title: SSRS show parameter values in header
body: <p>I am trying to add an expression in header section, which would display parameter value chosen by users (if everything selected, display "ALL" instead).  I am having trouble with "Property" parameter.</p>
  
  <p>Report structure:
  Report is a matrix report that shows data by year and grouped by refBuildingID.  Parameter "Property" is based on RefBuildingID.  I want to show these parameter values in header when users run this report. </p>
  
  <p>Expression:</p>
  
  <pre><code>=IIF(Countrows("BuildingID")=(Parameters!refBuildingID.Count),"All",Join(Parameters!refBuildingID.Value,", "))
  </code></pre>
  
  <p>Error:
  The Value expression for the text box ‘Textbox31’ has a scope parameter that is not valid for an aggregate function.  The scope parameter must be set to a string constant that is equal to either the name of a containing group, the name of a containing data region, or the name of a dataset.  </p>
  
  <p>Is there a way to get around this error?</p>
  
  <p><a href="https://i.stack.imgur.com/STYye.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/STYye.png" alt="enter image description here"></a></p>