Self-Service Dog

Intermittent PageRequestManagerParserErrorException With Full Postback

I have a project with a bunch of ajaxified Telerik controls within several nested UpdatePanel controls. As part of an Excel export process, I have a button which upon clicking performs a full postback, generates a spreadsheet from a RadGrid, and finally prompts the user to download the file. 

Once in a while, though, the export process would throw the dreaded and unhelpful Sys.WebForms.PageRequestManagerParserErrorException exception.

Eilon Lipton's helpful post gave a handy checklist of things not to do, but in spite of ensuring all the common causes were accounted for, it would still occur occasionally.

I tried implementing a workaround shared by a colleague and thus far it appears to have helped. This involves setting the OnClientClicked event handler of the export button so that it triggers a brief pause prior to initiating the full postback using a client-side Javascript.

function onExportClick(sender, args)
{
     setTimeout(
          function () 
          { 
               __doPostBack(sender.id, "_MyButtonID") 
          }, 
          500
     );   
}

The error itself points you in the right direction, but some more context sure would be nice. Anyway, apparently this script mitigates situations where the Telerik controls are still in the process of being rendered when the full postback is initiated, corrupting the Response header in the process. 

Distinct, Random Table Rows With Entity Framework

For the purpose of setting up various unit and functional tests in a C# project, I needed to grab some random rows from a database table to analyze. However, rather than just any random batch of records, I specifically wanted to grab a single, random member of each and every unique type.

I created some code which does this handily in TSQL, but for my C# project I needed to do the same using entity framework.

The table consists of an auto-incrementing primary key column, a uniqueidentifier column, and a name column, and the data are a random number of several different types of flowers. Here's how to create a table in SQL with some test data.

-- Reduce clutter in the Results tab as records get inserted or updated.
SET NOCOUNT ON
GO

-- Variables.
DECLARE
 @count int,
 @max int,
 @innercount int,
 @innermax int,
 @name varchar(100)

-- Table variable definition.
DECLARE @Flowers TABLE
(
 ID int IDENTITY (1, 1) PRIMARY KEY NOT NULL,
 Guid uniqueidentifier,
 Name varchar(100)
)

-- Start by inserting several flower names.
INSERT INTO @Flowers (Name)
SELECT 'Black Lotus'
UNION ALL
SELECT 'Dandelion'
UNION ALL
SELECT 'Rose'
UNION ALL
SELECT 'Tulip'
UNION ALL
SELECT 'Orchid' 

-- Set up a WHILE loop based on the number of flowers named in the table.
SELECT @count = 1, @max = COUNT(Name)
FROM @Flowers

-- Iterate through the list of flower names.
WHILE @count <= @max
BEGIN
 
 SELECT @name = Name
 FROM @Flowers
 WHERE ID = @count

 -- Set up another WHILE loop with a minimum value of 1, and a maximum
 -- value randomly picked from between 0 and 99, made effectively 1 to
 -- 100 by adding +1 to the random value.
 SET @innercount = 1
 SET @innermax = ( ABS(CAST(CAST(NEWID() AS VARBINARY) AS int)) % 99 ) + 1

 WHILE @innercount <= @innermax
 BEGIN

  -- Insert a new record for the current flower Name along with a 
  -- randomly chosen GUID value.
  INSERT INTO @Flowers (Guid, Name)
  SELECT NEWID(), @name

  SET @innercount = @innercount + 1

 END 

 -- Just to indicate how many of each type got inserted.
 PRINT 'Inserted ' + CAST(@innercount AS varchar(4)) + ' ' + @name

 SET @count = @count + 1

END

-- Remove the placeholders with no guid assigned.
DELETE
FROM @Flowers
WHERE Guid IS NULL

Now, I can use the following query in TSQL to grab a single unique member of each type of flower:

-- Return a single member picked at random of each group of flowers.
SELECT DISTINCT
 Name,
 (SELECT TOP 1 GUID FROM @Flowers WHERE Name = f.Name ORDER BY NEWID())
FROM @Flowers f
GROUP BY Name 

      

So now how do we reproduce the above results using entity framework? Like so:

public void Main()
{       
       var result = new List();

       // Declare a new instance of Random.
       var rand = new Random();
 
       // Query to determine how many unique instances of each flower name live in the table.
       var flowerCounts = Flowers
          .GroupBy(f => f.Name)
          .Select(f => new { name = f.Key, count = f.Count() });
 
     // Broad query to grab a list of flower names and unique identifiers.
       var flowers = Flowers
          .Select(f => new FlowerName { name = f.Name, uniqueIdentifier = f.Guid.ToString() });
  
       // Iterate through the list of flower names and their respective counts.
       foreach (var flowerCount in flowerCounts)
       {
          // Here we pick a random number between 1 and the number of flowers of the current name.
          var skip = rand.Next(1, flowerCount.count);
 
          // Now, we refine the broad query to group by flowers of a particular name, skipping
          // according to the randomly determined skip interval, and then grabbing the first. 
          // Then we just order by name.
          var uniqueSingleton = flowers
              .Where(f => f.name == flowerCount.name)
              .GroupBy(a => a.name, (key, s) => s.Skip(skip).First())
              .OrderBy(f => f.name);
  
          foreach (var combo in uniqueSingleton)
          {
               Console.WriteLine(string.Format("{0}: {1}", combo.name, combo.uniqueIdentifier)
               + Environment.NewLine);
          }
       } 
  }

  public class FlowerName
  {
       public string name{ get; set; }

       public string uniqueIdentifier { get; set; }
  }

Now as with the SQL example above, executing the above results in a collection of distinct flower names along with a unique row identifier for each:

Stop Whining