VB.NET: impossible to use Extension method on System.Object instance

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.

http://blogs.msdn.com/b/vbteam/archive/2007/01/24/extension-methods-and-late-binding-extension-methods-part-4.aspx

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)