WPF

private void Validate()
{
    ClearErrors("CustomerName");

    if (string.IsNullOrWhiteSpace(Customer.CustomerName))
    {
        AddError("CustomerName", "The Customer Name cannot be empty.");
    }
}

<TextBox Grid.Row="1" 
            Grid.Column="0"
            Text="{Binding Customer.CustomerName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"
            IsEnabled="{Binding AreFieldsEnabled}">

<pre>
<Validation.ErrorTemplate>
    <ControlTemplate>
        <StackPanel>
            <AdornedElementPlaceholder x:Name="textBox" />
            <ItemsControl ItemsSource="{Binding}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding ErrorContent}" Foreground="Red" />
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </StackPanel>
    </ControlTemplate>
</Validation.ErrorTemplate>

const string propertyName = $"{nameof(Customer)}.{nameof(Customer.CustomerName)}";

ClearErrors(propertyName);

f (string.IsNullOrWhiteSpace(Customer.CustomerName))
{
    AddError(propertyName, "The Customer Name cannot be empty.");
}

<TextBox Grid.Row="1" 
            Grid.Column="0"
            Text="{Binding Customer.CustomerName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnNotifyDataErrors=True}"
            IsEnabled="{Binding AreFieldsEnabled}">

<pre>
<Validation.ErrorTemplate>
    <ControlTemplate>
        <StackPanel>
            <AdornedElementPlaceholder/>
            <ItemsControl ItemsSource="{Binding}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding ErrorContent}" Foreground="Red" />
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </StackPanel>
    </ControlTemplate>
</Validation.ErrorTemplate>

private void Customer_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    IsChanged = true;

    ValidateCustomerName();
}

private void ValidateCustomerName()
{
    ClearErrors(nameof(Customer.CustomerName));

    if (string.IsNullOrWhiteSpace(Customer.CustomerName))
    {
        AddError(nameof(Customer.CustomerName), "The Customer Name cannot be empty.");
    }
}

Quote:

GetErrors method returns an IEnumerable that contains validation errors for the specified property (when the propertyName parameter isn’t equal to null or empty string) or for the entire entity (when the propertyName parameter is equal to null or empty string)

Evilfish2000 wrote:

How can I make the button work on my progress bar, and still have the IsHitTestVisible="False" on the ListView?

AppHost = Host.CreateDefaultBuilder()
                .ConfigureServices((hostContext, services) =>
                {
                    services.AddDbContext<MooseDBContext>(options =>
                    {
                        options.UseSqlServer(_connString);
                    });

                    services.AddSingleton<IEventAggregator, EventAggregator>();
                    services.AddSingleton<IApplicationSecurity, ApplicationSecurity>();
                    services.AddSingleton<MainWindowViewModel>();
                    services.AddSingleton<MainWindowView>();
                    services.AddSingleton<LoginViewModel>();
                    services.AddSingleton<LoginView>();
                })
                .Build();
}

// ApplicationSecurity assembly:
public interface IApplicationSecurityContext
{
    DbSet<YourEntityType> YourEntityTypes { get; }
}

public class ApplicationSecurity : IApplicationSecurity
{
    public ApplicationSecurity(IApplicationSecurityContext context)
    {
        ArgumentNullException.ThrowIfNull(context);
        Context = context;
    }
    
    private IApplicationSecurityContext Context { get; }
    ...
}

// MooseDBContext assembly:
public class MooseDBContext : DbContext, IApplicationSecurityContext
{
    ...
}

services.AddDbContext<MooseDBContext>(options =>
{
    options.UseSqlServer(_connString);
});

// When we request the IApplicationSecurityContext service,
// return the registered MooseDBContext service:
services.AddScoped<IApplicationSecurityContext>(sp => sp.GetRequiredService<MooseDBContext>());

public static IServiceCollection AddContextInterfaces<TContext>(this IServiceCollection services) where TContext : DbContext
{
    ArgumentNullException.ThrowIfNull(services);
    
    foreach (Type t in typeof(TContext).GetInterfaces().Except(typeof(DbContext).GetInterfaces()))
    {
        services.AddScoped(t, sp => sp.GetRequiredService<TContext>());
    }
    
    return services;
}

...

services.AddDbContext<MooseDBContext>(options =>
{
    options.UseSqlServer(_connString);
});

services.AddContextInterfaces<MooseDBContext>();