferenceNone diffNone = difference as AssemblyDifferenceNone;
        if (diffNone != null)
        {
            TaskDialog.Show("Revit", "Assemblies are identical");
            return;
        }

        AssemblyDifferenceConfiguration diffConfig = difference as AssemblyDifferenceConfiguration;
        if (diffConfig != null)
        {
            TaskDialog.Show("Revit", "Elements are not arranged in space relative to each other in the same way");
            return;
        }

        AssemblyDifferenceMemberCount diffMemberCount = difference as AssemblyDifferenceMemberCount;
        if (diffMemberCount != null)
        {
            TaskDialog.Show("Revit", "Elements counts differ: " + diffMemberCount.Count1 + " & " + diffMemberCount.Count2);
            return;
        }

        AssemblyDifferenceNamingCategory diffNameCat = difference as AssemblyDifferenceNamingCategory;
        if (diffNameCat != null)
        {
            int catInt1 = diffNameCat.NamingCategoryId1.IntegerValue;
            int catInt2 = diffNameCat.NamingCategoryId2.IntegerValue;
            TaskDialog.Show("Revit", "Naming categories differ: " + 
                doc.Settings.Categories.get_Item((BuiltInCategory)catInt1).Name + " & " +
                doc.Settings.Categories.get_Item((BuiltInCategory)catInt2).Name);
            return;
        }

        AssemblyDifferenceMemberDifference memberDifference = difference as AssemblyDifferenceMemberDifference;
        if (memberDifference != null)
        {
            AssemblyMemberDifference assemblyMemberDifference = memberDifference.MemberDifference as AssemblyMemberDifference;

            AssemblyMemberDifferentCategory differentCategory = assemblyMemberDifference as AssemblyMemberDifferentCategory;
            if (differentCategory != null)
            {
                int int1 = differentCategory.CategoryId1.IntegerValue;
                int int2 = differentCategory.CategoryId2.IntegerValue;
                TaskDialog.Show("Revit", "Element categories differ: " +
                    doc.Settings.Categories.get_Item((BuiltInCategory)int1).Name + " & " +
                    doc.Settings.Categories.get_Item((BuiltInCategory)int2).Name);
                return;
            }
            AssemblyMemberDifferentGeometry differentGeometry = assemblyMemberDifference as AssemblyMemberDifferentGeometry;
            if (differentGeometry != null)
            {
                TaskDialog.Show("Revit", "Member geometry differs");
                return;
            }
            AssemblyMemberDifferentParameters differentParameters = assemblyMemberDifference as AssemblyMemberDifferentParameters;
            if (differentParameters != null)
            {
                TaskDialog.Show("Revit", "Member parameters differs");
                return;
            }
            AssemblyMemberDifferentType differentType = assemblyMemberDifference as AssemblyMemberDifferentType;
            if (differentType != null)
            {
                TaskDialog.Show("Revit", "Element types differ: " + doc.GetElement(differentType.TypeId1).Name + " & " + doc.GetElement(differentType.TypeId2).Name);
                return;
            }
        }
        return;
}

' Identify the difference, if any, between two assembly instances
Private Sub FindAssemblyDifferences(instance1 As AssemblyInstance, instance2 As AssemblyInstance)
   Dim doc As Autodesk.Revit.DB.Document = instance1.Document
   Dim difference As AssemblyDifference = AssemblyInstance.CompareAssemblyInstances(instance1, instance2)

   Dim diffNone As AssemblyDifferenceNone = TryCast(difference, AssemblyDifferenceNone)
   If diffNone IsNot Nothing Then
      TaskDialog.Show("Revit", "Assemblies are identical")
      Return
   End If

   Dim diffConfig As AssemblyDifferenceConfiguration = TryCast(difference, AssemblyDifferenceConfiguration)
   If diffConfig IsNot Nothing Then
      TaskDialog.Show("Revit", "Elements are not arranged in space relative to each other in the same way")
      Return
   End If

   Dim diffMemberCount As AssemblyDifferenceMemberCount = TryCast(difference, AssemblyDifferenceMemberCount)
   If diffMemberCount IsNot Nothing Then
      TaskDialog.Show("Revit", ("Elements counts differ: " + diffMemberCount.Count1 & " & ") + diffMemberCount.Count2)
      Return
   End If

   Dim diffNameCat As AssemblyDifferenceNamingCategory = TryCast(difference, AssemblyDifferenceNamingCategory)
   If diffNameCat IsNot Nothing Then
      Dim catInt1 As Integer = diffNameCat.NamingCategoryId1.IntegerValue
      Dim catInt2 As Integer = diffNameCat.NamingCategoryId2.IntegerValue
      TaskDialog.Show("Revit", ("Naming categories differ: " + doc.Settings.Categories.Item(DirectCast(catInt1, BuiltInCategory)).Name & " & ") + doc.Settings.Categories.Item(DirectCast(catInt2, BuiltInCategory)).Name)
      Return
   End If

   Dim memberDifference As AssemblyDifferenceMemberDifference = TryCast(difference, AssemblyDifferenceMemberDifference)
   If memberDifference IsNot Nothing Then
      Dim assemblyMemberDifference As AssemblyMemberDifference = TryCast(memberDifference.MemberDifference, AssemblyMemberDifference)

      Dim differentCategory As AssemblyMemberDifferentCategory = TryCast(assemblyMemberDifference, AssemblyMemberDifferentCategory)
      If differentCategory IsNot Nothing Then
         Dim int1 As Integer = differentCategory.CategoryId1.IntegerValue
         Dim int2 As Integer = differentCategory.CategoryId2.IntegerValue
         TaskDialog.Show("Revit", ("Element categories differ: " + doc.Settings.Categories.Item(DirectCast(int1, BuiltInCategory)).Name & " & ") + doc.Settings.Categories.Item(DirectCast(int2, BuiltInCategory)).Name)
         Return
      End If
      Dim differentGeometry As AssemblyMemberDifferentGeometry = TryCast(assemblyMemberDifference, AssemblyMemberDifferentGeometry)
      If differentGeometry IsNot Nothing Then
         TaskDialog.Show("Revit", "Member geometry differs")
         Return
      End If
      Dim differentParameters As AssemblyMemberDifferentParameters = TryCast(assemblyMemberDifference, AssemblyMemberDifferentParameters)
      If differentParameters IsNot Nothing Then
         TaskDialog.Show("Revit", "Member parameters differs")
         Return
      End If
      Dim differentType As AssemblyMemberDifferentType = TryCast(assemblyMemberDifference, AssemblyMemberDifferentType)
      If differentType IsNot Nothing Then
         TaskDialog.Show("Revit", ("Element types differ: " + doc.GetElement(differentType.TypeId1).Name & " & ") + doc.GetElement(differentType.TypeId2).Name)
         Return
      End If
   End If
   Return
End Sub