How to Pass a Parameter using a RelayCommand

Question

In spite of reading countless web sites, I cannot get my RelayCommand to handle a parameter properly. I'm worki8ng on a login window:

<Window
    x:Class="Login"
    x:Name="Login"
    ....
    <Button
        Command="{Binding Path=Login}"
        CommandParameter="{Binding ElementName=Login}"
        Content="Login" />
    ....
</Window>

I am using the RelayCommand provided by the CommunityToolkit.Mvvm, Here is the Command implemented in the ViewModel.

Private ReadOnly Property _Login As New RelayCommand(AddressOf PerformLogin)

Public ReadOnly Property Login As RelayCommand
    Get
        Return _Login
    End Get
End Property

Private Sub PerformLogin(win As Object)
    <<Process user inputs>>
End Sub

The compile -time error is

Method 'Private Sub PerformLogin(win As Object)' does not have a signature compatible with delegate 'Delegate Sub Action()'.

I understand the error but none of the numerous fixes I've tried have worked. What is the right way to accomplish this?

Answer 1

Hi Roger,
in PerformLogin(win As Object) you can check parameter "win" where you get CommandParameter. Try following demo:

<Window x:Class="WpfApp1.Window046"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp046"
        mc:Ignorable="d"
        Title="Roger Schlueter_230313" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <StackPanel>
    <Label x:Name="Login" Content="First Element for Login" Margin="10"/>
    <Label x:Name="Logout" Content="Second Element for Logout" Margin="10"/>
    <Button
        Command="{Binding Path=Login}"
        CommandParameter="{Binding ElementName=Login}"
        Content="Login" Margin="10"/>
    <Button
        Command="{Binding Path=Login}"
        CommandParameter="{Binding ElementName=Logout}"
        Content="Logout" Margin="10"/>
  </StackPanel>
</Window>

ViewModel:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace WpfApp046
{
	public class ViewModel
	{
		public ICommand Login { get => new RelayCommand(CmdExec, CanCmdExec); }

		private void CmdExec(object obj)
		{
			var element = obj as Label;
			switch (element?.Name)
			{
				case "Login":
					MessageBox.Show("Login pressed");
					break;
				case "Logout":
					MessageBox.Show("Logout pressed");
					break;
				default:
					break;
			}
		}

		private bool CanCmdExec(object obj) => true;


	}
#pragma warning disable CS8767 // Nullability of reference types in type of parameter doesn't match implicitly implemented member (possibly because of nullability attributes).
#pragma warning disable CS8625 // Cannot convert null literal to non-nullable reference type.
	public class RelayCommand : ICommand
	{
		private readonly Predicate<object> _canExecute;
		private readonly Action<object> _action;
		public RelayCommand(Action<object> action) { _action = action; _canExecute = null; }
		public RelayCommand(Action<object> action, Predicate<object> canExecute) { _action = action; _canExecute = canExecute; }
		public void Execute(object o) => _action(o);
		public bool CanExecute(object o) => _canExecute == null ? true : _canExecute(o);
		public event EventHandler CanExecuteChanged
		{
			add { CommandManager.RequerySuggested += value; }
			remove { CommandManager.RequerySuggested -= value; }
		}
	}
}

Result:

Answer 2

Would you mind adding a custom RelayCommand? It can pass parameters.

<TextBlock x:Name="Logintxt" Text="nico" Height="100" Width="200"/>
        <Button    Command="{Binding Path=Login}"  CommandParameter="{Binding ElementName=Logintxt ,Path=Text }"
        Content="Login" />

Public Class MainWindow

    Public Sub New()
        InitializeComponent()

        DataContext = Me
    End Sub

    Private ReadOnly Property _Login As New MyRelayCommand(AddressOf PerformLogin)

    Public ReadOnly Property Login As MyRelayCommand
        Get
            Return _Login
        End Get
    End Property

    Private Sub PerformLogin(ByVal obj As Object)
        MessageBox.Show(obj + "hello")
    End Sub
End Class

Public Class MyRelayCommand

    Implements ICommand

    Private ReadOnly _execute As Action(Of Object)

    Private ReadOnly _canExecute As Predicate(Of Object)

    Public Sub New(ByVal execute As Action(Of Object))

        Me.New(execute, Nothing)

    End Sub


    Public Sub New(ByVal execute As Action(Of Object), ByVal canExecute As Predicate(Of Object))

        If execute Is Nothing Then

            Throw New ArgumentNullException("execute")

        End If

        _execute = execute

        _canExecute = canExecute

    End Sub

    Public Function CanExecute(ByVal parameter As Object) As Boolean Implements ICommand.CanExecute

        Return If(_canExecute Is Nothing, True, _canExecute(parameter))

    End Function

    Public Custom Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged

        AddHandler(ByVal value As EventHandler)

            AddHandler CommandManager.RequerySuggested, value

        End AddHandler

        RemoveHandler(ByVal value As EventHandler)

            RemoveHandler CommandManager.RequerySuggested, value

        End RemoveHandler

        RaiseEvent(ByVal sender As System.Object, ByVal e As System.EventArgs)

        End RaiseEvent

    End Event

    Public Sub Execute(ByVal parameter As Object) Implements ICommand.Execute

        _execute(parameter)

    End Sub

End Class

The result:

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