<Window x:Class="MyApp.Windows.Views.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:services="clr-namespace:MyApp.Windows.Services"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:prism="http://prismlibrary.com/"
        mc:Ignorable="d"
        x:Name="Window"
        prism:ViewModelLocator.AutoWireViewModel="True">
    <Window.Resources>
        <services:OpenFileDialogService x:Key="OpenFileDialogService" Title="Select File" Filter="Text File (*.txt)|*.txt|All Files (*.*)|*.*" DefaultFilterIndex="0" Multiselect="False" />
    </Window.Resources>
    <i:Interaction.Triggers>
        <prism:InteractionRequestTrigger SourceObject="{Binding SelectFileInteractionRequest, Mode=OneWay}">
            <services:PopupOpenFileDialogAction DialogService="{StaticResource OpenFileDialogService}" Owner="{Binding ElementName=Window}" />
        </prism:InteractionRequestTrigger>
    </i:Interaction.Triggers>
    <StackPanel Orientation="Vertical">
        <TextBox Text="{Binding Filename}"/>
        <Button Content="_Select File..." Command="{Binding SelectFileCommand}"/>
    </StackPanel>
</Window>

namespace MyApp.Windows.ViewModels
{
    using System;
    using System.Linq;
    using System.Windows;
    using System.Windows.Input;
    using Prism.Mvvm;
    using Prism.Commands;
    using Prism.Interactivity.InteractionRequest;
    using MyApp.Windows.Services;

    public class MainWindowViewModel : BindableBase
    {
        private string filename;

        public MainWindowViewModel()
        {
            this.SelectFileCommand = new DelegateCommand(this.SelectFile);
            this.SelectFileInteractionRequest = new InteractionRequest<OpenFileDialogInteractionContext>();
        }

        public string Filename
        {
            get => this.filename;
            set => this.SetProperty(ref this.filename, value);
        }

        public ICommand SelectFileCommand { get; }

        public InteractionRequest<OpenFileDialogInteractionContext> SelectFileInteractionRequest { get; }

        private void SelectFile()
        {
            this.RaiseSelectFileInteractionRequest();
        }

        private void RaiseSelectFileInteractionRequest()
        {
            var context = new OpenFileDialogInteractionContext();

            this.SelectFileInteractionRequest.Raise(
                context,
                c => this.UpdateFilename(c));
        }

        private void UpdateFilename(OpenFileDialogInteractionContext context)
        {
            if (context.Confirmed)
            {
                this.Filename = context.Filenames.First();
            }
        }
    }
}

namespace MyApp.Windows.Services
{
    using System;
    using System.Windows;
    using Microsoft.Win32;

    public class OpenFileDialogService
    {
        public string Filter { get; set; } = string.Empty;
        public string Title { get; set; } = string.Empty;
        public int DefaultFilterIndex { get; set; } = 0;
        public bool Multiselect { get; set; }

        public string[] ShowDialog(Window owner)
        {
            OpenFileDialog dialog = new OpenFileDialog()
            {
                Title = this.Title,
                Filter = this.Filter,
                FilterIndex = this.DefaultFilterIndex,
                Multiselect = this.Multiselect
            };

            var dialogResult = owner != null ? dialog.ShowDialog(owner) : dialog.ShowDialog();

            if (!dialogResult.HasValue || !dialogResult.Value)
            {
                return null;
            }

            return dialog.FileNames;
        }

        public string[] ShowDialog()
        {
            return this.ShowDialog(null);
        }
    }
}

namespace MyApp.Windows.Services
{
    using System;
    using System.Collections.Generic;
    using Prism.Interactivity.InteractionRequest;

    public class OpenFileDialogInteractionContext : Confirmation
    {
        public IEnumerable<string> Filenames { get; set; }
    }
}

namespace MyApp.Windows.Services
{
    using System;
    using System.Windows;
    using System.Windows.Interactivity;
    using Prism.Interactivity.InteractionRequest;

    public class PopupOpenFileDialogAction : TriggerAction<DependencyObject>
    {
        public static readonly DependencyProperty DialogServiceProperty = DependencyProperty.Register(
          nameof(DialogService), typeof(OpenFileDialogService), typeof(PopupOpenFileDialogAction), new PropertyMetadata());

        public static readonly DependencyProperty OwnerProperty = DependencyProperty.Register(
          nameof(Owner), typeof(Window), typeof(PopupOpenFileDialogAction), new PropertyMetadata());

        public OpenFileDialogService DialogService
        {
            get { return (OpenFileDialogService)this.GetValue(PopupOpenFileDialogAction.DialogServiceProperty); }
            set { this.SetValue(PopupOpenFileDialogAction.DialogServiceProperty, value); }
        }

        public Window Owner
        {
            get { return (Window)this.GetValue(PopupOpenFileDialogAction.OwnerProperty); }
            set { this.SetValue(PopupOpenFileDialogAction.OwnerProperty, value); }
        }

        protected override void Invoke(object parameter)
        {
            if (this.DialogService == null)
            {
                return;
            }

            var interaction = (InteractionRequestedEventArgs)parameter;
            var context = (OpenFileDialogInteractionContext)interaction.Context;

            context.Filenames = this.Owner !=null? this.DialogService.ShowDialog(this.Owner) : this.DialogService.ShowDialog();
            context.Confirmed = context.Filenames != null;

            interaction.Callback?.Invoke();
        }
    }
}