C＃SQL：无法启用约束。 一或多个行包含违反非空，唯一或外键约束的值

Question

I'm new to SQL in C# (and SQL in general).

I'm making this simple users management program (which is part of a bigger thing), where you have the option to register a new account, and view all the SQL Data Base content in a DataGrid.

I can add users, (Read/write from the DB) but for some reason, when I load the WPF page with the DataGrid, it crashed when it tries to load it and gives the error:

Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.

I tried to google an answer, but I couldn't find one, or understand what should I do.

this is my C# code:

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Data;
    using System.Windows.Documents;
    using System.Windows.Input;
    using System.Windows.Media;
    using System.Windows.Media.Imaging;
    using System.Windows.Shapes;
    //SQL
    using System.Configuration;
    using System.Data.SqlClient;
    using System.Data;
    namespace FMS_Csharp_GUI
    {
    /// <summary>
    /// Interaction logic for UsersBD.xaml
    /// </summary>
   public partial class UsersBD : Window
   {
        SqlConnection connection = new SqlConnection();
        string sqlConnectionString;
        public UsersBD()
    {
        InitializeComponent();
        sqlConnectionString = ConfigurationManager.ConnectionStrings["FMS_Csharp_GUI.Properties.Settings.UsersConnectionString"].ConnectionString;
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {

        FMS_Csharp_GUI.UsersDataSet usersDataSet = ((FMS_Csharp_GUI.UsersDataSet)(this.FindResource("usersDataSet")));
        // Load data into the table UsersTable. You can modify this code as needed.
        FMS_Csharp_GUI.UsersDataSetTableAdapters.UsersTableTableAdapter usersDataSetUsersTableTableAdapter = new FMS_Csharp_GUI.UsersDataSetTableAdapters.UsersTableTableAdapter();
  /*code crash in this line: */      usersDataSetUsersTableTableAdapter.Fill(usersDataSet.UsersTable); 
        System.Windows.Data.CollectionViewSource usersTableViewSource = ((System.Windows.Data.CollectionViewSource)(this.FindResource("usersTableViewSource")));
        usersTableViewSource.View.MoveCurrentToFirst();
    }

    private void btnAddUser_Click(object sender, RoutedEventArgs e)
    {

        string query = "INSERT INTO UsersTable VALUES (@username, @password, @IsAdmin);";

        //get input
        string username = txtUsername.Text;
        string password = txtPassword.Text;
        bool admin = chckAdmin.IsChecked == true;

        using(connection = new SqlConnection(sqlConnectionString))
        using(SqlCommand command = new SqlCommand(query,connection)) {

            connection.Open();

            command.Parameters.AddWithValue("@username", username);
            command.Parameters.AddWithValue("@password", password);
            command.Parameters.AddWithValue("@IsAdmin", admin);

            command.ExecuteScalar();
        }

    }
}
}

this is my SQL DB code:

CREATE TABLE [dbo].[UsersTable] (
[Id]          INT        IDENTITY (1, 1) NOT NULL,
[Name]        NCHAR (15) NOT NULL,
[PasswordMD5] NCHAR (32) NOT NULL,
[IsAdmin]     BIT        DEFAULT ((0)) NOT NULL,
PRIMARY KEY CLUSTERED ([Id] ASC)
);

this is my data base:

Thanks a lot! PS FMS_Csharp_GUI is the name of the program.

Answer 1

For some reason I don't konw it didn't work. but now it does, after I tried to do this :

            using (connection = new SqlConnection(sqlConnectionString))
        using (SqlDataAdapter adapter = new SqlDataAdapter("SELECT ID,Name,PasswordSHA256, IsAdmin"
            + " From UsersTable", connection))
        {
            //get data
            DataTable user = new DataTable();
            adapter.Fill(user);
            usersTableDataGrid.DataContext = user;
        }

then I highlighted this code, and used the one above, and it worked!

maybe getting it this why made it work?