How to Avoid a RelayCommand Error

Question

In a ViewModel for a wpf window I have the following command:

Private ReadOnly Property _AddVehicle As New RelayCommand(AddressOf OnAddVehicle())
Public ReadOnly Property AddVehicle As RelayCommand
	Get
		Return _AddVehicle
	End Get
End Property

I am using the nomional version of RelayCommand. Because I have Option Strict On, I get the following error:

Option Strict On does not allow narrowing in implicit type conversions between method 'Private Sub OnAddVehicle()' and delegate 'Delegate Sub Action(Of Object)(obj As Object)'.

I know I can fix this by using this for the argument to RelayCommand:

... RelayCommand(Sub() OnAddVehicle())

However, there are other factors that are easily met by using the "AddressOf" argument. So my questions are: Why am I getting the error? Is there a way to use the "AddressOf" argument to RelayCommand?

Answer 1

Hi,@ RogerSchlueter-7899. Welcome to Microsoft Q&A. The issue you're facing with the RelayCommand and AddressOf is related to the fact that AddressOf in VB.NET includes parentheses when used with a method, turning it into a delegate invocation. This can cause a type mismatch in cases where the delegate's signature doesn't match the method's signature. In your case, the RelayCommand expects a delegate with the signature Action(Of Object), but when you use AddressOf OnAddVehicle(), you are essentially trying to create a delegate with the signature Action(Of Object) that invokes the result of OnAddVehicle().

Private ReadOnly Property _AddVehicle As New RelayCommand(AddressOf OnAddVehicle)
Public ReadOnly Property AddVehicle As RelayCommand
    Get
        Return _AddVehicle
    End Get
End Property
Private Sub OnAddVehicle()
    ' Your implementation here
End Sub

By removing the parentheses after OnAddVehicle, you're passing the method itself as a delegate to the RelayCommand constructor, rather than trying to invoke it immediately. Alternatively, as you've mentioned, you could also use a lambda expression to achieve the same result:

Private ReadOnly Property _AddVehicle As New RelayCommand(Sub() OnAddVehicle())
Public ReadOnly Property AddVehicle As RelayCommand
    Get
        Return _AddVehicle
    End Get
End Property
Private Sub OnAddVehicle()
    ' Your implementation here
End Sub

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Answer 2

Hi Roger,
you must check constructors in RelayCommand class. RelayCommand class can look like this:

Public Class RelayCommand
  Implements ICommand
#Region "Fields"
  ' delegate to execute
  Private ReadOnly _execute As Action(Of Object)
  Private ReadOnly _canExecute As Predicate(Of Object)
#End Region ' Fields
#Region "Constructors"
  ''' <summary>
  ''' in ctor get delegate to execute
  ''' </summary>
  ''' <param name="execute">The execution logic.</param>
  Public Sub New(ByVal execute As Action(Of Object))
    Me.New(execute, Nothing)
  End Sub
...

Parameter in AddressOf must be only the name of method (without parantheses)

	Private ReadOnly Property _AddVehicle As New RelayCommand(AddressOf OnAddVehicle)
	Public ReadOnly Property AddVehicle As RelayCommand
		Get
			Return _AddVehicle
		End Get
	End Property

	''' <summary>
	''' There's no constructor in RelayCommand class with this signature and method is not necessary
	''' </summary>
	Private Sub OnAddVehicle()
		''
	End Sub

	''' <summary>
	''' This method work for RelayCommand class
	''' </summary>
	''' <param name="par"></param>
	Private Sub OnAddVehicle(par As Object)
		''
	End Sub