I am storing some program values in my Web.config file and would like to use them in my Code.
When i try to set the value like this.
Private Const Security As String = ConfigurationManager.AppSettings("jwtKey")
i get the error Constant expression is required. Is there a way to make this work or do i have to assign the value to each function in my controller which needs access to this constant.
An option is to use the ReadOnly attribute:
Private ReadOnly Security As String = ConfigurationManager.AppSettings("jwtKey")
From the MSDN article:
Specifies that a variable or property can be read but not written.
Just what you are describing, assign a value to a variable but do not allow it to be changed.
This what a constructor is for
Class MyController
Private Const Security As String
Public Sub New
Security = ConfigurationManager.AppSettings("jwtKey")
End Sub
If you're using DI, you could pass all the relevant options in as a single object
Related
Class cacheable(Of T As haveTimeStampandMainStringKey)
Public ReadOnly Property Cache As T
Public ReadOnly Property timestamp As Date
Public Shared Function create(cache1 As T) As cacheable(Of T)
Dim a = New cacheable(Of T)
a._Cache = cache1
a._timestamp = Now
Dim key = T.mainkey 'this things fail to compile
Return a
End Function
End Class
Interface haveTimeStampandMainStringKey
ReadOnly Property TimeStamp As DateTime
ReadOnly Property mainKey As String
End Interface
Basically I want class cacheable to work only with classes that support haveTimeStampandMainStringKey
Yet
Dim key = T.mainkey produces an error
Clearly T supports haveTimeStampandMainStringKey interface. So I should be able to access T.mainkey. I can't. Why? What's wrong with the code?
Why?
It doesn't work because T is a type, not an instance. You need to have an instance to refer to mainKey. You probably want either a.Cache.mainKey or cache1.mainKey.
(If you really want something Shared rather than something attached to an instance, unfortunately, there isn't a good way to do it as it's not supported by .NET except through various reflection-based approaches, see various lamentations about the absence of "static interfaces" over the years.)
When you implement an Interface in your Class the arguments are automatically named RHS as shown on MDSN https://msdn.microsoft.com/en-us/library/office/gg264387.aspx
For example, if I create IInterface as so:
Public Property Let Value1(strValue1 As String)
End Property
Public Property Let Value2(strValue2 As String)
End Property
And implement it, the class would look like this:
Implements IInterface
Private Property Let IInterface_Value1(RHS As String)
End Property
Private Property Let IInterface_Value2(RHS As String)
End Property
It's a best practice to name your arguments in such a way as to provide some level of abstraction and make it easier to read and write code. I can actually change the arguments to whatever I want in the class after I've implemented the statements, as shown below, but my question is why does this happen? Is RHS a leftover from another language or is there a particular reason it's named like this?
Implements IInterface
Private Property Let IInterface_Value1(strValue1 As String)
End Property
Private Property Let IInterface_Value2(strValue2 As String)
End Property
The above compiles fine if I manually change it.
rhs stands for right hand side of operator = and lhs for left hand side of =. Why is this named like this here? Maybe its something which comes from c++ conventions. By the properties you have consider this code:
Dim test As IInterface
Set test = New ClassTest
test.Value1 = "rhsVal"
The new string value is actually on the right side of the = so therefor rhs.
I am working on a C#.net. I have a code of vb.net and I want to convert those code into C#.
I have done all my task but while running application it gives me a error of object not set to an object reference. below is my VB.net code. I have used third party ddl in my code so ExchangeList is a class of that dll.
Private WithEvents moExchanges As ExchangeList
Private Sub RequestChartData()
Trace.WriteLine("Init")
moExchanges = moHost.MarketData.Exchanges
End Sub
Now below is my C#.code
Private Host moHost;
Private ExchangeList moExchanges;
private void RequestChartData()
{
Trace.WriteLine("Init");
moExchanges = moHost.MarketData.Exchanges;
}
Thanks.
It's not possible to tell you with 100% certainty the source of your object not set to an object reference. without seeing more code, but the error appears to be telling you that moHost is null - in other words, you haven't created an instance of the object yet.
