AvaloniaTaskFive/Auth/ViewModels/CalcVM.cs

using Auth.Models;
using Avalonia.Media;
using Avalonia.Styling;
using ReactiveUI;
using System;
using System.Drawing;
using System.Security.Cryptography.X509Certificates;
using Avalonia.Controls;
using Avalonia.Controls.Shapes;
using System.Linq;
using Avalonia.Threading;
using System.Collections.Generic;
using System.ComponentModel;
using Avalonia.Input;
using System.Windows.Input;

namespace Auth.ViewModels
{
    public class CalcVM : ReactiveObject
    {
        public float? chislo1;
        public float? chislo2;
        public string result = "";
        public static List<string> _operation = Models.data.Operation;
        private string _selectedOperation = _operation[0];
        public string znaki = "+";

        public List<String> Operation
        {
            get
            {
                return _operation;
            }
        }

        public string Znaki
        {
            get
            {
                return znaki;
            }
            set
            {
                this.RaiseAndSetIfChanged(ref znaki, value);
            }
        }

        public string SelectedOperation
        {
            get => _selectedOperation;
            set
            {
                this.RaiseAndSetIfChanged(ref _selectedOperation, value);
                if (SelectedOperation == _operation[0])
                {
                    Znaki = "+";
                }
                else if (SelectedOperation == _operation[1])
                {
                    Znaki = "-";
                }
                else if (SelectedOperation == _operation[2])
                {
                    Znaki = "*";
                }
                else if (SelectedOperation == _operation[3])
                {
                    Znaki = "/";
                }
            }
        }

        public void Resultat()
        {
            switch (SelectedOperation)
            {
                case "Сложение":
                    Result = (Chislo1 + Chislo2).ToString();
                    break;
                case "Вычитание":
                    Result = (Chislo1 - Chislo2).ToString();
                    break;
                case "Умножение":
                    Result = (Chislo1 * Chislo2).ToString();
                    break;
                case "Деление":
                    Result = Chislo2 != 0 ? (Chislo1 / Chislo2).ToString() : float.NaN.ToString();
                    break;
            }
        }

        public string Result
        {
            get
            {
                return result;
            }
            set
            {
                this.RaiseAndSetIfChanged(ref result, value);
            }
        }

        public float? Chislo1
        {
            get => chislo1;
            set
            {              
                this.RaiseAndSetIfChanged(ref chislo1, value);
            }
        }

        public float? Chislo2
        {
            get => chislo2;
            set
            {
                this.RaiseAndSetIfChanged(ref chislo2, value);
            }
        }        
    }
}