C SqlDataReader doesn t have a constructor?

sqldatareader is used to read the data obtained from the database line by line.

sqldatareader sdr = ;

As for those classes, there is no constructor. I don't think it's useful to die.

General interfaces, abstract classes, and static classes do not have constructors, and those with a single design pattern do not use constructors to get objects.

All classes have constructors, but some classes have private constructors.

Even if the constructor is not explicitly declared, the system provides a default parameterless constructor.

c Why doesn't sqldatareader have a constructor? "This is a question you can ask Microsoft

What types of classes do not have constructors?? "This question you can ask Microsoft MSDN

Whether there is a constructor or not is entirely up to the person to decide the relationship to the thing and the performance of the problem.

There is a design concept here, the landlord can use it first, and then with the accumulation of knowledge, I believe that the landlord will have a new experience, now to tell you this why, that why, in fact, it doesn't have much effect, some things need knowledge and practice and the accumulation of experience, in order to experience. If the landlord is interested, you can take a look at some books on design.

All classes have a default parameterless constructor as long as they don't write constructors.

If you write a constructor with parameters, the argumentated constructor will override the default parameterless constructor.

sqldatareader myread = myread = ;

What does that mean?

Changed to. sqldatareader myread = ;

The fieldcount property returns the number of columns in the current row, and your SQL query only for the column St Brithday will naturally always return 1Let's talk about what you mentioned.

within two cases. Appearance.

1.There are no records that qualify.

Then the query result is empty, and the number of rows in the recordset should be 0. You can determine that if it returns true, it means that a record has been queried, otherwise it means that there is no set of records that meet the criteria.

2.Another record that meets the criteria, but a column value is null, so it is judged:

if(read["st_brithday"] ==

string sql = "select (case when ast_brithday is null then '' else ast_brithday end) as ast_brithday from students where st_id = 8";

datatable dttemp=new datatable ()

using (sqldatareader objreader =

In this way, if there are no records that match the conditions of st id = 8, the result of the data is empty, while (will not be executed, if there are records that meet the criteria, but the student's birthday is not filled in yes"null"value, an empty string will be returned without an error.

If it is true, bai must have data, but this data may be zhidbnull

After dbnull is converted to c language, the dao is a null object, if(

I don't know what function you want to implement, so I can only remind you like this!

Determine whether there is a record to see if a row of data is returned, to see if the result is greater than 0, greater than 0 is a record. Do you understand?

When the landlord can read the queried data using the read() method of the sqldatareader object, it is judged that the sqldatareader object [column name]==null. That's fine.

Open and slow SQLSer 2008 to create a new GEEK database.

Open Visual Studio to create a new Windows Forms application.

Write the following under the query: Which grip is written**.

string connection string = server=.; database=geek;integrated security=true";

string sql = select * from table login where id >@id; ";

sqlparameter parameter username = new sqlparameter("@id", textbox_;

using (sqlconnection connection object = new sqlconnection (connection string)).

using (sqlcommand execution object = new sqlcommand(sql, join object)).

Execution object. parameter username);

Connect objects. open();

using (sqldatareader read object = execution object. executereader())

textbox data. text = null;

while (reads the object.) read())

for (int i = 0;i < Read the object. fieldcount; i++)

Enter 1 to look it up.

Enter 5 to call for a query.

This way we get the results we want.

This is impossible, theoretically almost all classes have constructors, that is the key to his instantiation, but it is not excluded that it has constructs, but does not show, there are indeed many classes that do not need to be instantiated and do not need constructors, but he must exist, empty or not displayed ......

c classes and structs all have constructors, and at least have default parameterless constructors.

Suffice it to say that abstract classes do not recommend the existence of a public constructor, everything else should have one or more constructors. Even static classes can have a constructor that is only used for initialization.

c Each class has a constructor that can be defined by itself, otherwise it will be automatically generated.

public form1()

That's it, yours don't have it?

Why do you want to return sqldatareader, in this case, the database connection cannot be closed in the data layer, and the reader can only be closed in the foreground, which is not good, it is recommended to return the list

Your **,Returns Reader,Why does it become sqldataadapter when you use it?,It should still be sqldatareader.。

using(sqldatareader sdr =}

Don't worry, debug it and see if there is any value here. Execute ystr in the database; Then ydr[0] as string is changed to ydr[0].tostring().

Refresh after getting the value?

You've just set up a read, but you haven't started reading it yet, and it's reading 1 line, usually with while(

Add breakpoint step debugging, if the reader does have a value then consider the page event cycle, if the reader has no value, then check the connection and SQL

By the way, your SQL statement returns the number of matches between the username and the password.

It is a single-row, single-column dataset.

As I understand it.

sqldatareader dr=;

sqldatareader dr declares dr at this time, but dr is null

For example, sqldatareader is a white peifang.

DR can be seen as an open space.

A DR object is returned that is assigned to the declared DR

A renovated house, and then painted a gourd in the same way, and built a white pei room for you, and renovated it.

If the returned dataset doesn't have data, then it's just a white room for you, and there's no content.

class cl = new class();

class cl declares that it is null inside

new class();A class structure is constructed.

To put it simply, this is post-declaration initialization.

sqldatareader dr=;

This is a post-declaration assignment.

If; Returns null

Then the DR still has no structure.

This is a member function of cd that returns a value to be assigned directly to dr, which is allowed, because the default constructor of sqldatareader has no parameters, and the return value of executereader() is directly assigned after the object is generated.