DOT NET Interview Questions - DotNetSpider

(A) What is difference between abstract classes and
interfaces?
Following are the differences between abstract and interfaces :-
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Abstract classes can have concrete methods while interfaces have no methods
implemented.
Interfaces do not come in inheriting chain , while abstract classes come in
inheritance.
(B) What is a delegate ?
Delegate is a class that can hold a reference to a method or a function.Delegate class has
a signature and it can only reference those methods whose signature is compliant with the
class.Delegates are type-safe functions pointers or callbacks.
Below is a sample code which shows a example of how to implement delegates.
Public Class FrmDelegates
Inherits System.Windows.Forms.Form
Public Delegate Sub DelegateAddString()
Private Sub FrmDelegates_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub AddString()
lstDelegates.Items.Add(“Running AddString() method”)
End Sub
Private Sub cmdDelegates_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles cmdDelegates.Click
Dim objDelegateAddString As DelegateAddString
objDelegateAddString = AddressOf AddString
objDelegateAddString.Invoke()
End Sub
End Class
In the above there is a method called “AddString()” which adds a string to a listbox.You
can also see a delegate declared as :-
Public Delegate Sub DelegateAddString()
This delegate signature is compatible with the “AddString” method.When i mean
compatibility that means that there return types and passing parameter types are same.Later
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