basic design

.gitignore

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+bin/
+obj/
+.idea/
+.vs/
+tmp/
+temp/
+packages/
+*.user

SharpCalculator.Avalonia/App.axaml

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+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             x:Class="SharpCalculator.Avalonia.App"
+             RequestedThemeVariant="Dark">
+    <Application.Styles>
+        <SimpleTheme />
+    </Application.Styles>
+</Application>

SharpCalculator.Avalonia/App.axaml.cs

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+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+
+namespace SharpCalculator.Avalonia;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.MainWindow = new MainWindow();
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}

SharpCalculator.Avalonia/MainWindow.axaml

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+<Window xmlns="https://github.com/avaloniaui"
+        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"
+        mc:Ignorable="d" Width="400" Height="500"
+        x:Class="SharpCalculator.Avalonia.MainWindow"
+        Title="SharpCalculator"
+        FontFamily="Consolas"
+        FontSize="24">
+    <Grid Margin="16">
+        <Grid.RowDefinitions>4* 1* 4*</Grid.RowDefinitions>
+        <TextBox Name="Input" IsReadOnly="True"></TextBox>
+            <TextBlock Name="Output" Grid.Row="1" VerticalAlignment="Center">= 0</TextBlock>
+        <Grid Grid.Row="2">
+            <Grid.ColumnDefinitions> * * * * </Grid.ColumnDefinitions>
+            <Grid.RowDefinitions> * * * * * </Grid.RowDefinitions>
+            <Button Grid.Row="1" Grid.Column="0" Name="SevenButton" Click="NumberButton_OnClick">7</Button>
+            <Button Grid.Row="1" Grid.Column="1" Name="EightButton" Click="NumberButton_OnClick">8</Button>
+            <Button Grid.Row="1" Grid.Column="2" Name="NineButton" Click="NumberButton_OnClick">9</Button>
+            
+            <Button Grid.Row="2" Grid.Column="0" Name="FourButton" Click="NumberButton_OnClick">4</Button>
+            <Button Grid.Row="2" Grid.Column="1" Name="FiveButton" Click="NumberButton_OnClick">5</Button>
+            <Button Grid.Row="2" Grid.Column="2" Name="SixButton" Click="NumberButton_OnClick">6</Button>
+            
+            <Button Grid.Row="3" Grid.Column="0" Name="OneButton" Click="NumberButton_OnClick">1</Button>
+            <Button Grid.Row="3" Grid.Column="1" Name="TwoButton" Click="NumberButton_OnClick">2</Button>
+            <Button Grid.Row="3" Grid.Column="2" Name="ThreeButton" Click="NumberButton_OnClick">3</Button>
+            
+            <Button Grid.Row="4" Grid.Column="0" Name="ZeroButton" Click="NumberButton_OnClick">0</Button>
+            <Button Grid.Row="4" Grid.Column="1" Name="PointButton" Click="NumberButton_OnClick">.</Button>
+            <Button Grid.Row="4" Grid.Column="2" Name="PowerButton" Click="OperationButton_OnClick">^</Button>
+            
+            <Button Grid.Row="1" Grid.Column="3" Name="DivideButton" Click="OperationButton_OnClick">/</Button>
+            <Button Grid.Row="2" Grid.Column="3" Name="MultiplyButton" Click="OperationButton_OnClick">*</Button>
+            <Button Grid.Row="3" Grid.Column="3" Name="SubstractButton" Click="OperationButton_OnClick">-</Button>
+            <Button Grid.Row="4" Grid.Column="3" Name="AddButton" Click="OperationButton_OnClick">+</Button>
+            
+            <Button Grid.Row="0" Grid.Column="3" Name="EqualButton"  Click="OperationButton_OnClick">=</Button>
+            <Button Grid.Row="0" Grid.Column="2" Name="ClearButton"  Click="ClearButton_OnClick">CLR</Button>
+        </Grid>
+    </Grid>
+</Window>

