R – Using reflection for generically typed delegate and interfaces

netreflectionvb.net

I have a bunch of classes that each have a property called Sequence. This property is implemented from an interface called ISequenced. For this example lets call one of these classes A.
When I have a List(of A), I want to be able to sort them using the standard List.Sort(addressof delegate) where the delegate is a standard function that takes in two ISequenced objects and compares their sequence numbers and returns a boolean flag, rather than declaring a function for each individual class that implements ISequenced.

E.g.

Dim li as List(of A) = GetValues()

li.Sort(addressof SortBySeq)

...

Public Function SortBySeq(ByVal ob1 as ISequenced, ByVal ob2 as ISequenced) as Boolean
   return ob1.Sequence.CompareTo(ob2.Sequence)
End If

EDIT: Using the above gives me the following error:

"Overload resolution failed because no
accessible 'Sort' can be called with
these arguments:
'Public Sub Sort(comparison As System.Comparison(Of A))': Option
Strict On does not allow narrowing in
implicit type conversions between
method 'Public Function SortBySeq(ob1
As ISequenced, ob2 As ISequenced) As
Integer' and delegate 'Delegate
Function Comparison(Of A)(x As A, y As
A) As Integer'.
'Public Sub Sort(comparer As System.Collections.Generic.IComparer(Of
A))': 'AddressOf' expression cannot be
converted to
'System.Collections.Generic.IComparer(Of
A)' because
'System.Collections.Generic.IComparer(Of
A)' is not a delegate type."

How would I declare the function in order to do this (if it is even possible in VB.NET?

Thanks,
Dane.

Best Answer

Implement an IComparer. Everything you need is here.

Edit: Oh, I see what the rub is. All the classes that have the Sequence property must implement an interface to be comparable. E.g. use IHaveSequence and do an IComparer< IHaveSequence>. IHaveSequence's definition should be obvious.

Related Solutions

Java – reflection and why is it useful

The name reflection is used to describe code which is able to inspect other code in the same system (or itself).

For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.

So, to give you a code example of this in Java (imagine the object in question is foo) :

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.

There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html

And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.

Update from a comment:

The ability to inspect the code in the system and see object types is not reflection, but rather Type Introspection. Reflection is then the ability to make modifications at runtime by making use of introspection. The distinction is necessary here as some languages support introspection, but do not support reflection. One such example is C++

.net – How to extend ADO.NET Entity Framework objects with partial classes

Is this what you're trying to do - create a method that applies a filter to Client queries.

I don't know vb.net, so don't trust this free-hand code 100%.

Partial Public Class Client
  Public Shared Function IsWashington(query As IQueryable(Of Client)) As IQueryable(Of Client)
    Return query.Where(Function(someClient) someClient.LastName = "Washington")
  End Function
End Class

later, some calling code.

IQueryable(Of Client) someQuery = dc.ClientSet.AsQueryable
someQuery = Client.IsWashington(someQuery)

Label1.Content = someQuery.First.FirstName

Hope this works!

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