I am adding a feature to an existing VB .Net application that involves retrieving data from a .Net web service. The web service returns an array of Locations. A Location is pretty simple, it has 3 properties – an integer and two strings.
So that the rest of my application does not have to be dependent on this web service, I would like to create my own Location type within my application. My thought is I could call a method that returns a generic list of my Location type which internally calls the web service and populates the list I return. That way, if the data source for Locations changes in the future to something other than the web service, I only have to fix the method instead of fixing all the callers.
So I created my own Location that has identical properties as the service Location. But I don’t seem to be able to cast the array of service locations into a generic list of my locations. I also tried casting a single service Location into one of my Locations and that didn’t work either.
So is casting an option or do I have to loop through each service Location and assign each property to a new one of my Locations? Or is there a completely different way to go about this?
By default you will not be able to cast one Location to another. They are completely unrelated types and thus cannot be the subject of casting. You can make it possible to cast though by defining a custom operator for the application version of CType.
' Location for application
Public Class Location
...
Public Shared Operator Widening CType(p1 as Namespace.Of.WebService.Location) As Location
Dim loc = ConvertWebServiceLocationToApplicationLocation
return loc
End Operator
End Class
This allows you to now do a CType operation between a WebService Location the Application Location.
Casting the array though, simply won't be possible. There is no way to define a conversion operator for arrays so they can't use the above trick. But you can write a quick and dirty function for this purpose
Public Shared Function ConvertArray(ByVal arr As Namespace.Of.WebServiec.Location()) As Location()
Dim newArray(arr.Length) As Location
For i as Integer = 0 To arr.Length - 1
newArray(i) = CType(arr(i), Location)
Next
return newArray
End Function
Casting will not work because these are not the same type. Even if two types look exactly the same, you cannot cast from one to the other, unless you define a CType operator which describes how to transform an object from one type into an object from another type.
Even then, you cannot cast a List(of Type1) into a List(Of Type2) directly.
You will have to loop through and create a new object of your class.
Related
Maybe superfluous, but some intro...
I am rewriting an add-in for my CAD-application (using VB.NET).
This add-in reads, via an API, a bunch of metadata from a file, presents it in a Form. This data can then be (partially) changed and written back to the file.
This metadata is accessible in a consistent way, however the data type is not the same everywhere (String, Currency, Date, Boolean, Long and IPictureDisp).
Currently I have a much too complex class with several arrays. I thought it might be smarter to create a structure. The problem is the varying data type.
Is it possible to define a structure with a member with varying datat type, or am I forced to define a different structure for each data type?
You have a few options...
1: Use Object
Nice and simple, every data type inherits from Object - so if your struct contains a property of type Object, you can put pretty much any data type in there
From the docs:
The Object data type can point to data of any data type, including any object instance your application recognizes. Use Object when you do not know at compile time what data type the variable might point to.
However, this does mean that you will get next to no help from the compiler when you are trying to write code using this property. You will also probably have to cast any time you need to do anything type-specific
2: Generic Types
This will not fit situations where you are not sure of the type. You can create a generic struct using the Of syntax.
You'd create it as so:
Structure MyStructure(Of T)
'our changing type
Dim MyCustomData As T
'...alongside regular types
Dim Name As String
Dim OtherThing As Integer
End Structure
and then when you need to create the structure, you'd simply pass the type in and assign the value
Dim struct As New MyStructure(Of Integer)
struct.MyCustomData = 123
Dim struct2 As New MyStructure(Of String)
struct2.MyCustomData = "a"
I've been given a collection of WCF services to use which are all the same, save for the way certain properties of the objects returned by the services are named. If I have services A, B, and C, they each return nearly identical objects except that the objects' properties have been changed to reflect the service from whence they came. So, service A returns an object called responseA with a property responseA.AValidation and B returns a similar object with a property called responseB.BValidation.
Now, as much as might like to make changes to the services, I can't. Which means I'm stuck writing the exact same code for each service all to check for various possible error conditions,
If responseA.AErrors.Length > 0
...
If responseB.BErrors.Length > 0
...
and so on
What I'd love is to write one generic class that can use any of these services and has a single method to perform the above checking on any of them However, I'm stumped on how to check the value of a property when the name of that property can be slightly different from class to class.
So, is it possible to return the value of a property that I don't know the name of until runtime?
Here's what I've tried so far
Dim responseType As Type = responseA.GetType()
For Each Prop In responseType.GetProperties()
If Prop.Name.Contains("ErrorInfoTypes") Then
Dim errorTypes()
errorTypes = Prop.GetValue(GetType(Array), Nothing)
If errorTypes.Length > 0 Then
'Deal with the fact there are errors.
End If
Exit For
End If
Next
This complies, but .GetProperties returns no properties. What am I missing?
I have a class call CalcArray that has an array of doubles called Amounts(), and two ints, StartPeriod and EndPeriod.
The user almost always wants to interact with the items in the array, not the Periods or the object itself. So ideally, I'd like:
property AnAmount() as CalcArray 'So the user can talk to the object if they need to
property AnAmount(i as Integer) as Double 'So the user can just get the value directly
This seems to work sometimes and not others. Is this simply a syntax issue? or is such an overload not possible?
You can do this with a function returning a different based on how it is called. Especially since you have a param, a function might be more appropriate:
Public Function AnAmount(Of T)(parm As SomeType) As T
to use it:
Dim n as Decimal
n = AnAmount(Of Decimal)(foo)
Its very useful as a way to avoid returning an object and then have to use CType to convert the return. In this case, an amount implies a value type, but the function would accept Point, Rectangle etc as T, so you might need to check valid type requests.
