What Are xmlns and xmlns:x in WPF? Understanding XML Namespace Declarations

In WPF (Windows Presentation Foundation), xmlns and xmlns:x are XML namespace declarations used in XAML files. They define the scope of XML namespaces, which are used to distinguish between elements and attributes that might have the same name but are used in different contexts. xmlns : The Default XML Namespace Definition : xmlns stands for XML namespace. It is used to declare the default namespace for the elements in the XAML file. Purpose : In WPF, it typically maps to the .NET namespaces that contain the classes and controls used in WPF applications. This default namespace allows you to use the WPF controls and elements without prefixing them. Example : <Window x:Class="WpfApp.MainWindow"    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"    ... > <Grid>        <Button Content="Click Me"/>     </Grid> </Window> Here, xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" sets...

Validation By Exception in Wpf using MVVM

View

<Window x:Class="WpfPrismTutorial.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfPrismTutorial"
        mc:Ignorable="d"
        prism:ViewModelLocator.AutoWireViewModel="True"
        xmlns:prism="http://prismlibrary.com/"
        xmlns:valid="clr-namespace:WpfPrismTutorial.Validations"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <ControlTemplate x:Key="errorTemplate">
            <StackPanel Orientation="Horizontal">
            <AdornedElementPlaceholder x:Name="ErrorHolder" >
                <Border BorderBrush="red" BorderThickness="1" />
            </AdornedElementPlaceholder>
            <TextBlock  Text="{Binding [0].ErrorContent}" VerticalAlignment="Center" HorizontalAlignment="Right" Foreground="Red"/>
        </StackPanel>

        </ControlTemplate>
    </Window.Resources>
    <Grid VerticalAlignment="Center" HorizontalAlignment="Left">
     
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Validation by Exception" Margin="5,5"/>
            <TextBox Text="{Binding MainWindowModel.ValidationByException,UpdateSourceTrigger=PropertyChanged,
                        ValidatesOnExceptions=True}" Validation.ErrorTemplate="{StaticResource errorTemplate}"  Height="30" Width="300"/>
        </StackPanel>
     
    </Grid>
</Window>


ViewModel 

 public class MainWindowViewModel : BindableBase
    {
        private MainWindowModel mainWindowModel = new MainWindowModel();

        public MainWindowModel MainWindowModel
        {
            get { return mainWindowModel; }
            set { SetProperty(ref mainWindowModel, value); }
        }
    }


Model

 public class MainWindowModel : BindableBase
    {
        #region Validation By Exception

        private string _validationByException = string.Empty;

        public string ValidationByException
        {
            get { return _validationByException; }
            set
            {
                if (string.IsNullOrEmpty(value))
                    throw new Exception("Validation By Exception should not empty");
                SetProperty(ref _validationByException, value);
            }
        }
        #endregion


    }

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