How can I pass two command parameters using a WPF binding?

Question

I'm writing code for wpf app using MVVM which has following UI;

when I enter name and age of someone in the app, message box will say:
"Happy Birthday, {Name}!, Age: {Age}"

Following is code for the app:
<Command>

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Input;

namespace WpfApp2.ViewModel
{
public class MyCommand : ICommand
{
public event EventHandler CanExecuteChanged;
private Action<string,string> _execute;

    public MyCommand(Action<string,string> execute)  
    {  
        _execute = execute;  
    }  

    public bool CanExecute(object parameter)  
    {  
        return true;  
    }  

    public void Execute(object parameter)  
    {  
        var values = (object[])parameter;  
        _execute.Invoke(values[0] as string ,values[1] as string);  
    }  
}  

}

<ViewModel>

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;

namespace WpfApp2.ViewModel
{
public class MyViewModel
{
//public string nameText { get; set; }
//public string ageText { get; set; }

    public MyCommand DisplayBirthdayCommand { get; private set; }  

    public MyViewModel()  
    {  
        DisplayBirthdayCommand = new MyCommand(DisplayBirthday);  
    }  

    private void DisplayBirthday(string nameText, string ageText)  
    {  
        MessageBox.Show(string.Format("Happy Birthday, {0}!  Age: {1}", nameText,Convert.ToInt32(ageText)+1), $"{nameText}'s Birthday");  
    }  
}  

}

To pass multiple command parameter to Command, I made converter:
<converter>

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Windows.Data;

namespace WpfApp2.ViewModel
{
public class TextBox
{
public string FirstBoxValue { get; set; }
public string SecondBoxValue { get; set; }
}

public class MyConverter : IMultiValueConverter  
{  
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)  
    {  
        return new TextBox()  
        {  
            FirstBoxValue = values[0].ToString(),  
            SecondBoxValue = values[1].ToString()  
        };  
    }  

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)  
    {  
        throw new NotImplementedException();  
    }  
}  

}

I didn't get any error till I run app. when I run the app, I got following error:

I couldn't figure what's wrong with this code. How can I fix this?
if someone give me an advice, I'd be very appreciated.

thanks,
seihyung

Answer 1

Hi,
you can use CommandParameter binded to DataContext (=MyViewModel). Try following demo with yours code:

XAML MainWindow:

<Window x:Class="WpfApp2.MainWindow"
        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:WpfApp2"
        xmlns:vm="clr-namespace:WpfApp2.ViewModel"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="211">
  <Window.DataContext>
    <vm:MyViewModel/>
  </Window.DataContext>
  <Window.Resources>
    <Style TargetType="Label">
      <Setter Property="Margin" Value="5"/>
    </Style>
    <Style TargetType="TextBox">
      <Setter Property="Margin" Value="5"/>
    </Style>
    <Style TargetType="Button">
      <Setter Property="Margin" Value="5"/>
    </Style>
  </Window.Resources>
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
      <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <Label Grid.Row="0" Grid.Column="0" Content="Name:"/>
    <TextBox Grid.Row="0" Grid.Column="1" Text="{Binding nameText}"/>
    <Label Grid.Row="1" Grid.Column="0" Content="Age:"/>
    <TextBox Grid.Row="1" Grid.Column="1" Text="{Binding ageText}"/>
    <Button Grid.Row="2" Grid.Column="1" Content="Birthday" 
            Command="{Binding DisplayBirthdayCommand}" CommandParameter="{Binding}"/>
  </Grid>
</Window>

MyCommand:

using System;
using System.Windows.Input;

namespace WpfApp2.ViewModel
{
  public class MyCommand : ICommand
  {
    public event EventHandler CanExecuteChanged;
    private Action<object> _execute;
    public MyCommand(Action<object> execute) => _execute = execute;
    public bool CanExecute(object parameter) => true;
    public void Execute(object parameter) => _execute.Invoke(parameter);
  }
}

MyViewModel:

using System;
using System.Windows;

namespace WpfApp2.ViewModel
{
  public class MyViewModel
  {
    public string nameText { get; set; }
    public string ageText { get; set; }
    public MyCommand DisplayBirthdayCommand { get; private set; }
    public MyViewModel()
    {
      DisplayBirthdayCommand = new MyCommand(DisplayBirthday);
    }
    private void DisplayBirthday(object state)
    {
      MyViewModel vm = state as MyViewModel;
      MessageBox.Show(string.Format("Happy Birthday, {0}!  Age: {1}", vm.nameText, Convert.ToInt32(vm.ageText) + 1), $"{vm.nameText}'s Birthday");
    }
  }
}