Using VB.Net
I am using the CallByName() function to call certain subroutines based on a selection made by the user. The options for the subroutines come from a dropdown menu which gets it data from a table in a database. The CallByName function requires the starting class and the subroutine name as well as the method type ex: CallByName(class,subroutine,type.method). In my table I have a column for the class and a column for the subroutine. So these get read by my program as strings. I'm trying to use these variables in the function and while the subroutine string works fine, I can't seem to convert the class string into a proper class object that can be used by the CallByName function.
My code looks like:
Dim base_class as string = myReader(0)
Dim subroutine as string = myReader(1)
base_class = CType(base_class, class)
'* what I need, but doesn't work ^^^
CallByName(base_class, subroutine, CallType.Method)
How can I convert the class variable read in from the DB into something that I can use in the function?
Thanks.
Edit: The code that I ended up using that worked based on Jimi's response:
Dim t As Type = Type.GetType(base_class)
Dim obj = Activator.CreateInstance(t)
CallByName(obj, subroutine , CallType.Method)
Related
I am trying to find a particular user from a mongodb collection that matches the given id. Folliwng is my VB.Net code. However, I keep getting the error 'Public member 'Find' on type 'MongoCollectionImpl(Of BsonDocument)' not found.'
Public Function GetCollectionByName(ByVal collectionName As String)
Dim db As IMongoDatabase = DBcontext()
Dim collection As IMongoCollection(Of BsonDocument)
collection = db.GetCollection(Of BsonDocument)(collectionName)
Return collection
End Function
Public Function GetUser(ByVal id As String)
Dim filter = Builders(Of BsonDocument).Filter.Eq(Of String)("ID", id)
Dim collection = GetCollectionByName("Users")
Dim list = collection.Find(filter).ToList()`<<<<<<<<<<<<<<<<<<<<<<<<<<<<<ERROR here
Return list
End Function
Firstly, you must have Option Strict Off for that code to even compile. That's bad. You should immediately turn Option Strict On in the project properties and address all the issues it raises. One of those will be the fact that your GetCollectionByName has no return type declared. That means that here:
Dim list = collection.Find(filter).ToList()
that collection variable is implicitly type Object and you are relying on late binding when calling that Find method because the Object class has no such method. As a result, you get no help from Intellisense and Intellisense would have told you what members were and were not available if you were doing this properly.
Regardless, you still could have made it work if you had actually read the documentation for the types you're using to see what members they have. Here is the documentation for the interface you're using in that GetCollectionByName method and, I don't know about you but I don't see any Find method listed there. There is a FindSync method, so maybe that's what you actually want. If you had Option Strict On and used proper types every where, Intellisense would have shown you that.
You should also turn Option Strict On in the IDE options, so that it is On for all future projects.
I had a look at some documentation for the MongoCollectionImpl for Java and there appears to be a find method there but that doesn't necessarily mean that the same method is available in .NET and you aren't working directly with that class anyway. You are working with the IMongoCollection so you should only be working with members of that interface. Basically, your code would need to look more like the below with Option Strict On:
Public Function GetCollectionByName(ByVal collectionName As String) As IMongoCollection(Of BsonDocument)
Dim db As IMongoDatabase = DBcontext()
Dim collection As IMongoCollection(Of BsonDocument)
collection = db.GetCollection(Of BsonDocument)(collectionName)
Return collection
End Function
Public Function GetUser(ByVal id As String) As List(Of BsonDocument)
Dim filter = Builders(Of BsonDocument).Filter.Eq(Of String)("ID", id)
Dim collection = GetCollectionByName("Users")
Dim list = collection.FindSync(filter).ToList()
Return list
End Function
You may want to declare the GetUser method as type IList(Of BsonDocument) if you want to work with interfaces. You probably ought to rename that method or change the implementation too. If a method is returning a list then the name should not indicate that it returns a single item.
I have read that String was a "reference type", unlike integers. MS website
I tried to test its behavior.
Sub Main()
Dim s As New TheTest
s.TheString = "42"
Dim z As String = s.GimmeTheString
z = z & "000"
Dim E As String = s.TheString
s.GimmeByref(z)
end sub
Class TheTest
Public TheString As String
Public Function GimmeTheString() As String
Return TheString
End Function
Public Sub GimmeByref(s As String)
s = TheString
End Sub
End Class
So I expected :
z is same reference as TheString, thus TheString would be set to "42000"
Then Z is modified by reference by GimmeByref thus Z is set to whatever TheString is
Actual result:
Z="42000"
E="42"
TheString="42"
What point am I missing?
I also tried adding "ByRef" in GimmeByRef : yes obviously the GimmeByRef does work as expected, but it also does if I put everything as Integer, which are said to be "Value type".
Is there any actual difference between those types?
The confusion comes about because regardless of type, argument passing in VB is pass by value by default.
If you want to pass an argument by reference, you need to specify the argument type as ByRef:
Public Sub GimmeByref(ByRef s As String)
You also need to understand the difference between mutating a value and re-assigning a variable. Doing s = TheString inside the method doesn’t mutate the value of the string, it reassigns s. This can obviously be done regardless of whether a type is a value or reference type.
The difference between value and reference types comes to bear when modifying the value itself, not a variable:
obj.ModifyMe()
Strings in .NET are immutable and thus don’t possess any such methods (same as integers). However, List(Of String), for instance, is a mutable reference type. So if you modify an argument of type List(Of String), even if it is passed by value, then the object itself is modified beyond the scope of the method.
Strings are immutable, every time you do a change it creates a new "reference" like if New was called.
