WPF

1. Read the question carefully
2. Understand that English isn't everyone's first language so be lenient of bad spelling and grammar
3. If a question is poorly phrased then either ask for clarification, ignore it, or mark it down. Insults are not welcome
4. If the question is inappropriate then click the 'vote to remove message' button

1. Choose the correct forum for your message. Posting a VB.NET question in the C++ forum will end in tears.
   
   
2. Be specific! Don't ask "can someone send me the code to create an application that does 'X'. Pinpoint exactly what it is you need help with.
   
   
3. Keep the subject line brief, but descriptive. eg "File Serialization problem"
   
   
4. Keep the question as brief as possible. If you have to include code, include the smallest snippet of code you can.
   
   
5. Be careful when including code that you haven't made a typo. Typing mistakes can become the focal point instead of the actual question you asked.
   
   
6. Do not remove or empty a message if others have replied. Keep the thread intact and available for others to search and read. If your problem was answered then edit your message and add "[Solved]" to the subject line of the original post, and cast an approval vote to the one or several answers that really helped you.
   
   
7. If you are posting source code with your question, place it inside <pre></pre> tags. We advise you also check the "Encode "<" (and other HTML) characters when pasting" checkbox before pasting anything inside the PRE block, and make sure "Use HTML in this post" check box is checked.
   
   
8. Be courteous and DON'T SHOUT. Everyone here helps because they enjoy helping others, not because it's their job.
   
   
9. Please do not post links to your question into an unrelated forum such as the lounge. It will be deleted. Likewise, do not post the same question in more than one forum.
   
   
10. Do not be abusive, offensive, inappropriate or harass anyone on the boards. Doing so will get you kicked off and banned. Play nice.
    
    
11. If you have a school or university assignment, assume that your teacher or lecturer is also reading these forums.
    
    
12. No advertising or soliciting.
    
    
13. We reserve the right to move your posts to a more appropriate forum or to delete anything deemed inappropriate or illegal.

<i:Interaction.Triggers>
    <i:EventTrigger EventName="AddingNewItem">
        <i:InvokeCommandAction Command="{Binding DataContext.NewItemAddedCommand, 
            RelativeSource={
                RelativeSource FindAncestor, AncestorType=Window}}"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

<DataGrid Grid.Row="2"
          Grid.Column="0"
          ItemsSource="{Binding ColumnInfos}"
          HorizontalGridLinesBrush="LightGray"
          VerticalGridLinesBrush="LightGray"
          AutoGenerateColumns="False"
          CanUserAddRows="True"
          CanUserDeleteRows="True"
          CanUserReorderColumns="False"
          CanUserResizeColumns="False"
          CanUserSortColumns="False"
          BorderBrush="SteelBlue"
          BorderThickness="1"
          HeadersVisibility="Column"
          HorizontalScrollBarVisibility="Auto"
          VerticalScrollBarVisibility="Auto"
          VerticalAlignment="Stretch"
          Margin="5,0,5,5">

    <DataGrid.Resources>
        <local:BindingProxy x:Key="proxy" Data="{Binding IsColumnWidthVisibile}"/>
    </DataGrid.Resources>

    <DataGrid.Columns>

        <DataGridTemplateColumn>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Button Command="{Binding DeleteColumnCommand, RelativeSource={RelativeSource FindAncestor,
                                                                    AncestorLevel=1, AncestorType={x:Type Window}}}"
                            CommandParameter="{Binding}"
                            Margin="2"
                            Height="18"
                            Width="18"/>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>

        <DataGridTextColumn Header="Column Name"
                            Binding="{Binding ColumnName}"
                            Width="*"/>

        <DataGridTextColumn Header="Width"
                            Binding="{Binding ColumnWidth}"
                            Width="65"
                            Visibility="{Binding Data, Converter={StaticResource boolToVisibilityConverter}, Source={StaticResource proxy}}"/>

    </DataGrid.Columns>

</DataGrid>

System.InvalidCastException: 'Unable to cast object of type 'MS.Internal.NamedObject' to type 'WPFDataGrid.ColumnInfoEntity'.'

