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New Message on BDOTNET
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From: Sitaraman
Message 6 in Discussion
Hi Rajesh. That was a good explanation, but a few corrections... " you will
find that the constructor for DataReader is private " : The Access modifier of the
constructor of a DataReader is not "Private" but "Friend"(internal in C#). Actually
the signature of the contructor of a DataReader is as follows
VB.Net
Friend Sub New(ByVal command As SqlCommand)
End Sub
C#
internal SqlDataReader(SqlCommand command)
{
}
Friend(internal in C#) Access Modifier for the element basically means that "elements
are accessible from within the program that contains their declaration and from
anywhere else in the same assembly"
" when you call a public method ExecuteReader of DataReader" : ExecuteReader
is a method of the SQLCommand and not the DataReader. Note that in the previous
point, i had mentioned that the constructor of the DataReader has a "Friend"
Specifier. This means tht whereas you cannot use a new for DataReader due to the
"Friend" access specifier, the SQLCommand can very well use a "new DataReader()", as
SQLCommand is in the same assembly as that of SQLDataReader.
"This is infact an example of singleton pattern" : This is not a Singleton in
any way. Singleton is basically a mechanism where "you make an instance of an object
globally available and guarantee that only one instance of the class is created".
DataReader is no way Singleton, neither by design nor by implementation. The way a
Singleton is implemented is as follows
<MSDN_Snip>
1. Making the class create a single instance of itself.
2. Allowing other objects to access this instance through a class method
that returns a reference to the instance. A class method is globally
3. accessible.
Declaring the class constructor as private so that no other object can
create a new instance.
</MSDN_Snip>
Lets see whether the DataReader conforms to the afore -mentioned points
1. It doesnt conform : You can have multiple instances of DataReaders.
2. It doesnt conform : A Class method that returns an instance of its
own is nowhere provided in DataReader. The method for returning an
instance of a DataReader is actually provided by another class, i.e.
SQLCommandand there is no concept of Single instance as such
3. It doesnt conform : DataReader Class Constructor is not "Private",
but a Friend. So other objects, provided they are in the same
assembly CAN create an instance of the DataReader.
So DataReader was never meant to be and is NOT a Singleton. To verify this, Create 2
DataReader Objects, populate them thru SQLCommand.executereader and compare them using
the Object.ReferenceEquals method. If DataReaders had been Singleton, it should
return a true. But on using ReferenceEquals u will find that it returns a false,
whcih means there are multiple instances!!!
So the reason why you cannot use a new for a DataReader is because of the fact that
its constructor has a "Friend" Access modifier. When u want a DataReader populated u
use the SQLCommand's ExecuteReader method which will perform certain operations(not
going into the details here) and create a new instance of the DataReader internally
and return it to u. So every call to the SQLCommand.executereader will result in one
new instance of SQLDataReader object.
hth
regards,
sr
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