系统找不到，Xamarin.Forms 找不到

Question

Severity Code Description Project File Line Suppression State Error CS0246 The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) Beer Garden1 C:\Users\henry\source\repos\Beer Garden1\Beer Garden1\Beer Garden1\ViewModel\LoginViewModel.cs 3 Active

Severity Code Description Project File Line Suppression State Error CS0246 The type or namespace name 'Xamarin' could not be found (are you missing a using directive or an assembly reference?) Beer Garden1 C:\Users\henry\source\repos\Beer Garden1\Beer Garden1\Beer Garden1\ViewModel\LoginViewModel.cs 7 Active

I am currently getting the above errors and unsure as to why. I have spent hours looking through stack overflow and google and nothing is working,

Clean solution, rebuild, build reload has not worked.

Repairing visual studio has not worked

Im stumped please anything to help will be appreciated!

using Beer_Garden1.Services;
using Beer_Garden1.View;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text;
using Xamarin.Forms;

namespace Beer_Garden1.ViewModel
{
    public class LoginViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
    public Command cmdLogin { get; set;  }
    public Command cmdCreateAccount { get; set; }
    public Command cmdForgotPassword { get; set; }
    public Command cmdSetting { get; set; }

    ILoginService ilog = DependencyService.Get<ILoginService>();

    public LoginViewModel()
    {
        cmdLogin = new Command(gotoMainPage);
        cmdCreateAccount = new Command(gotoCreateAccount);
        cmdForgotPassword = new Command(gotoForgotPassword);
        cmdSetting = new Command(gotoSetting);
        
    }

    private void gotoSetting(object obj)
    {
        App.Current.MainPage.Navigation.PushAsync(new SettingPage());
    }

    private void gotoForgotPassword(object obj)
    {
        App.Current.MainPage.Navigation.PushAsync(new ForgotPasswordPage());
    }

    private void gotoCreateAccount(object obj)
    {
        App.Current.MainPage.Navigation.PushAsync(new CreateAccountPage());
    }

    private void gotoMainPage(object obj)
    {
        if (ilog.login(UserName, Password))
        {
            App.Current.MainPage.Navigation.PushAsync(new MainPage());
        }
        else
        {
            LoginMessage = "Please enter a valid user name and password!";
            TurnLoginMessage = true;
        }

    }
    //-------------------------------------------------------------
    private string userName;
    public string UserName
    { get 
        { 
            return userName; 
        } 
        set 
        { 
            userName = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("UserName"));
        } 
    }

    private string password;
    public string Password
    {
        get
        {
            return password;
        }
        set
        {
            password = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Password"));
        }
    }

    private string loginMessage;
    public string LoginMessage
    {
        get
        {
            return LoginMessage;
        } set
        {
            loginMessage = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("LoginMessage"));
        }
    }

    private bool turnLoginMessage = false;
    public bool TurnLoginMessage
    {
        get
        {
            return turnLoginMessage;
        }
        set
        {
            turnLoginMessage = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("TurnLoginMessage"));
        }
    }
}

}

_________________________________________________________________________________UPDATE::

I managed to fix the package source issue and it is now accepting xamarin.forms and system but is throwing erros under using System.Collections.Generic; using System.ComponentModel; using System.Text;

When restoring NuGet Packages I get the following error

NU1603: Beer Garden1.iOS depends on Xamarin.Forms (>= 4.8.0.1451) but Xamarin.Forms 4.8.0.1451 was not found. An approximate best match of Xamarin.Forms 5.0.0.2012 was resolved.
NU1102: Unable to find package System.Numerics.Vectors with version (>= 4.5.0)
  - Found 2 version(s) in Microsoft Visual Studio Offline Packages [ Nearest version: 4.1.1 ]
NU1603: Beer Garden1.Android depends on Xamarin.Essentials (>= 1.5.3.2) but Xamarin.Essentials 1.5.3.2 was not found. An approximate best match of Xamarin.Essentials 1.6.1 was resolved.
NU1603: Beer Garden1.Android depends on Xamarin.Forms (>= 4.8.0.1451) but Xamarin.Forms 4.8.0.1451 was not found. An approximate best match of Xamarin.Forms 5.0.0.2012 was resolved.
NU1102: Unable to find package System.Numerics.Vectors with version (>= 4.5.0)
  - Found 2 version(s) in Microsoft Visual Studio Offline Packages [ Nearest version: 4.1.1 ]
NuGet package restore failed. Please see Error List window for detailed warnings and errors.
Time Elapsed: 00:00:00.1797576

Answer 1

I fixed my issue in the end, I deleted the NuGet folder from %appdata% This was then recreated back with its defaults on VS startup and simply I had to run a NuGet Package restore and it removed all 98 of my errors and 13 of my warnings.

I hope this comes in handy for anyone running the same issue as it was hard for me to find a solution myself online.

Thank you everyone for your help!