<pre><ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:MyTextBoxEx">

<pre>
<SolidColorBrush x:Key="CaptionNormalColor" Color="Green"/>
<SolidColorBrush x:Key="CaptionMouseOverColor" Color="Purple"/>

<SolidColorBrush x:Key="TextNormalColor" Color="Red"/>
<SolidColorBrush x:Key="TextMouseOverColor" Color="Blue"/>

<Style TargetType="{x:Type local:TextBoxEx}">

    <Setter Property="Padding" Value="1"/>

    <Setter Property="Template">

        <Setter.Value>

            <ControlTemplate TargetType="{x:Type local:TextBoxEx}">

                <StackPanel Orientation="Vertical">

                    <Border Padding="{Binding Padding, RelativeSource={RelativeSource TemplatedParent}}"
                            Margin="{Binding Margin, RelativeSource={RelativeSource TemplatedParent}}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}">

                        <StackPanel Orientation="Vertical">

                            <TextBlock x:Name="caption"
                                       Foreground="{StaticResource CaptionNormalColor}"
                                       Text="{TemplateBinding Caption}"/>

                            <TextBox x:Name="text"
                                     Foreground="{StaticResource TextNormalColor}"
                                     Text="{TemplateBinding Text}"/>

                        </StackPanel>

                    </Border>

                </StackPanel>

                <ControlTemplate.Triggers>

                    <Trigger Property="IsMouseOver" Value="true">
                        <Setter TargetName="caption" Property="TextElement.Foreground" Value="{StaticResource CaptionMouseOverColor}"/>
                        <Setter TargetName="caption" Property="TextElement.FontWeight" Value="Bold"/>

                        <Setter TargetName="text" Property="TextElement.Foreground" Value="{StaticResource TextMouseOverColor}"/>
                        <Setter TargetName="text" Property="TextElement.FontStyle" Value="Italic"/>
                    </Trigger>

                </ControlTemplate.Triggers>

            </ControlTemplate>

        </Setter.Value>

    </Setter>

</Style>

public class _ViewModelBase : BindableBase, INotifyDataErrorInfo
{
    #region Events
    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
    #endregion

    #region Private Fields
    public readonly Dictionary<string, List<string>> _validationErrors = new Dictionary<string, List<string>>();
    #endregion

    #region Properties
    public bool HasErrors => _validationErrors.Any();
    #endregion

    #region Public Methods
    public IEnumerable GetErrors(string propertyName)
    {
        return _validationErrors.ContainsKey(propertyName) ? _validationErrors[propertyName] : null;
    }
    #endregion

    #region Protected Methods
    protected void AddError(string propertyName, string error)
    {
        if (!_validationErrors.ContainsKey(propertyName))
        {
            _validationErrors[propertyName] = new List<string>();
        }

        if (!_validationErrors[propertyName].Contains(error))
        {
            _validationErrors[propertyName].Add(error);
            RaiseErrorsChanged(propertyName);
        }
    }

    protected void ClearErrors(string propertyName)
    {
        if (_validationErrors.ContainsKey(propertyName))
        {
            _validationErrors.Remove(propertyName);
            {
                RaiseErrorsChanged(propertyName);
            }
        }
    }
    #endregion

    #region Private Methods
    private void RaiseErrorsChanged(string propertyName)
    {
        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
    }
    #endregion
}

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

    <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>

</TextBox>

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

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

}

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

    Validate();
    RaiseChangedEvent();
}

1. How would you validate that a list has items in it?
2. I have 2 comboboxes with the same collection of items in it. How can I validate that they both have differt items selected?

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:Marois.Framework.WPF.Controls">

<pre>
<SolidColorBrush x:Key="disabledBackgroundBrush" Color="Gray"/>

<Style TargetType="{x:Type local:MaroisFilePicker}">

    <Setter Property="Template">

        <Setter.Value>

            <ControlTemplate>

                <Border x:Name="border"
                        Background="{TemplateBinding Background}" 
                        BorderBrush="{TemplateBinding BorderBrush}" 
                        BorderThickness="{TemplateBinding BorderThickness}" 
                        CornerRadius="{TemplateBinding BorderThickness}">

                    <Grid>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>

                        <TextBlock Text="{Binding Caption}"
                                   x:Name="caption"
                                   Margin="2"/>

                        <TextBox Grid.Column="1"
                                 x:Name="textbox"
                                 Text="{Binding File}"/>

                        <Button Grid.Column="2"
                                x:Name="button"
                                Command="{Binding SelectedFileCommand}"/>

                    </Grid>

                </Border>

                <ControlTemplate.Triggers>
                    <Trigger Property="IsEnabled" Value="False">
                        <Setter TargetName="button" Property="Background" Value="{StaticResource disabledBackgroundBrush}" />
                    </Trigger>

                </ControlTemplate.Triggers>
            </ControlTemplate>

        </Setter.Value>

    </Setter>

</Style>