title: Expression to Sum on DataSet field in Textbox for SSRS2008
body: <p>I have what I thought was a very basic SSRS scenario. I have two datasets in my SSRS 2008 report; dsMTD and dsYTD. They both contain the same columns, the only difference is the SQL statement that fills them both (but I don't imagine that's really relevant to the problem).</p>
  
  <p>I have a textbox on my report, and I wanted to show the SUM of a certain column within the report. I tried to make my expression as follows:</p>
  
  <pre><code>=Sum(Fields!cost.Value, "dsMTD").ToString()
  </code></pre>
  
  <p>This works fine in SSRS 2005, but the expression in SSRS2008 gives an error of "Unknown Collection Member" on the field reference in my expression.</p>
  
  <p>Am I not doing something right? I've recently installed CU6 for SQL Server 2008 on my machine, and wonder if that has changed something?</p>