Detecting if a field exists in a page template

That's because GetValue("isOverview") will be null in other pages.

Use this:

<% if(DocumentContext.CurrentDocument.GetValue("isOverview") != null ? DocumentContext.CurrentDocument.GetValue("isOverview").ToString() == "True" : false) { %>

or even better:

<% if(CMS.Helpers.ValidationHelper.GetBoolean(DocumentContext.CurrentDocument.GetValue("isOverview"), false)) { %>

1. If this column allows null values then It might be complicated I would look into {%CurrentDocument.TypeInfo. |(identity)GlobalAdministrator%} to see if there is something there to check the existence of column.

2. the other way would be to create a custom macro where you can catch an exception when you try to access property of document i.e CurrentDocument.GetValue("isOverview"):

bool isOverview; try { isOverview = (bool)CurrentDocument.GetValue("isOverview"); } catch(Exception exception) { // column does not exist }

Ideally you don't want extra sql query to make sure if column is there.

3rd way I would hard code the list of document types that have this custom column and simply check in macro if your current document type (class) is in this list then access column.

Actually it is kinda ugly but you can do it with 2 line macro:

{%isColumnExist = CurrentDocument.isOverview|(identity)GlobalAdministrator%}

The only issue is that if isOverview exists it must have a value different from null. isColumnExist is null after the 1st macro if it is failing. In the second macro you check if it has any value.

P.S. Still the idea is not to have a SQL query to check column existence in the document type.