Sort episerver class

Rewriter that sorts properties based on Order and Group

// Define other methods and classes here
public class Rewriter : CSharpSyntaxRewriter
{
	private string LastGroup = null;
	private int? Index = null;	
	
	public override SyntaxNode VisitPropertyDeclaration(PropertyDeclarationSyntax node)
	{
		
		var display = node.AttributeLists.SelectMany(x => x.Attributes.Where(y => y.Name.ToString() == "Display")).FirstOrDefault();
		if (display == null){
			return base.VisitPropertyDeclaration(node);
		}	
		
		var order = display.ArgumentList.Arguments.FirstOrDefault(x => x.NameEquals.Name.ToString() == "Order");
		var groupname = display.ArgumentList.Arguments.FirstOrDefault(x => x.NameEquals.Name.ToString() == "GroupName");
				
		int thisIndex;
		if (LastGroup == null || (groupname != null && groupname.Expression.ToString() != LastGroup)){
			thisIndex = 100;			
			Index = 110;
		}
		else {
			thisIndex = Index.Value;
			Index += 10;
		}
		if (groupname == null){
			if (LastGroup == null){
				LastGroup = "Dummy";
			}
		} else {
			LastGroup = groupname.Expression.ToString().Trim();
		}
		
		
		var orderLiteral = SyntaxFactory.LiteralExpression(SyntaxKind.NumericLiteralExpression, SyntaxFactory.Literal(thisIndex));
		
		if (Index == null){
			node = node.ReplaceNode(display, display.ArgumentList.AddArguments(SyntaxFactory.AttributeArgument(SyntaxFactory.LiteralExpression(SyntaxKind.NumericLiteralExpression, SyntaxFactory.Literal(thisIndex)))));
		}
		else {
			node = node.ReplaceNode(order, order.WithExpression(orderLiteral));
		}
		
		return base.VisitPropertyDeclaration(node);
	}
}