So when you try to call MarketData.Exchanges on the object (so you can assign it to moExchanges, it's throwing the error.
Find moHost in your code and make sure you have that you have assigned an instance of that object to it (by calling it's constructor, like moHost = new Host() or whatever the constructor is), and this should fix your error.
UPDATE
You never initialize moHost. This line:
private Host moHost;
Simply declares the object - at this point it's null. So when you try to access any instance methods/properties, you get the object not set to an object reference error.
You need to create an instance of moHost by calling it's constructor, something like this:
private Host moHost = new Host();
The constructor may require parameters - take a look at the documentation for the third-party DLL (intellisense in the IDE may also tell you what parameters, if any, are needed).
This will take care of your error.
()Instead of:
Private Host moHost;
Private ExchangeList moExchanges;
I think you need to write it as:
Private Host moHost = New Host();
Private ExchangeList moExchanges = New ExhangeList();
NOTE: This is not a duplicate of VB.NET equivalent of C# property shorthand?. This question is about how to have different access rights on getter and setter of a VB auto-property; e.g public getter and private setter. That question is about the syntax for auto-property (and does not mention this issue).
I am trying to convert an auto Property (public getter and private setter) from C# to VB.NET.
But after conversion VB.NET is maintaining a private field.
C# code
class DemoViewModel
{
DemoViewModel (){ AddCommand = new RelayCommand(); }
public ICommand AddCommand {get;private set;}
}
VB.NET equivalent from code converter is
Class DemoViewModel
Private Sub New()
AddCommand = New RelayCommand()
End Sub
Public Property AddCommand() As ICommand
Get
Return m_AddCommand
End Get
Private Set
m_AddCommand = Value
End Set
End Property
Private m_AddCommand As ICommand
End Class
VB.NET code generates private backing field.
Is it possible to get rid of this back field in source code (like c#)? How?
Without this feature, VB.NET source will have lots of such redundancy.
Using VB.NET, if you want to specify different accessibility for the Get and Set procedure, then you cannot use an auto-implemented property and must instead use standard, or expanded, property syntax.
Read MSDN: https://learn.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/procedures/auto-implemented-properties
If getter and setter have same accessibility, e.g. both are Public, then you can use the auto-property syntax, e.g.:
Public Property Prop2 As String = "Empty"
In VB.NET it's
Public ReadOnly Property Value As String
Then to access the private setter, you use an underscore before your property name
Me._Value = "Fred"
since the answer(s) above hold(s), you may introduce a Public Prop to expose the Private one. This may not be a nice solution but still less code, than expanded Property syntax
Private Property internalprop as object
Public Readonly Property exposedprop as Object = internalprop
Is there a way to have one variable per class in vbscript?
If not what is the best way to emulate it? Prefixing a global variable declared next to the class?
Also is there a way to declare static/class methods(for a static constructor) or am I force to prefix a function?
In languages that support class-level/static data or methods you can
associate/bind data or methods explicitly to the set of objects defined by the class. So you can have Customer.Count and Product.Count and a plain Count (or ##Count) in Customer code will access the right number.
use such data or method without having an instance of the class (yet).
VBScript does not support static data or methods. You have to use global data or functions/subs and do the associating in your mind (perhaps with a little help from a naming convention). Accessing these 'static'=global elements without an object is trivial, but - obviously - should be done with care.
You can embed one or more singleton objects or code references (GetRef()) in your objects to bind them closer to the class, but that will increase the size of the instances.
You can do something like this to sort of emulate a static class:
Class Defines_
Public Sub DoSomethingUseful
End Sub
End Class
Dim Defines : Set Defines = New Defines_
...
Defines.DoSomethingUseful
This can be used to give you something analogous to constructors (really, factory methods):
Class Something
Private mValue
Public Property Get Value : Value = mValue : End Property
Public Property Let Value(x) : mValue = x : End Property
End Class
Class SomethingFactory_
Public Function Create(value)
Set Create = New Something
Create.Value = value
End Function
End Class
Dim SomethingFactory : Set SomethingFactory = New SomethingFactory_
...
Dim something : Set something = SomethingFactory.Create(5)