Trigger validation on click of Search button

Question

I'm building an MVVM Light WPF app using Visual Studio 2015 Update 1. I have the following two search fields: cmbSearchColumn and txtSearchValue. Neither can be blank when the user clicks the Search button. Note that I've got ValidationRules set for both fields.

Here's the relevant XAML:

<TextBlock Grid.Row="1"
           Grid.Column="0"
           Style="{StaticResource FieldLabel}">
    Search Column
</TextBlock>
<StackPanel Grid.Row="1"
            Grid.Column="1"
            Style="{StaticResource ValidationStackPanel}">
    <ComboBox x:Name="cmbSearchColumn"
              DisplayMemberPath="MemberName"
              IsEditable="True"
              ItemsSource="{Binding SearchColumns}"
              SelectedValuePath="MemberValue"
              Style="{StaticResource ComboBoxStyle}">
        <ComboBox.SelectedItem>
            <Binding Mode="TwoWay"
                     Path="SelectedColumn}"
                     UpdateSourceTrigger="Explicit">
                <Binding.ValidationRules>
                    <helpers:NotEmptyStringValidationRule 
  Message="Search Column cannot be blank." ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </ComboBox.SelectedItem>
    </ComboBox>
    <TextBlock Style="{StaticResource FieldLabelError}" 
  Text="{Binding (Validation.Errors)[0].ErrorContent, ElementName=cmbSearchColumn}" />
</StackPanel>
<TextBlock Grid.Row="2"
           Grid.Column="0"
           Padding="0 0 9 9"
           Style="{StaticResource FieldLabel}">
    Search Value
</TextBlock>
<StackPanel Grid.Row="1"
            Grid.Column="1"
            Style="{StaticResource ValidationStackPanel}">
    <TextBox x:Name="txtSearchValue" Style="{StaticResource FieldTextBox}">
        <TextBox.Text>
            <Binding Mode="TwoWay"
                     Path="SearchValue"
                     UpdateSourceTrigger="Explicit">
                <Binding.ValidationRules>
                    <helpers:NotEmptyStringValidationRule 
  Message="Search Value cannot be blank." ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <TextBlock Style="{StaticResource FieldLabelError}" 
  Text="{Binding (Validation.Errors)[0].ErrorContent, ElementName=txtSearchValue}" />
</StackPanel>   
<Button Grid.Row="4"
    Grid.Column="1"
    Command="{Binding SearchEmployeesRelayCommand}"
    Content="Search"
    Style="{StaticResource FieldButton}" />

When the app loads, it immediately displays the error next to the fields, saying that they cannot be blank. However, I need to trigger the validation on them only when the user clicks the Search button.

How do I do this? Thanks.

Answer 1

You can use INotifyDataErrorInfo

Note that INotifyDataErrorInfoworks with custom rules added to the binding. The custom rule and the code for RelayCommand are not included in this answer.

Sample implementation:

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;

public class PropertyErrors : INotifyDataErrorInfo
{
    private static readonly IReadOnlyList<object> EmptyErrors = new object[0];
    private readonly Action<DataErrorsChangedEventArgs> ownerOnErrorsChanged;
    private readonly Type type;
    private readonly ConcurrentDictionary<string, List<object>> propertyErrors = new ConcurrentDictionary<string, List<object>>();

    public PropertyErrors(INotifyDataErrorInfo owner, Action<DataErrorsChangedEventArgs> ownerOnErrorsChanged)
    {
        this.ownerOnErrorsChanged = ownerOnErrorsChanged;
        this.type = owner.GetType();
    }

    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

    public bool HasErrors => this.propertyErrors.Count > 0;

    public IEnumerable GetErrors(string propertyName)
    {
        Debug.Assert(this.type.GetProperty(propertyName) != null, $"The type {this.type.Name} does not have a property named {propertyName}");
        List<object> errors;
        return this.propertyErrors.TryGetValue(propertyName, out errors)
            ? errors
            : EmptyErrors;
    }

    public void Add(string propertyName, object error)
    {
        Debug.Assert(this.type.GetProperty(propertyName) != null, $"The type {this.type.Name} does not have a property named {propertyName}");
        this.propertyErrors.AddOrUpdate(
            propertyName,
            _ => new List<object> { error },
            (_, errors) => UpdateErrors(error, errors));

        this.OnErrorsChanged(new DataErrorsChangedEventArgs(propertyName));
    }

    public void Remove(string propertyName, Predicate<object> filter)
    {
        Debug.Assert(this.type.GetProperty(propertyName) != null, $"The type {this.type.Name} does not have a property named {propertyName}");
        List<object> errors;
        if (this.propertyErrors.TryGetValue(propertyName, out errors))
        {
            errors.RemoveAll(filter);
            if (errors.Count == 0)
            {
                this.Clear(propertyName);
            }
        }
    }

    public void Clear(string propertyName)
    {
        Debug.Assert(this.type.GetProperty(propertyName) != null, $"The type {this.type.Name} does not have a property named {propertyName}");
        List<object> temp;
        if (this.propertyErrors.TryRemove(propertyName, out temp))
        {
            this.OnErrorsChanged(new DataErrorsChangedEventArgs(propertyName));
        }
    }

    protected virtual void OnErrorsChanged(DataErrorsChangedEventArgs e)
    {
        this.ErrorsChanged?.Invoke(this, e);
        this.ownerOnErrorsChanged(e);
    }

    private static List<object> UpdateErrors(object error, List<object> errors)
    {
        if (!errors.Contains(error))
        {
            errors.Add(error);
        }

        return errors;
    }
}

using System;
using System.Collections;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;

public class ViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{
    private readonly PropertyErrors errors;
    private string searchText;

    public ViewModel()
    {
        this.SearchCommand = new RelayCommand(this.Search);
        this.errors = new PropertyErrors(this, this.OnErrorsChanged);
    }

    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

    public event PropertyChangedEventHandler PropertyChanged;

    public string SearchText
    {
        get { return this.searchText; }
        set
        {
            if (value == this.searchText)
            {
                return;
            }

            this.searchText = value;
            this.errors.Clear(nameof(this.SearchText));
            this.OnPropertyChanged();
        }
    }

    public bool HasErrors => this.errors.HasErrors;

    public ICommand SearchCommand { get; }

    public IEnumerable GetErrors(string propertyName) => this.errors.GetErrors(propertyName);

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    private void Search()
    {
        if (string.IsNullOrEmpty(this.searchText))
        {
            this.errors.Add(nameof(this.SearchText), "Search text cannot be empty");
            return;
        }

        MessageBox.Show("searching");
    }

    protected virtual void OnErrorsChanged(DataErrorsChangedEventArgs e)
    {
        this.ErrorsChanged?.Invoke(this, e);
    }
}

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
        <RowDefinition />
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
    <TextBlock Grid.Row="0"
               Grid.Column="0"
               Text="Search text" />
    <TextBox x:Name="SearchTextBox"
             Grid.Row="0"
             Grid.Column="1">
        <TextBox.Text>
            <Binding Path="SearchText"
                     UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <local:MustStartWithValidationRule StartsWith="a" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <ItemsControl Grid.Row="0"
                  Grid.Column="2"
                  Margin="6,0,0,0"
                  ItemsSource="{Binding Path=(Validation.Errors),
                                        ElementName=SearchTextBox}">
        <ItemsControl.ItemTemplate>
            <DataTemplate DataType="{x:Type ValidationError}">
                <TextBlock Foreground="Red"
                           Text="{Binding ErrorContent}" />
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>

    <Button Grid.Row="1"
            Grid.Column="1"
            Command="{Binding SearchCommand}"
            Content="Search" />
</Grid>