Why do we use the reference of abstract class (or base class) to create object of it's sub-class. eg: TextWriter is the abstract class for StreamWriter & StreamWriter.
TextWriter writer = new StreamWriter();
why can't we simply use :
StreamWriter writer = new StreamWriter();
We all know that, we cannot create the object of Abstract class.
But why it is so?
I mean, can anyone explain it with real time example?
I am trying to compile the following code and i am getting the error:
Cannot create instance of abstract class .
m_objExcel = new Excel.Application();
m_objBooks = (Excel.Workbooks)m_objExcel.Workbooks;
m_objBook = (Excel._Workbook)(m_objBooks.Add(m_objOpt));
m_objSheets = (Excel.Sheets)m_objBook.Worksheets;
m_objSheet = (Excel._Worksheet)(m_objSheets.get_Item(1));
// Create an array for the headers and add it to cells A1:C1.
object[] objHeaders = {"Order ID", "Amount", "Tax"};
m_objRange = m_objSheet.get_Range("A1", "C1");
m_objRange.Value = objHeaders;
m_objFont = m_objRange.Font;
m_objFont.Bold=true;
// Create an array with 3 columns and 100 rows and add it to
// the worksheet starting at cell A2.
object[,] objData = new Object[100,3];
Random rdm = new Random((int)DateTime.Now.Ticks);
double nOrderAmt, nTax;
for(int r=0;r<100;r++)
{
objData[r,0] = "ORD" + r.ToString("0000");
nOrderAmt = rdm.Next(1000);
objData[r,1] = nOrderAmt.ToString("c");
nTax = nOrderAmt*0.07;
objData[r,2] = nTax.ToString("c");
}
m_objRange = m_objSheet.get_Range("A2", m_objOpt);
m_objRange = m_objRange.get_Resize(100,3);
m_objRange.Value = objData;
// Save the Workbook and quit Excel.
m_objBook.SaveAs(m_strSampleFolder + "Book2.xls", m_objOpt, m_objOpt,
m_objOpt, m_objOpt, m_objOpt, Excel.XlSaveAsAccessMode.xlNoChange,
m_objOpt, m_objOpt, m_objOpt, m_objOpt);
m_objBook.Close(false, m_objOpt, m_objOpt);
m_objExcel.Quit();
Error 1279 Cannot create an instance of the abstract class or interface 'System.Web.Mvc.FileResult'
[Code]....
I am using MVC 2. The same code works in my onather application. I have no idea about this error.
Configuration Error Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately. Parser Error Message: Cannot create an abstract class.
Source Error:
Line 150: <providers>
Line 151: <clear/>
Line 152: <add name="NDMSMembershipProvider"
type="CSW.Web.Security.NDMSMemberProvider"/>
Line 153: </providers>
Line 154: </membership>
Source File: C:DevCSW2srcCSW.Webweb.config Line: 152
We have a Static class that 's called CData in our asp.net c# app.It handles all the data layer CRUD functionality on our pages.
Problem is, were are losing the intellisense reference in our pages to it.For example: we would expect to type CData.(dot) and have a list of methods and properties available to us.
Nope. Not there.
I kind of thought it was because the file was getting too long at 3000 lines of code, since some other static classes were working ok.
So then I thought that I'd try wrapping the CData Class in a namespace. That worked, but of course I had to put an include statement of the top of each page that's using CDATA, also, now some of the other developers are getting errors in there pages complaining about missing methods that are in CDATA, but their pages are not seeing.
I am trying to develop logic to add dynamic controls to page via class method. So I want to know how can we access the ASP.Net Page object reference in class method.
View 1 RepliesGiven an instance of a control SomeControl that is in a some file (could be a usercontrol/ascx, or an aspx) how can I get a reference to the class to which it belongs?
In a usercontrol, SomeControl.NamingContainer seems to be reliable.
In a page that inherits from a MasterPage, this will return a reference to the ContentPlaceHolder, not the actual class that defines the aspx. What I would want in that case is SomeControl.Page
Parent will return the parent control in the heirarchy so isn't much use.
Is there any method for getting this directly regardless of what kind of thing it is?
When to use Abstract class and when to use Interface class.
View 10 Repliesin my default.aspx page i have a dropdown List and a textbox with a submit button below that there are 2 listbox... and the dropdown list holds the names of the listbox
my logic here is to select an item from the dropdown list and put some text in the text box and submit the form which will add an item to the listbox selected.. but when i do this i get an error saying Object reference not set to an instance of an object. i tried to figure out the problem and found that when i remove the reference to the Site Master Page it works fine and when i undo and apply my reference back to the Site Master Page i get the same error.
[Code]....
When I get an Object Ref error, it can sometimes be a real pain to find out which variable is causing the error (when you can't debug). Is there a way for this error to throw the classname that isn't assigned?
View 2 RepliesFrom the following URL i got some doubts about the Recommendations for using Abstract class vs interfaces
[URL]
1. If you anticipate creating multiple versions of your component, create an abstract class. Abstract classes provide a simple and easy way to version your components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, you must create a whole new interface. { Is there any example for this t ounderstand throughly ?} If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class. If you want to provide common, implemented functionality among all implementations of your component, use an abstract class. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members. { Is there any example for this t ounderstand throughly?
