Add application files

App.xaml

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+<Application x:Class="DotNetVersionCheck.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:DotNetVersionCheck"
+             Startup="Application_Startup">
+    <Application.Resources></Application.Resources>
+</Application>

App.xaml.cs

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+using System;
+using System.CommandLine;
+using System.CommandLine.Invocation;
+using System.Linq;
+using System.Windows;
+
+namespace DotNetVersionCheck
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+        private VersionInfo VersionInfo { get; set; } = new VersionInfo();
+
+        private void Application_Startup(object sender, StartupEventArgs e)
+        {
+            var rootCommand = new RootCommand(description: "Reports .NET runtime version information and triggers a prompt to install .NET Core runtime if applicable")
+            {
+                new Option(new[] { "--no-window" }, "Don't display window, write version information to console"),
+                new Option(new[] { "--silent" }, "Don't display window or write to console. Useful to only trigger .NET Core install if necessary")
+            };
+
+            rootCommand.Handler = CommandHandler.Create<bool, bool>(RootCommand);
+            int result = rootCommand.Invoke(e.Args);
+
+            if (result == 1 || e.Args.Any(a => a.Contains("--help") || a.Contains("-h") || a.Contains("--version")))
+            {
+                Shutdown();
+            }
+            else
+            {
+                var wnd = new MainWindow() { VersionInfo = VersionInfo };
+                wnd.Show();
+            }
+        }
+
+        private int RootCommand(bool noWindow, bool silent)
+        {
+            if (noWindow)
+            {
+                Console.WriteLine($".NET Runtime Version: {VersionInfo.RuntimeVersion}");
+                Console.WriteLine($".NET Target Version: {VersionInfo.TargetFramework}");
+            }
+
+            if (noWindow || silent)
+                return 1;
+
+            return 0;
+        }
+    }
+}

AssemblyInfo.cs

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+using System.Windows;
+
+[assembly:ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+                                     //(used if a resource is not found in the page,
+                                     // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+                                              //(used if a resource is not found in the page,
+                                              // app, or any theme specific resource dictionaries)
+)]

MainWindow.xaml

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+<Window x:Class="DotNetVersionCheck.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:DotNetVersionCheck"
+        mc:Ignorable="d"
+        Title=".NET Version Check" Height="200" Width="600"
+        ResizeMode="NoResize"
+        Loaded="Window_Loaded" WindowStartupLocation="CenterOwner" WindowStyle="SingleBorderWindow">
+    <Window.Resources>
+        <Style x:Key="labelStyle" TargetType="Label">
+            <Setter Property="FontSize" Value="16"/>
+            <Setter Property="Margin" Value="10,0,10,0"/>
+            <Setter Property="VerticalAlignment" Value="Center"/>
+        </Style>
+        <Style x:Key="buttonStyle" TargetType="Button">
+            <Setter Property="Height" Value="25"/>
+            <Setter Property="Width" Value="80"/>
+            <Setter Property="HorizontalAlignment" Value="Right"/>
+            <Setter Property="VerticalAlignment" Value="Center"/>
+            <Setter Property="Margin" Value="0,0,15,10"/>
+        </Style>
+    </Window.Resources>
+    <Grid>
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition Width="3*"/>
+            <ColumnDefinition Width="5*"/>
+        </Grid.ColumnDefinitions>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="5*"/>
+            <RowDefinition Height="5*"/>
+            <RowDefinition Height="4*"/>
+        </Grid.RowDefinitions>
+        <Label Content="Running .NET Version:" Grid.Column="0" Style="{StaticResource labelStyle}" />
+        <Label x:Name="labelRuntimeVersion" Content="[version]" Grid.Column="1" Grid.Row="0" Style="{StaticResource labelStyle}" />
+        <Label Content="Target .NET Version:" Grid.Column="0" Grid.Row="1" Style="{StaticResource labelStyle}" />
+        <Label x:Name="labelTargetVersion" Content="[version]" Grid.Column="1" Grid.Row="1" Style="{StaticResource labelStyle}" />
+        <Button x:Name="closeButton" Content="_Close" Grid.Column="1" Grid.Row="2" Style="{StaticResource buttonStyle}" Click="CloseButton_Click" />
+    </Grid>
+</Window>

MainWindow.xaml.cs

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+using System.Windows;
+
+namespace DotNetVersionCheck
+{
+    /// <summary>
+    /// Interaction logic for MainWindow.xaml
+    /// </summary>
+    public partial class MainWindow : Window
+    {
+        public MainWindow()
+        {
+            InitializeComponent();
+        }
+
+        public VersionInfo VersionInfo { get; set; }
+
+        private void Window_Loaded(object sender, RoutedEventArgs e)
+        {
+            labelRuntimeVersion.Content = VersionInfo?.RuntimeVersion;
+            labelTargetVersion.Content = VersionInfo?.TargetFramework;
+        }
+
+        private void CloseButton_Click(object sender, RoutedEventArgs e)
+        {
+            Close();
+        }
+    }
+}

VersionInfo.cs

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+using System;
+using System.Reflection;
+using System.Runtime.Versioning;
+
+namespace DotNetVersionCheck
+{
+    public class VersionInfo
+    {
+        private string _targetFramework;
+        private string _runtimeVersion;
+
+        public string TargetFramework
+        {
+            get
+            { 
+                if (_targetFramework == null)
+                {
+                    _targetFramework = Assembly
+                        .GetEntryAssembly()?
+                        .GetCustomAttribute<TargetFrameworkAttribute>()?
+                        .FrameworkName;
+                }
+
+                return _targetFramework;
+            }
+        }
+        public string RuntimeVersion
+        {
+            get
+            {
+                if (_runtimeVersion == null)
+                    _runtimeVersion = Environment.Version.ToString();
+
+                return _runtimeVersion;
+            }
+        }
+    }
+}

dotnet-version-check.csproj

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+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
+
+  <PropertyGroup>
+    <!-- Needs to be WinExe to trigger install prompt for .NET Core. This does prevent console output though so have to work around that -->
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <RootNamespace>DotNetVersionCheck</RootNamespace>
+    <UseWPF>true</UseWPF>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="System.CommandLine" Version="0.3.0-alpha.20054.1" />
+  </ItemGroup>
+
+</Project>

dotnet-version-check.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.29613.14
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "dotnet-version-check", "dotnet-version-check.csproj", "{56EFF347-3480-4868-B13B-C7C275940DF0}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{56EFF347-3480-4868-B13B-C7C275940DF0}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{56EFF347-3480-4868-B13B-C7C275940DF0}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{56EFF347-3480-4868-B13B-C7C275940DF0}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{56EFF347-3480-4868-B13B-C7C275940DF0}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {5F06B3CB-EA97-4662-A68D-216BE3E8B977}
+	EndGlobalSection
+EndGlobal