|
Hi,
Could someone tell me the purpose/good situations where override and new keywords are used in function declaration.
I got the primary idea through versioning sample(given below for your reference). But I am sure,there is more value than what I can I understand through this sample.
Thanks
Vinay.
The sample :
--------------
//Copyright (C) 2000 Microsoft Corporation. All rights reserved.
// versioning.cs
// CS0114 expected
public class MyBase
{
public virtual string Meth1()
{
return "MyBase-Meth1";
}
public virtual string Meth2()
{
return "MyBase-Meth2";
}
public virtual string Meth3()
{
return "MyBase-Meth3";
}
}
class MyDerived : MyBase
{
// Overrides the virtual method Meth1 using the override keyword:
public override string Meth1()
{
return "MyDerived-Meth1";
}
// Explicitly hide the virtual method Meth2 using the new
// keyword:
public new string Meth2()
{
return "MyDerived-Meth2";
}
// Because no keyword is specified in the following declaration
// a warning will be issued to alert the programmer that
// the method hides the inherited member MyBase.Meth3():
public string Meth3()
{
return "MyDerived-Meth3";
}
public static void Main()
{
MyDerived mD = new MyDerived();
MyBase mB = (MyBase) mD;
System.Console.WriteLine(mB.Meth1());
System.Console.WriteLine(mB.Meth2());
System.Console.WriteLine(mB.Meth3());
}
}
|
|
|
|
|
To begin with, both these topics are discussed in the online help, as well as probably hundreds of indy websites around the globe, but I'll try to give a brief reply that might help clarify things:
1. A subclass method marked with "override" indicates that the programmer has a subclass in which he does not desire to (only) use the base class implementation of a particular method. The developer may still choose to execute the base implementation (e.g. base.SomeVirtualMethod() ) but he can choose to completely replace - or "override" - the base implementation if desired.
2. A subclass method marked with "new", or one that bears the same name as a base method which has no "override" modifier on it, "hides" (C#) or "shadows" (VB) the base method. Subclass methods that hide base methods can (like overrides) also completely replace the functionality of the base implementation.
The main difference between "override" and "new", and certainly the one that underscores the danger in using "new" indescriminately, is in regards to execution of the virtual methods.
If you have overridden base methods, it does not matter how your variables are declared - it's the way they are instantiated that matters, so even if an instance is "upcasted" to the base type, the overridden implementation will be called. This is very important - it's part of what makes concepts like polymorphism work.
However, using "new" has some potentially unexpected results.
Assuming we are using your example above, and we write:
MyDerived md = new MyDerived();
string foo = md.Meth2();
We would get the expected result; the derived class' implementation of Meth2, or "MyDerived-Meth2".
However, if we wrote:
MyBase mb = new MyDerived();
string foo = mb.Meth2();
We would actually get the base class implementation of Meth2's return, or "MyBase-Meth2".
Careless or uninformed use of hiding can have very serious implications and confusing results. In general, override should "always" be used. I put "always" in quotes because it's impossible to say "always" and get away with it. There are some possible cases where hiding methods is acceptable. For instance if you had a third party assembly to which you had no source, and one of its types had a method that was non-virtual, and did something that made your life impossible, then *maybe* hiding the base implementation would be okay.
I can't think of a case for hiding a virtual base method, though, except human error.
I hope this helps.
The most exciting phrase to hear in science, the one that heralds the most discoveries, is not 'Eureka!' ('I found it!') but 'That's funny...’
|
|
|
|
|
The textbox is already set to MultiLine = true;
How can I move to a new line in a textbox using code?
Thanks
|
|
|
|
|
myTextBox.Text += "This is a new line" + "\r\n";
This means: The box uses both new-line and carriage-return characters to determine where each line ends.
Cheers
Sid
|
|
|
|
|
You can either append "\r\n" to your Text or append the string returned by Environment.NewLine. Using the Environment.NewLine is preferred for code portability.
RageInTheMachine9532
"...a pungent, ghastly, stinky piece of cheese!" -- The Roaming Gnome
|
|
|
|
|
I've create my own class and i've instanced an object in this way:
MyClass mObjcet = new MyClass();
...
i use it, now i want release all the memory, how?
|
|
|
|
|
The garbage collector does it for you when there are no longer any references to that object.
|
|
|
|
|
You don't. The Garbage Collector will free the memory whenever it gets around to it (it's pretty lazy), or if the system is running low on memory.
You can force a garbage collection, but don't try it unless you know exactly what you're doing, why, and what the side-effects of doing so are. The GC has it's own built in scheduling and optimization system built in. Forcing collections whenever YOU feel like it needs to be done can actually degrade system performance.
