User-1760427068 posted
Quick note, the original code used <, not < at the start of tags.
If you haven't worked with C# before, notice that almost everything ends with a semicolon. Take notes and write down which "things" don't to help you remember later.
Since I don't know your background (or that of future readers), I hope I error on the side of explaining too much. I also hope most people looking at the full example can understand a lot of it just from seeing it.
The front page (aspx) only adds three things that weren't previously there: several asp:buttons, a div element that includes runat="server" and some <br /> tags to insert blank lines.
The runat="server" must be incluced in a control or an element on the front page in order for the "things" on the code behind page to "see" the control or the element. Forgetting the runat="server" is a common error for a beginner.
There are nine lines at the top that begin with the word "using". These are namespaces. It is sometimes necessary to add a namespace at the top so that program will load something it needs in order for a particle piece of code to function.
For example, "CommandType" would not function without the namespace System.Data. In this example, the first five namespaces were placed there automatically when the page was created. The remaining four namespaces were added manually.
After each manually added namespace there is a comment, beginning with two forward slashes. The comment indicates which Code or piece of code made it necessary to include the namespace. Prior to adding the namespace, a jagged red line would appear
under the word. This red warning underline disappeared about two seconds after the namespace was added.
The line beginning with public partial class was put there automatically, and it is put there for every program.
If you have variables that you want to use between several subroutines you define them after this public partial class line.
In C#, the subroutine shown below:
protected
void Page_Load(object sender,
EventArgs e)
{
}
is added to the page automatically. Anything in this subroutine will happen when the page loads. The only thing our program does on load is to put some text in the div. To do this, we take the id name of the div element, type
.InnerHtml =, and then type the text we want within quotes. (more on this later).
Notice next that there are several protected void subroutines with names like SummationButton_Click. The program goes to one of these after the button (look for the same name in the button) is clicked. Ignore the (object sender, EventArgs
e) for now, noting only that it is identical in each button click subroutine.
There are several types of variables that you will want to use (I'm assuming you already know the meanings): double, int, string. We didn't use any strings in this example.
For work with percentages, variables of the type double were used (strangely, if I did division of an integer by an integer, the result came to an integer even though that particular variable was given the type 'double'.
One quick note, when using doubles, they are set to "real" when defining the type for a field in a table.
When we come to the part where the code behind must communicate to the table in the database, we use
SqlConnection (just copy the entire string for now) and SqlCommand. The example always uses a 'conn' SqlConnection and 'cmd' for the SqlCommand. You could use different words.
Glance on down and notice SqlDataReader, and still further down, a while loop. When starting out, you'll probably just want to copy all these lines, paste them in, and then make some changes.
cmd.CommandText is where you write the SQL statement. A SQL statement provides answers to several questions: What table do you want to query? What fields in this table contain the data that you need? Do you want to retrieve
only records that meet a criterium or criteria (as opposed to retrieving every record)? Or course there is even more, but this should be enough initially.
The SQL statements in the examples are short and simple. The word Numbers is the name of a table. The words number1 and number2 refer to fields in the table Numbers.
Before looking at what is inside the reader loop, quickly note that reader.Read() has a corresponding reader.Close(). Too bad they didn't make it .Open() and .Close() like they did for opening and closing a connection--conn.Open() and conn.Close();
A reader will scan through the table once, selecting rows that meet the criteria. You can't turn it around and have it go backwards, or anything like that.
Within the while loop, notice that a structure like reader["field"] is used to pull text out of what stored in a field, in a record. I say text at this point because until you do something to it, the program doesn't know the type of the data (integer,
string, double, datetime, etc.) To specify a type, you "convert" the text to a type. If the content in the field something is an integer:
Convert.ToInt32(reader["something"]);
Convert.ToString(reader["anotherthing"]);
Convert.ToDouble(reader["somenumber"]);
I've found that if something is going into a string, it isn't always necessary to convert to string. However, I'd recommend doing it every time to avoid the trouble of having to track down and find a problem. Conversion is necessary if text
is a number and you want to use it for math.
The SQL statement is slightly different for when we want the sum of a column:
"SELECT Sum(number1) FROM Numbers"
You might guess there are other things where you do something by typing a word and then writing the field name within parentheses. Yes.
Sum is an example of a SQL aggregate function (see
http://www.w3schools.com/sql/sql_functions.asp).
Now, instead of using ExecuteReader(), we use ExecuteScalar().
If the result of a SQL statement would be just one number, it is convenient to use ExecuteScalar(), add a conversion and set a variable equal to that expression.
What would happen if the SQL statement was written in a way that it DIDN'T result in a single value and you used ExecuteScalar()? It wouldn't crash. It would take the first result and give that to you, and ignore all the others.
The above covers what is present in the example.
BONUS FEATURES:
We put data into a div element using .InnerHtml (example: dividname.InnerHtml = "This is some text";)
You can do something similar with .Text to put a string into a TextBox (example: textboxidname.Text = "Some text";)
Or you can take content from a TextBox (example: string mystring = textboxidname.Text;)
Regarding Executions, you've seen ExecuteReader() and ExecuteScalar(). There are others but the next one you will want to find is ExecuteNonQuery(). This third executioner is used when you want to use SQL statements to do operations on your database
such as INSERT INTO, UPDATE, DELETE, etc.
FINALLY
If it turns out that you are a VB person, a lot of the above will be the same, or at least similar. I'm happy to repeat the example in VB for you.
Regards,
-Larry
