Unexpected behavior with ISBLANK and CONTAINS functions in Visualforce Page

Question

I've reduced my issue to the following example:

<apex:page >
  <apex:variable var="blank" value="" />
  <p> Contains(blank, 'foo'): <apex:outputText value="'TRUE'" rendered="{!CONTAINS(blank, 'foo')}" /></p>
  <p> Not(Contains(blank, 'foo')): <apex:outputText value="'TRUE'" rendered="{!NOT(CONTAINS(blank, 'foo'))}" /></p>
  <p> NULLVALUE(blank, 'x'): <apex:outputText value="{! NULLVALUE(blank,'XXXXX')}" /></p>
  <p> BLANKVALUE(blank, 'x'): <apex:outputText value="{! BLANKVALUE(blank,'XXXXX')}" /></p>
  <p> ISBLANK(blank, 'x'): <apex:outputText value="{! ISBLANK(blank)}" /></p>
</apex:page>

Which produces:

Contains(blank, 'foo'):'TRUE'
Not(Contains(blank, 'foo')):'TRUE'
NULLVALUE(blank, 'x'):
BLANKVALUE(blank, 'x'):
ISBLANK(blank, 'x'):false

How can Contains(blank, 'foo') and Not(Contains(blank, 'foo')) both return true? Why doesn't BLANKVALUE or ISBLANK recognize the var blank as being blank?

I took this a step further, and created this controller:

public class formulaTestController {
  public string getNullValField() { return null;}
  public string getBlankValField() {return ''; }
}

If I add this controller to the page and change the variable to <apex:variable var="blank" value="{!blankValField}" />, I get the same results as above. If I change it to <apex:variable var="blank" value="{!nullValField}" />, I get this:

Contains(blank, 'foo'):'TRUE'
Not(Contains(blank, 'foo')):'TRUE'
NULLVALUE(blank, 'x'):XXXXX
BLANKVALUE(blank, 'x'):XXXXX
ISBLANK(blank, 'x'):true 

So it appears that BLANKVALUE and ISBLANK really operate on nulls, but not on blanks. Is this expected? Especially w/r/t BLANKVALUE, as NULLVALUE also exists. And why do the Contains() and (Not(Contains()) tests still produce the same result?!

In the end, I need to reliably handle a controller value that may be blank or null, treating these as special cases in my VF logic. How can I do so?

EDIT: Per suggested answer, I tried wrapping Contains test in IF(). Note that I moved the formula into outputText's Value to make the result explicit:

<apex:page >
  <apex:variable var="blank" value="" />
  <p> IF(CONTAINS(blank, 'foo'),true, false): <apex:outputText value="{!IF(CONTAINS(blank, 'foo'),true, false)}" /></p>
  <p> IF(NOT(CONTAINS(blank, 'foo')),true, false): <apex:outputText value="{!IF(NOT(CONTAINS(blank, 'foo')),true, false)}" /></p>
  <p> IF(CONTAINS(blank, 'foo'),'true', 'false'): <apex:outputText value="{!IF(CONTAINS(blank, 'foo'),'true', 'false')}" /></p>
  <p> IF(NOT(CONTAINS(blank, 'foo')),'true', 'false'): <apex:outputText value="{!IF(NOT(CONTAINS(blank, 'foo')),'true', 'false')}" /></p>
</apex:page>

No change:

IF(CONTAINS(blank, 'foo'),true, false):false
IF(NOT(CONTAINS(blank, 'foo')),true, false):false
IF(CONTAINS(blank, 'foo'),'true', 'false'):false
IF(NOT(CONTAINS(blank, 'foo')),'true', 'false'):false

Answer 1

When I changed the page to be this i.e. 1) change var to be a blank field on Account and 2) AND the output of CONTAINS with TRUE, it seems to work as expected:

  <apex:page standardController="Account">
  <apex:variable var="blank" value="{!Account.Fax}" />
  <p> Contains(blank, 'foo'): <apex:outputText value="'TRUE'" rendered="{! TRUE && CONTAINS(blank, 'foo')}" /></p>
  <p> Not(Contains(blank, 'foo')): <apex:outputText value="'TRUE'" rendered="{! TRUE && NOT(CONTAINS(blank, 'foo'))}" /></p>
  <p> NULLVALUE(blank, 'x'): <apex:outputText value="{! NULLVALUE(blank,'XXXXX')}" /></p>
  <p> BLANKVALUE(blank, 'x'): <apex:outputText value="{! BLANKVALUE(blank,'XXXXX')}" /></p>
  <p> ISBLANK(blank, 'x'): <apex:outputText value="{! ISBLANK(blank)}" /></p>
</apex:page>

I have a feeling that the functions are attuned to working with sObject fields rather than a dynamic variable (or even if a variable, at least initialised to an sObject field) in Visualforce.

Contains(blank, 'foo'):

Not(Contains(blank, 'foo')):

NULLVALUE(blank, 'x'):XXXXX

BLANKVALUE(blank, 'x'):XXXXX

ISBLANK(blank, 'x'):true

So its almost as if the output of CONTAINS is somehow messing up, although it seems to be alright after you AND it with TRUE, which in essence is just transparent and lets the actual value of it pass through.

Answer 2

{!IF(CONTAINS(adjustmentType,'Inventory Count'),'true','false')}

As far as Contains function is concerned ,I usually use with If function with contains here and explicitly assign true and false and it works for me .

Answer 3

This is actually an issue with your use of <apex:variable>. According to the documentation, the value parameter is supposed to refer to an object. Think of it more as an alias than anything else, to simplify your later references. For example, after <apex:variable var="r" value="{!Custom_Object__c.Related_Object__r}"/>, you can easily refer to fields r.foo, r.bar and r.baz.

So, in your example, the blank alias is failing, so trying to use it in other functions leads to inconsistent results.