You may be bumping into the limitation that a function or property cannot vary by only the return type. In general if the signature has changed, the output type can change also on an overload. Look out also for the limitation for using default properties requires an argument. In some cases class inheritance is the issue, properties and functions being shadowed may explicitly be required to nominate Shadows, Overloads, Overrides etc. or the shadowing will be disallowed by the language.
If these don't cover the cases you've seen, try to catch an example of the problem and study all locations of the same named property in your solution, reporting the results here.
I'm really struggling to get my head around the idea of working with a Class Library and I'm pretty much at the point of just maintaining separate projects with duplicate classes rather than a shared class library.
I've created a solution that contains a single class library project and two web applications. My main problems are the connection strings. These are held/declared in the web projects and I'm having to pass them into the class library every time I perform any kind of data access. I sort of understand why I should do this so I'm going with it for the moment.
This has now led me to a problem/question with lazy loading. I'm using lazy loading for the following property:
Public Property KeyRelationshipManager() As Employee
Get
If _keyRelationshipManager Is Nothing Then
_keyRelationshipManager = Employee.GetEmployee(_keyRelationshipManagerStaffNumber)
Return _keyRelationshipManager
Else
Return _keyRelationshipManager
End If
End Get
Set(ByVal value As AECOM.Employee)
_keyRelationshipManager = value
End Set
End Property
Because this property is using the function:
Employee.GetEmployee
I need to pass in the connection string to that function.
This means I would need to pass the connection string in to the property every time I use it so I could pass it into the function.
Is this correct? It doesn't 'feel' right to me because I'm going to have to adjust a huge number of functions and property and pass through the connections string.
Why are you passing the connection string in to the class library? Use ConfigurationManager.ConnectionStrings["myClassLibraryConnection"] in your class library. As long as you have that connection string in both your host applications' config files, it should be fine. It's the web.config files' jobs to bind the configuration of different class libraries to form a single application.
I am attempting to create a generic function that the students in my introductory VB .NET course can use to search a single dimension array of a structure.
My structure and array look like this:
Private Structure Survey
Dim idInteger As Integer
Dim membersInteger As Integer
Dim incomeInteger As Integer
Dim stateString As String
Dim belowPovertyLevelBoolean As Boolean
End Structure
Private incomeSurvey(199) As Survey
My generic function header looks like:
Private Function FindSurveyItem(Of xType As Structure)
(ByVal surveyIDInInt As Integer, ByVal surveyArrayIn() As xType) As Integer
??????
End Function
My call to the function looks like:
If FindSurveyItem(Of Survey)(CInt(idTextBox.Text), incomeSurvey) <> -1 Then
My question is: Is there a way to reference the individual structure fields in the array from inside the function? I was trying to make it generic so that the student could simply pass their array into the function - their structure may be named differently than mine and the field names may be different.
I suspect there are other ways to deal with this situation, but I was trying to keep it to just a simple single-dimension array of a structure. I don't think it is possible to do what I want, but I wondered what others thought.
Is there a way to reference the individual structure fields in the array from inside the function?
Generic instead of an array you need a collection type. Add LINQ Code:
Dim Surveys = From svys In xType
Where svys.idInteger = surveyIDInInt
Select svys
For Each rSurveys In svys
'''' Your Code
Next
This is rough answer fill in the details (I know imagine LINQ without SQL DB!!)
If you have a genric type parameter T you are only able to access members of instances of T that are known to exist at compile time. As every type derives from Object you have only the members of Object availiable - ToString(), GetType(), GetHashCode(), and Equals().
If you want to access other members you have to constrain what T is allowed to be. In your situation a interface would be the way to go. See MSDN for details.
You could also try to use reflection or check the actual type at runtime an perform a cast. The first is hard to impossible to do if you do not know much about the types you will get. And the later requires you to know possible types at compiletime and will not work in your situation, too.
Another way might be to pass a delegate to the search method that performs the actual comparison.
What you're looking for are predicates, if using ,net 3.5
dim arr() as structure
Array.Find(arr, function(item)(item.MyMember = MemberToMatch))
More combersome in earlier versions, see here for more info
The point being, that your function would look very like an implementation of Array.Find (if you didn't want to use the function provided), and the students would need to write their own predicate function.
No, there isn't. You can't know the type at compile time, therefore you cannot access members of that type. You would need change from a structure to a class that must implement IComparable so that you can use CompareTo between the item you pass in and the array you are passing in.
Though it's not entirely clear what you are trying to do within your method so I'm guessing by the name of the method.
You can use reflection to get those fields, but in this case that wouldn't have much meaning. How would you know that the passed type has the field you're looking for? There are other problems with that code as well.
Instead, to do this I would normally create an interface for something like this that had a public ID property, and constrain my input to the function to implement that interface, or as others mentioned use a built-in feature in the clr.
But that may be ahead of where your students are. If you just want an example of a generic function, my suggestion is to show them a type-safe implementation of the old vb imediate if function:
Public Function IIf(Of T)(ByVal Expression As Boolean, ByVal TruePart As T, ByVal FalsePart As T) AS T
If Expression Then Return TruePart Else Return FalsePart
End Function
Note that this is obsolete, too, as in VS2008 and beyond you can use the new If operator instead, which will work better with type inference and won't try to evaluate the expression that isn't returned.