RageInTheMachine9532
"...a pungent, ghastly, stinky piece of cheese!" -- The Roaming Gnome
|
|
|
|
|
am developing an application that uploads an image to the database and other users can view this pic, uploading is ok, but the problem in displaying the pic from the database, heres my code.
image = (byte[])this.mData.Tables[0].Rows[x]["Image"];
System.IO.MemoryStream memStream = new System.IO.MemoryStrea(image);
System.Drawing.Image Img = System.Drawing.Image.FromStream(memStream);
the data type of the column "Image" in the database is "image"
everytime i run the code i get Argument Exception "Invalid parameter used." on the line
System.Drawing.Image Img = System.Drawing.Image.FromStream(memStream);<br />
i look everywhere on the net but they r using the same way i think, i dunno where the problem is.
any help ?
thanx
|
|
|
|
|
I use this instead:
OleDbCommand cmd = new OleDbCommand("select Image " +
"from Table where Name=@Name", conn);
OleDbParameter param = new OleDbParameter("@Name", OleDbType.VarChar);
param.Value = textBoxName.Text;
cmd.Parameters.Add(param);
OleDbDataReader reader = cmd.ExecuteReader();
reader.Read();
long length = reader.GetBytes(0, 0, null, 0, 0);
byte[] buffer = new byte[length];
length = reader.GetBytes(0, 0, buffer, 0, (int)length);
if (buffer.Length == length)
{
MemoryStream ms = new MemoryStream(buffer);
mImage = Image.FromStream(ms);
Adapt this code to your and enjoy.
}
|
|
|
|
|
am using dataset instead of the SqlDataReader, so how i retrive the column in byte format, casting ?
so it will be like
byte[] img = (byte[])mData.Tables[0].Rows[0]["image"];<br />
MemoryStream ms = new MemoryStream(img);<br />
System.Drawing.Image myImage = System.Drawing.Image.FromStream(ms);
then, i'll back the same point and to the same exception.
could u explain much more please.
thanx for ur reply
|
|
|
|
|
i also mentioned somethin, that everytime the length of the byte is 13
even if i changed the pic. i mean that this line of code always returns a bytes of length 13
byte[] img = (byte[])mData.Tables[0].Rows[0]["image"];
byte length is always 13
|
|
|
|
|
I have a local Udp port opened by a process. I need to share this port with another process. How Can I do it using UdpClient.
Regards
|
|
|
|
|
Uhhh... AFAIK, you can't! How is either app going to know that the message coming down the pipe is for it and not the other app?
Whoops! My bad! I was thinking TCP, not UDP...
RageInTheMachine9532
"...a pungent, ghastly, stinky piece of cheese!" -- The Roaming Gnome
|
|
|
|
|
I've been testing a few of my own apps in 2.0, and i'm having trouble with ones that minimise-to-tray
basically, after a lot of digging and hair pulling, i found that if you call this.Visible=false on a form to hide it, and later on call this.Visible=true to reshow it - all comboboxes on that form stop firing SelectedIndexChanged events. they work fine up to the point where you alter the Visible properly on the form that holds them.
does anyone know of a nice workaround I can use for this? (other than disposing the form on minimise and reconstructing a whole new one on the tray icon maximise event - thats just not possible for some of the apps i've got  ).
Thanks
Steve
|
|
|
|
|
I am working on A project that needs a dial-up connection to the internet;
i need away to make this connection
this is my e-mail "ahmed_sep_1983@hotmail.com" please any code ,i am a computer Engineering student and this is apart of my graduation project so please help me ;
any one,any thing;
"using C#"
|
|
|
|
|
:bigtime-sigh:
1) Your choice of thread title is a joke, please please please go and lookup "how best to get answers on technical forums" and pay specific attention to the section on picking a suitable title for your thread.
2) Your explaination of the problem you are facing is so far from suitable its unbelievable. (As above, but this time pay attention to the section on formulating your post).
3) The whole purpose of forums like this is that others can learn as well as yourself. Asking for an answer to be mailed directly to YOU shortcurcuits this. This will be covered in the above as well.
|
|
|
|
|
hi
i want to know that if there is any xml control that can display xml file in winform application or even if there is any html control
i found xml control and html control in web Application in c#
Saima
|
|
|
|
|
DataGrid[^] does that. It's not pretty, but it does the job. You have to bind the data source to a DataSet[^]object, which can in turn read and write XML.
|
|
|
|
|
will it give output like
<item>
<item#>1
coz i don't think so datagrid will do this and i need this output on the control
|
|
|
|
|
use internet explorer - there are a few articles on code project.
"When the only tool you have is a hammer, a sore thumb you will have."
|
|
|
|
|
Hi all,
I am developing a application where i convert all msi into a encrypted file. when they want to install the application, I decrypt the encrypted msi files and copy it to temp folder and install. After installation i have to delete that MSI file from Temp.
How to perform this. I couldnt get the msi running in the process it gives just msiexec.exe.
Please help urgent
Venky
|
|
|
|
|
Why bother?
Obviously you don't want people to see your MSI file.
But Windows Installer needs the unencrypted MSI file, so if somebody wants to get the unencrypted MSI file, writing a program that's watching Temp and copying the msi to a different location is a matter of minutes.
Or, if the installation takes more than about 5 seconds, you can even copy the MSI by hand.
So your efforts are just a waste of time IMHO...
mav
|
|
|
|
|
Hi,
I have a tab with 4 tab pages. Each page contains a RichTextBox.
I have no problems with the tab pages in the application when running on XP system.
on most windows 2000 systems it is fine as well but on some windows 2000 systems only the first tab is active and the other 3 are gray with no RichTextBox open on them!
Does anyone know what could be the reason for it? I tried everything I could (including trying .NET framework 2.0) but found no solution for this.
Please help.
Thanks.
avivhal
|
|
|
|
|
Hi all
I want to know as to how to write a program in C# to recieve characters from the internet port(like port 25)and store it in a file.
Can someone help me with the code or any help for such a programe?????????????!!!!!!!!!!!
Thanks.
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