SharpCalculator.Avalonia/MainWindow.axaml.cs

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+using System;
+using System.Globalization;
+using System.Text;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+
+namespace SharpCalculator.Avalonia;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+
+    private double _rezult;
+    private StringBuilder currentNumberB = new();
+
+    private double CompleteCurrentNumber()
+    {
+        var n = double.Parse(currentNumberB.ToString(), CultureInfo.InvariantCulture);
+        currentNumberB.Clear();
+        return n;
+    }
+
+    private void UpdateOutput()
+    {
+        Output.Text = "= " + _rezult;
+    }
+
+    private Func<double, double, double> prevOperation = (_, firstN) => firstN;
+    
+    private void TryUpdateRezult(Func<double, double, double> operation, string text)
+    {
+        if (currentNumberB.Length > 0)
+        {
+            _rezult = prevOperation(_rezult, CompleteCurrentNumber());
+            prevOperation = operation;
+            Input.Text += " " + text + " ";
+        }
+
+        UpdateOutput();
+    }
+
+    private void NumberButton_OnClick(object? sender, RoutedEventArgs e)
+    {
+        if (sender is not Button button)
+            throw new Exception();
+        string text = button.Content?.ToString()!;
+        Input.Text += text;
+        currentNumberB.Append(text);
+    }
+
+    private void OperationButton_OnClick(object? sender, RoutedEventArgs e)
+    {
+        if (sender is not Button button)
+            throw new Exception();
+        string text = button.Content?.ToString()!;
+        switch (text)
+        {
+            case "+":
+                TryUpdateRezult((a, b) => a + b, text);
+                break;
+            case "-":
+                TryUpdateRezult((a, b) => a - b, text);
+                break;
+            case "*":
+                TryUpdateRezult((a, b) => a * b, text);
+                break;
+            case "/":
+                TryUpdateRezult((a, b) => a / b, text);
+                break;
+            case "^":
+                TryUpdateRezult(Math.Pow, text);
+                break;
+            case "=":
+                TryUpdateRezult((a, _) => a, "");
+                break;
+            default:
+                throw new Exception("incorrect button text: " + text);
+        }
+    }
+
+    private void ClearButton_OnClick(object? sender, RoutedEventArgs e)
+    {
+        _rezult = 0;
+        UpdateOutput();
+        Input.Text = "";
+    }
+}

SharpCalculator.Avalonia/Program.cs

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+using Avalonia;
+using System;
+
+namespace SharpCalculator.Avalonia;
+
+class Program
+{
+    // Initialization code. Don't use any Avalonia, third-party APIs or any
+    // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+    // yet and stuff might break.
+    [STAThread]
+    public static void Main(string[] args) => BuildAvaloniaApp()
+        .StartWithClassicDesktopLifetime(args);
+
+    // Avalonia configuration, don't remove; also used by visual designer.
+    private static AppBuilder BuildAvaloniaApp()
+        => AppBuilder.Configure<App>()
+            .UsePlatformDetect()
+            .LogToTrace();
+}

SharpCalculator.Avalonia/SharpCalculator.Avalonia.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+    <PropertyGroup>
+        <OutputType>WinExe</OutputType>
+        <TargetFramework>net8.0</TargetFramework>
+        <LangVersion>12</LangVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>disable</ImplicitUsings>
+        <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
+        <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
+    </PropertyGroup>
+    
+    <ItemGroup>
+        <PackageReference Include="Avalonia" Version="11.0.5"/>
+        <PackageReference Include="Avalonia.Desktop" Version="11.0.5"/>
+        <PackageReference Condition="'$(Configuration)' == 'Debug'" Include="Avalonia.Diagnostics" Version="11.0.5"/>
+        <PackageReference Include="Avalonia.Themes.Simple" Version="11.0.5" />
+    </ItemGroup>
+</Project>

SharpCalculator.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SharpCalculator.Avalonia", "SharpCalculator.Avalonia\SharpCalculator.Avalonia.csproj", "{93E7D002-22D7-4C4F-AE6B-F1E0D938BCBA}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{93E7D002-22D7-4C4F-AE6B-F1E0D938BCBA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{93E7D002-22D7-4C4F-AE6B-F1E0D938BCBA}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{93E7D002-22D7-4C4F-AE6B-F1E0D938BCBA}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{93E7D002-22D7-4C4F-AE6B-F1E0D938BCBA}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal

nuget.config

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+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+    <packageSources>
+        <add key="local" value="./packages/" />
+    </packageSources>
+</configuration>