I just want to know that how can I utilize the concept of Abstract class, virtual class etc. in my shopping cart website. I have read the tutorial out there on internet and I saw some examples too, but those examples are so general that they dosen't fit into real world scenerio like I am searching for a shopping website. Same questions again and again comes to my mind that why to made a class only to give the declaration of methods and property.
View 4 Replieswhat is the function of abstract class, why design pattern need to build BLL and DAL
anyone give an example for my reference as I am getting strat to build my web-based project
I know what Singleton Pattern means and Abstract class means.What I wanted to know was how would this apply to real world.Could anyone give me any good example or simple explanation.Say I have a simple website, why would I use any of the above if any.Why would it simplify my architechture.
View 3 RepliesObject reference not set to an instance of an object.
iam getting this Error Just created the controls as an null and how to create the object for reference
1) as my analysis i got the information that i need to create the object reference with new keyword but how to do it script using c#
public TextBox txtEventDate = null;
public Calendar calEventDate = null;
public Label lblResult = null;
protected void calEventDate_SelectionChanged (Object sender,EventArgs e)
{
this.txtEventDate = this.FindControl("txtEventDate") as TextBox;
this.calEventDate = this.FindControl("calEventDate") as Calendar;
txtEventDate.Text = calEventDate.SelectedDate.ToString ("d");
}
protected void btnSubmit_Click(Object Sender,EventArgs e)
{
this.lblResult = this.FindControl("lblResult") as Label;
lblResult.Text = "You Picked" + txtEventDate.Text;
}
</script>
I am using grid view as mentioned below, it is giving me error "object reference not set", but if comment allowpaging and pagesize lines, it works. let me know whats wrong I am doing?
[Code]....
[Code]....
What is the use of abstract class design in real time projects
and how to use abstract clases and interfaces
and inheritense concepts
like big shoping portals or content managment,blogs etc
----> I have datatable which is passing to another page in session variable.
----> Now on another page i take the session variable into datatable.
datatable ds_table = new datatable();
ds_table = (datatable)session["table_value"];
----> so problem, is that , when i filtering some rows from ds_table . that taking effect in the session variable. if some rows deleted from ds_table. then it is also deleted from session variable.
----> so, anyone tell me why is this going to happene?
how to create object of HttpResponse class
HttpResponse response=new
HttpResponse (TextWriter
tx);
how to inplement this wrong code to right one & how to implement TextWriter.
I am developing a couple of small ASP.NET application and would like to know what pattern. approach do you use in your projects.
My projects involve databases, using Data access and Business logic layers.
The data-access approach that I was using so far is the following(I read in some book and liked it):
For DAL layer:
Creating an abstract class that will define all database manipulation methods to implement.
The abstract class will contain a static "Instance" property, that will load (if instance == null) an instance (Activator.CreateInstance) of the needed type (a class that implements it).
Creating a classes that implement this abstract class, the implementation will be according to the databases (SQL, mySQL and etc) in use.
With this I can create different implementation according to database in use.
For BLL layer:
A class that encapsulates all all retrieved fields , and static methods that will call the DAL classes.
I am attempting to bind a Repeater (but it could be a GridView or ListView) to a list of objects. The List's type is an abstract type, which has two different classes derived from it, both with different properties. Because they have different properties, I cannot just have one ItemTemplate. If I bind a control to a property of one type of class and the other type doesn't have it, it throws an error.
Here's where I'm at:
I cannot use <% if (whatever) { %> some stuff <% } else { %> some other stuff <% } %> because I cannot access the databound item to make the choice based on its type. I cannot use the <%# %> syntax, which lets me use the databound information, because you cannot code logic like if...then...else. I cannot (rather not) call a function and return a string with the code because what I want to render is complex and contains further nested databound controls. Has anyone found an ingenious way of doing if it is this type of object, display these controls, else display these other controls?
I have a product class which contains 11 public fields.
ProductId
ShortTitle
LongTitle
Description
Price
Length
Width
Depth
Material
Img
Colors
Pattern
The number of fields may grow with attributes for more specific product tyes. The description may contain a large amount of data. I want to create a slim version of this product class that only contains the data needed. I'm only displaying 4 of the 12 fields when listing products on a category page. It seems like a waste to retrieve all of the data when most of it isn't being used. I created a parent class of ProductListing that contains the 4 fields I need for the category page
ProductId
ShortTitle
Price
Img
Then created a class of Product that inherits from ProductListing containing all product data. It seems backwards as "ProductListing" is not a kind of "Product" but I just started reading about inheritance a few months ago so it's stil a little new to me. Is there a better way to get a slim object so I'm not pulling data I don't need? Is the solution I have in place fine how it is?
i need to create a instant of class by string value which store in hiddenfield.value()
string str = "myclass1";
var x = new <str>();
or
var x = new <hiddenfields1.value.tostring()>();
How to create object of class file (asp.net) using javascript and call is methods.
View 1 Replies