VB.NET: impossible to use Extension method on System.Object instance
Can I make an Extension method for all the subclasses of System.Object
(everything)?
Example:
<Extension>
Public Function MyExtension(value As Object) As Object
Return value
End Function
The above functions won't work for object instance:
Dim myObj1 As New Object()
Dim myObj2 = myObj1.MyExtension()
The compiler does not accept it, is the problem in my computer? :)
UPDATE
The problem seems to occur only in VB, where members of object are looked-up by reflection (late-bound).UPDATE AFTER ANSWERED
F开发者_StackOverflow社区YI, as vb has an advantage that C# lacks that is, members of imported Modules are imported to the global scope so you can still use this functions without their wrapper:Dim myObj2 = MyExtension(myObj1)
It seems like not supporting Extension methods on Object was a design decision in VB.
As a result, the only way we could prevent extension methods from completely breaking existing late bound code was to prevent them from being used on anything typed as object.
Link
See this question I asked some time ago. Basically, you can extend Object
in VB.NET if you want; but for backwards compatibility reasons, no variable declared as Object
will be able to use your extension method. This is because VB.NET supports late binding on Object
, so an attempt to access an extension method will be ignored in favor of trying to find a method of the same name from the type of the object in question.
So take this extension method, for example:
<Extension()>
Public Sub Dump(ByVal obj As Object)
Console.WriteLine(obj)
End Sub
This extension method could be used here:
' Note: here we are calling the Dump extension method on a variable '
' typed as String, which works because String (like all classes) '
' inherits from Object. '
Dim str As String = "Hello!"
str.Dump()
But not here:
' Here we attempt to call Dump on a variable typed as Object; but '
' this will not work since late binding is a feature that came before '
' extension methods. '
Dim obj As New Object
obj.Dump()
Ask yourself why extension methods don't work on dynamic
variables in C#, and you'll realize the explanation is the same.
You can not directly write an extension method for Object, but using generics you can achieve the same result:
<Extension()>
Public Function NullSafeToString(Of T)(this As T) As String
If this is Nothing Then
Return String.Empty
End If
Return this.ToString()
End Function
Note that you can call this as an extension method on everything except things that are declared to have the type Object. For those, you have to either call it directly (full proof) or call via casting (which could fail, as there is no univesal interface, so somewhat chancy).
jmoreno's answer cannot be used with Option Strict On
– it throws error:
BC30512 Option Strict On disallows implicit conversions from 'Object' to 'Integer'.
It needs context switch from class to extension module:
Dim text1 As String = MyExtModule.NullSafeToString(DataGridView1.Rows(0).Cells(0).Value)
Sure you can, though you might want to be sparing about what you do here so as not to clutter every object. An extension method I like using for Object is a method called IsIn() that functions similarly to the SQL IN() statement. It's nice to say things like:
If someString.IsIn("a", "b", "c") Then
DoSomething()
Else If someInt.IsIn(1, 2, 3) Then
DoSomethingElse()
Else If someObj.IsIn(1, "q", #7/1/2010#) Then
DoSomethingTheThirdWay()
End If
EDIT -
Added implementation of IsIn() extension method below to help commenter.
Imports System.Runtime.CompilerServices
Public Module ObjectExtensions
<Extension()>
Public Function IsIn(obj As Object, ParamArray values() As Object) As Boolean
For Each val As Object In values
If val.Equals(obj) Then Return True
Next
Return False
End Function
End Module
If you do too many extensions on object intellisence might become less useful, but it's perfectly valid.
Here's an example of an extension method on object for object information:
http://www.developer.com/net/csharp/article.php/3718806/NET-Tip-Using-Extension-Methods.htm
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