.NET cache stores only a reference -- and what to do about it

.NET provides the System.Web.Caching.Cache object as a way to persistently store data. This is useful, for example, if you retrieve some data from a database and want to refer to it repeatedly, perhaps from several different pages, without the overhead of accessing the database again each time.

When I attempted to store an instance of a class I had created with several members -- I discovered a quirk of the Cache object: adding such an object to the cache seems to store a reference to the object, not a copy of the object. An example will make this clear.

// define a class with some members
class Animal
{
public string species;
public string name;

public Animal(string s, string n)
{
species = s;
name = n;
}
}

// create a dog named Spot
Animal animal = new Animal("dog", "Spot");

// store Spot in the cache
HttpContext.Current.Cache.Insert("MyAnimal", animal);

// change Spot's name to Rover
animal.name = "Rover";

// get Spot from the cache
Animal animal2 = (Animal)HttpContext.Current.Cache.Get("MyAnimal");

// this is the cached object; the name should be Spot, but instead it's Rover ??!!
Label1.Text = animal2.name;

I expected the above code to set the label to Spot, but it actually sets it to Rover. Cache. Insert appears to store a reference to the object. I have yet to find anyplace this is mentioned in Microsoft's documentation. Kudos to Martin Bakiev of Penn State University for figuring this out.

Here's one way to get around the problem. Add another constructor to the Animal class. Use to create a copy of the object, and then store that copy in the cache. This has the desired effect, setting the label to Spot.


// define a class with some members
class Animal
{
public string species;
public string name;

public Animal(string s, string n)
{
species = s;
name = n;
}

public Animal(Animal a)
{
species = a.species;
name = a.name;
}
}

// create a dog named Spot
Animal animal = new Animal("dog", "Spot");

// store Spot in the cache
//HttpContext.Current.Cache.Insert("MyAnimal", animal);
HttpContext.Current.Cache.Insert("MyAnimal", new Animal(animal));

// change Spot's name to Rover
animal.name = "Rover";

// get Spot from the cache
Animal animal2 = (Animal)HttpContext.Current.Cache.Get("MyAnimal");

// this is the cached object; the name should be Spot, but instead it's Rover ??!!
Label1.Text = animal2.name;

That avoids the problem, but it's not very convenient if you want to store instances of many different classes in the cache; you'd need to create a new constructor for each class. I found a better solution: I wrote methods that use the .NET BinaryFormatter class to serialize an object on its way into the cache, and deserialize it on the way back out. I may be able to share that code in a future post.

Determining whether a numeric value has at most two decimal places

I was working on a .NET website and wanted to write some C# code to validate that a value input by the user was numeric and had at most two decimal places. This is useful, for example, when validating that the input represents a dollar amount.

I first tried this, which didn't work:
try
{
// must be numeric value
double d = double.Parse(s);
// max of two decimal places
if (100 * d != (int)(100 * d)) // max of two decimal places
return false;
return true;
}
catch
{
return false;
}

The above is unreliable because, since d is a floating-point number, 100 * d isn't always exactly equal to (int)(100 * d), even when d has two or fewer decimal places. For example, 100 * 1.23 might evaluate to, say, 122.9999999.

This post on StackOverflow offers several solutions, but none of them looked right for my purpose. Instead, I came up with this:
try
{
// must be numeric value
double d = double.Parse(s);
// max of two decimal places
if (s.IndexOf(".") >= 0)
{
if (s.Length > s.IndexOf(".") + 3)
return false;
}
return true;
catch
{
return false;
}

The same thing could be accomplished using a regular expression, if you prefer.

Reading a stored procedure's output variable in .NET

I recently wrote some C# code to call a stored procedure, pass some input and output parameters, read the resulting recordset, and then read the value of an output parameter. When I tried to read the output parameter's value, the result was null. It took a long time to figure out why.

It turns out that, when using SqlDataReader.ExecuteReader -- as opposed to ExecuteScalar or ExecuteNonQuery, you must close the reader before you can obtain the value of an output variable. (A tip of the hat to The Code Project for that information.)

Here's an example:

The SQL code:
CREATE PROCEDURE test
@foo INT,
@goo INT OUTPUT
AS
BEGIN
SET @goo = @foo * @foo
SELECT @goo AS hoo
END

First version of the C# -- wrong, output value is null:
string connectionString = "...";
using (SqlConnection conn = new SqlConnection(connectionString))
{
conn.Open();
SqlCommand cmd = new SqlCommand("test", conn);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@foo", 9);
SqlParameter outParam = cmd.Parameters.AddWithValue("@goo", 0);
outParam.Direction = ParameterDirection.Output;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
rdr.Read();

// obtain a value from the recordset
int result = Int32.Parse(rdr["hoo"].ToString());

// obtain the value of the output parameter
// reader is still open
// System.NullReferenceException occurs because the value is null
Label1.Text = cmd.Parameters["@goo"].Value.ToString();
} // reader is closed here
}

Second version of the C# -- right, output value is 81:
string connectionString = "...";
using (SqlConnection conn = new SqlConnection(connectionString))
{
conn.Open();
SqlCommand cmd = new SqlCommand("test", conn);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@foo", 9);
SqlParameter outParam = cmd.Parameters.AddWithValue("@goo", 0);
outParam.Direction = ParameterDirection.Output;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
rdr.Read();

// obtain a value from the recordset
int result = Int32.Parse(rdr["hoo"].ToString());

// obtain the value of the output parameter
// reader is still open
// System.NullReferenceException occurs because the value is null
} // reader is closed here
Label1.Text = cmd.Parameters["@goo"].Value.ToString();
}