Passing a parameter using RelayCommand defined in the ViewModel (from Josh Smith example)

Question

I would like to pass a parameter defined in the XAML (View) of my application to the ViewModel class by using the RelayCommand. I followed Josh Smith's excellent article on MVVM and have implemented the following.

XAML Code

        <Button 
        Command="{Binding Path=ACommandWithAParameter}"
        CommandParameter="Orange"
        HorizontalAlignment="Left" 
        Style="{DynamicResource SimpleButton}" 
        VerticalAlignment="Top" 
        Content="Button"/>

ViewModel Code

  public RelayCommand _aCommandWithAParameter;
  /// <summary>
  /// Returns a command with a parameter
  /// </summary>
  public RelayCommand ACommandWithAParameter
  {
     get
     {
        if (_aCommandWithAParameter == null)
        {
           _aCommandWithAParameter = new RelayCommand(
               param => this.CommandWithAParameter("Apple")
               );
        }

        return _aCommandWithAParameter;
     }
  }

  public void CommandWithAParameter(String aParameter)
  {
     String theParameter = aParameter;
  }
  #endregion

I set a breakpoint in the CommandWithAParameter method and observed that aParameter was set to "Apple", and not "Orange". This seems obvious as the method CommandWithAParameter is being called with the literal String "Apple".

However, looking up the execution stack, I can see that "Orange", the CommandParameter I set in the XAML is the parameter value for RelayCommand implemenation of the ICommand Execute interface method.

That is the value of parameter in the method below of the execution stack is "Orange",

  public void Execute(object parameter)
  {
     _execute(parameter);
  }

What I am trying to figure out is how to create the RelayCommand ACommandWithAParameter property such that it can call the CommandWithAParameter method with the CommandParameter "Orange" defined in the XAML.

Is there a way to do this?

Why do I want to do this? Part of "On The Fly Localization" In my particular implementation I want to create a SetLanguage RelayCommand that can be bound to multiple buttons. I would like to pass the two character language identifier ("en", "es", "ja", etc) as the CommandParameter and have that be defined for each "set language" button defined in the XAML. I want to avoid having to create a SetLanguageToXXX command for each language supporting and hard coding the two character language identifier into each RelayCommand in the ViewModel.

Answer 1

I don't understand why you have the extra complexity of specifying the lambda in the first place. Why not just do this:

if (_aCommandWithAParameter == null)
{           
    _aCommandWithAParameter = new RelayCommand<object>(CommandWithAParameter);
}

private void CommandWithAParameter(object state)
{
    var str = state as string;
}

Answer 2

You'll pass the param in the lambda to the command like so:

if (_aCommandWithAParameter == null)
{           
    _aCommandWithAParameter = new RelayCommand(               
        param => this.CommandWithAParameter(param)
        );        
}

Answer 3

Nothing posted here before worked for me.

Turns out, all answers are missing the <object> after RelayCommand!

This works for me:

public RelayCommand<object> OKCommand
{
    get
    {
        if (_okCommand == null)
            _okCommand = new RelayCommand<object>(OkCommand_Execute);
        return _okCommand;
    }
}
private RelayCommand<object> _okCommand = null;

private void OkCommand_Execute(object obj)
{
    Result = true;
}

If you want to use aCanExecute method, use the following code:

_okCommand = new RelayCommand<object>(OkCommand_Execute, OkCommand_CanExecute);

private bool OkCommand_CanExecute(object obj) { }

Answer 4

Here is a simple solution for the commandparameter as I was looking for help on the subject. I could not find anything online that was simple enough. The following solution works well when you are using a relaycommand. I had a few hyperlinks for which I needed to get the url value that was clicked using the command parameter.

Step 1: In your relay command, create a simple property that will hold the parameter object value. You could call it parametervalue or any name that you prefer.

public object ParameterValue
{
  get;
  set;
}

Step 2: In the Execute Method of the RelayCommand class, set the value or the property created above to the parameter from the Execute method.

readonly Action<object> m_execute;       // Action to execute

public void Execute(object parameter)
 {
   this.ParameterValue = parameter;
   m_execute(parameter);
 }

Step 3: Now when you can bind the CommandParameter in xaml to any value you want to retrieve when the command is executed. example:

<TextBlock>
  <Hyperlink Command="{Binding Path=NavigateUrlCmd}"
             CommandParameter="{Binding ElementName=tbwebsite, Path=Text}">
    <TextBlock Name="tbwebsite" Text="{Binding Path=website}"/>
  </Hyperlink>
</TextBlock> 

If you have a command called chickenCommand, when executed you could access the parameter: chickenCommand.ParameterValue

I hope this helps somebody. Thank you for all your previous help.

Answer 5

I am just trying to sell my point, check this out whether this works...

http://mywpf-visu.blogspot.com/2009/12/relay-command-how-to-pass-parameter.html

Answer 6

I cannot substitute a reference to the method name for the lamda expression withing a compile error. Apparently, and by no means surprisingly, a non-static method name reference cannot be used in place of a lambda. I hardly see it as "added complexity". Consistently passing lamdas makes sense to me.