A String object is called immutable (read-only), because its value
cannot be modified after it has been created. Methods that appear to
modify a String object actually return a new String object that
contains the modification. Ref
Your code basically does something like this:
Sub Main()
Dim a, b As String
a = "12"
b = a
a = a & "13"
Console.WriteLine(a) ' 1213
Console.WriteLine(b) ' 12
Console.ReadLine()
End Sub
I am familiar with this post: How to Return a result from a VBA Function but changing my code does not seem to help.
I want to write a simple function in VBA that allows to lowercase an input sentence. I wrote this:
Private Function Converter(inputText As String) As String
Converter = LCase(inputText)
End Function
Sub test()
Dim new_output As String
new_output = Converter("Henk")
MsgBox (new_output)
End Sub
I tried following the advice I found at another stackoverflow post. I made me change this:
Private Function Converter(inputText As String)
Set outputText = LCase(inputText)
End Function
Sub test()
Dim new_output As String
Set new_output = Converter("Henk")
MsgBox (new_output)
End Sub
However, now I get an error that an object is required. Why does it require an object now? I dont get it...
Set outputText = LCase(inputText)
The Set keyword is reserved for Object data types. Unlike VB.NET, String in VBA is a basic data types.
So you dont Set a variable to a string. Drop the second version of your code altogether. It doesn't make sense. That "advice" was probably in another context.
To fix your first version
1- Assign the returned result to the name of the function Converter
2- It would be beneficial to specify explicitly the return type, as String. Currently it is a Variant that always embeds a String, so better make it explicit:
' vvvvvvvvv
Private Function Converter(inputText As String) As String
Converter = LCase(inputText) ' <------------ assign return to name of function
End Function
I would like to to copy that values of an array into a Structure.
Example:
' The Array
Dim Columns(2) As String
' The Structure
Private Structure Fields
Public FName As String
Public LName As String
Public Email As String
End Structure
' I would like to map it like so:
Fields.FName = Columns(0)
Fields.LName = Columns(1)
Fields.Email = Columns(2)
Obviously I could write a function if it was so simple, but really there are over 25 columns and it's a pain to write a function that would map it.
Is there some way to do this?
There really is no simple way that will work in all cases. What you are complaining is too much effort is the only way to guarantee that it will work in all cases.
That said, if you can guarantee that the number of elements in the array matches the number of properties/fields in the structure/class and that they are in the same order and of the same types then you could use Reflection in a loop, e.g.
Private Function Map(source As Object()) As SomeType
Dim result As New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return result
End Function
EDIT:
The code I have provided works as is if SomeType is a class but, as I missed the first time around, not for a structure. The reason is that structures are value types and therefore a copy of the original object is being sent to SetValue, so the field value never gets set on that original object. In theory, to prevent a copy being created, you should be able to simply box the value, i.e. wrap it in an Object reference:
Private Function Map(source As Object()) As SomeType
Dim result As Object = New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return DirectCast(result, SomeType)
End Function
As it turns out though, the VB compiler treats that a little differently than the C# compiler treats the equivalent C# code and it still doesn't work. That's because, in VB, the boxed value gets unboxed before being passed to the method, so a copy is still created. In order to make it work in VB, you need to use a ValueType reference instead of Object:
Private Function Map(source As Object()) As SomeType
Dim result As ValueType = New SomeType
Dim resultType = result.GetType()
Dim fields = resultType.GetFields()
For i = 0 To source.GetUpperBound(0)
fields(i).SetValue(result, source(i))
Next
Return DirectCast(result, SomeType)
End Function
I have a requirement where I need to have a "type safe" way of accessing property names, without actually instantiating an object to get to the property. To give an example, consider a method that takes as arguments a list of IMyObject and a string that represents a property name (a property that exists in IMyObject).
The methods implementation will take the list and access all the objects in the list using the property name passed... for some reason or another, we won't dwell on that!!
Now, I know that you can do this using an instantiated object, something like ...
Dim x as MyObject = nothing
Dim prop As PropertyInfo = PropHelper.GetProperty(Of MyObject)(Function() x.MyProperty)
Where my helper method uses reflection to get the name of the property as a string - there are numerous examples of this flying around on the web!
But I don't want to have to create this pointless object, I just want to do something like MyObject.MyProperty! Reflection allows you to iterate through a types properties and methods without declaring an object of that type... but I want to access a specific property and retrieve the string version of its name without iteration and without declaring an object of that type!
The main point here is that although I am trying to get the property name as a string... this is done at run time... at compile time, I want this to be type safe so if someone changes the property name, the compilation will break.
Can anyone help in this quest!?!
So here is a quick code-listing to demonstrate the answer that I was looking for:
Imports System.Linq.Expressions
Public Class A
Public Prop1 As String
Public Prop2 As Integer
End Class
Public Class Form1
Public Function GetPropertyNameB(Of TModel, TProperty)(ByVal [property] As Expression(Of Func(Of TModel, TProperty))) As String
Dim memberExpression As MemberExpression = DirectCast([property].Body, MemberExpression)
Return memberExpression.Member.Name
End Function
Public Sub New()
InitializeComponent()
Dim propertyName As String = GetPropertyNameB(Function(myObj As A) myObj.Prop1)
Dim propertyName2 As String = GetPropertyNameB(Function(myObj As A) myObj.Prop2)
MsgBox(propertyName & " | " & propertyName2)
End
End Sub
End Class
You may be able to pass the property in as a simple lamdba expression, and take it in the method as an expression tree. You should be able to analyze the expression tree to get the string name of the property, but it the lambda expression will fail to compile if the property name changes. Check out this page for more details:
http://msdn.microsoft.com/en-us/library/bb397951.aspx
You can make use of the NameOf function:
Dim fieldName = nameOf(MyClass.MyField)