无法创建 ViewModel 类型的实例

我是 MVVM 和 WPF 的新手。

我创建了一个简单的加法应用程序，它获取两个数字作为输入并在数据库中添加给定的数字并在文本框中提供结果。

该应用程序运行良好。

但是 windows.xaml 抛出以下错误

无法创建 ViewModel 类型的实例

视窗.xaml

<Window x:Class="addition.Window1"
         xmlns:vm="clr-namespace:addition.ViewModel" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Window.DataContext>
        <vm:ViewModel/>
    </Window.DataContext>
    <Grid>

        <Label Height="28" Margin="28,54,0,0" Name="Number1" VerticalAlignment="Top" HorizontalAlignment="Left" Width="48">Number</Label>
        <TextBox Height="28" Margin="112,56,46,0"  Text ="{Binding Path = FirstArgument}"   Name="textBox1" VerticalAlignment="Top" />
        <Label Margin="28,106,0,128" Name="Number2" Width="58" HorizontalAlignment="Left">Number1</Label>
        <TextBox Height="28" Margin="112,120,46,120" Text ="{Binding  Path = secondargument}" Name="textBox2" />
        <Label Height="28" Margin="28,0,0,75" Name="label1" VerticalAlignment="Bottom" HorizontalAlignment="Left" Width="58">Number2</Label>
        <TextBox Height="23" Margin="112,0,46,68" Name="textBox3" Text="{Binding Path = Addedargument}" VerticalAlignment="Bottom" />
        <Button Height="23"  HorizontalAlignment="Left" Margin="39,0,0,16" Name="button1" VerticalAlignment="Bottom" Width="75" Command="{Binding AddNew}">Button</Button>
    </Grid>
</Window>

当我在视图模型中实例化数据库连接类时发生错误。当我注释掉 databaseconnetion 类时，问题得到解决。

视图模型.cs：

using addition.Model;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel.Design;
using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
using System.ComponentModel;

namespace addition.ViewModel
{
    class ViewModel : INotifyPropertyChanged
    {
        private Number n1 = new Number();
        int num, num1;
//The issue is resolved when i comment the below instantiation

        databaseconnection d1 = new databaseconnection();

        public RelayCommand AddNew { get; set; }

        private string _number1;

        public string FirstArgument
        {

            get { return this._number1; }
            set
            {
                this._number1 = value;
                if (int.TryParse(_number1.ToString(), out num))
                {
                    this.n1.number1 = num;
                    this.OnPropertyChanged("FirstArgument");

                }
                else { MessageBox.Show("The given Value is not a Number "); }

            }
        }
        private string _number2;

        public string secondargument
        {
            get { return this._number2; }

            set
            {
                this._number2 = value;
                if (int.TryParse(_number2.ToString(), out num1))
                {
                    this.n1.number2 = num1;
                    this.OnPropertyChanged("secondargument");
                }
                else { MessageBox.Show("The given Value is not a Number "); }

            }
        }

        private string _number3;

        public string Addedargument
        {
            get { return this._number3; }
            set
            {
                this._number3 = value;
                this.OnPropertyChanged("Addedargument");
            }
        }

        public ViewModel()
        {

                AddNew = new RelayCommand(o => AddNumbers());

        }


        public void AddNumbers()
        {

                d1.data(this);


        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
        #endregion

    }


    class databaseconnection
    {
        static string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MYConnectionString"].ConnectionString;
        public void data(ViewModel m1)
        {
            string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MYConnectionString"].ConnectionString;

//The query is prone to sql injection 

        string sql = "SELECT " + "( cast( " + m1.FirstArgument +" as int) + " + "cast( " + m1.secondargument + " as int) )" + " as Addedargument";
            // MessageBox.Show(sql);
            DataSet ds = new DataSet();
            using (var connection = new SqlConnection(connectionString))
            {
                using (var command = new SqlCommand(sql, connection))
                {
                    SqlDataAdapter dataadapter = new SqlDataAdapter(command);
                    connection.Open();
                    using (SqlDataReader reader = command.ExecuteReader())
                    {

                        if (reader.Read())
                        {

                            m1.Addedargument = reader["Addedargument"].ToString();
                        }
                    }
                }
            }
        }
    }
}

问题：

• 由于我是通过 mvvm 接触 wpf 的新手，因此我不确定数据库连接是否应该发生在 ViewModel 或 Model 中。请让我知道我的方法是否正确。

• 为什么即使程序和逻辑在运行时运行良好，我在实例化类时也会收到错误。问题是由于错误，我无法设计应用程序。错误发生在

   <Window.DataContext>
         <vm:ViewModel/>
     </Window.DataContext>