How to access a control using its variable ID? ASP.net and VB

I have a number of checkbox controls dynamically built on Page_Load.  Each checkbox control's ID is programmatically created using a primary key pulled from a database.  
The ID for each checkbox control is created like this:   checkbox.ID = "CheckboxID" & UserReader("fldPrimaryKey")
At runtime, I want to determine if the checkbox is checked or not.  I am able to pull the correct value and assign it to the variable fldPrimaryKey . This is my code that presently does not work.
If ("CheckboxID" & fldPrimaryKey).checked = True Then    lots of happy code... End If 
My question is 'how do I correct the above code?' or 'is this a stupid way of checking to see if the control is checked?'.
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