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Jag Reehal on Agile Development, ASP.NET MVC, Silverlight and all manner of good stuff

Using IDataErrorInfo for validation in MVVM with Silverlight and WPF

Posted on | July 7, 2010 | 1 Comment

In this post we will be looking at how validation can be done by implementing the IDataErrorInfo interface for a calculator we have been building as part of the Silverlight refactoring series.

The IDataErrorInfo interface gives you the ability to do validation without throwing exceptions.

The full solution for this post can be downloaded here.

Pre IDataErrorInfo

As you can see in the code below we are throwing exceptions in the setters for the two values we want to add together.

public string  FirstValue
{
    get { return _firstValue; }
    set
    {
        _firstValue = value;
        try
        {
            int.Parse(_firstValue);
        }
        catch (Exception)
        {
            throw new Exception("That's not a number");
        }
        OnPropertyChanged("FirstValue");
    }
}
public string  SecondValue
{
    get { return _secondValue; }
    set
    {
        _secondValue = value;
        try
        {
            int.Parse(_secondValue);
        }
        catch (Exception)
        {
            throw new Exception("That's not a number");
        }
        OnPropertyChanged("SecondValue");
    }
}

Implementing the IDataErrorInfo interface

The IDataErrorInfo interface consists of two properties.

string this[string columnName] {get;}
string Error {get;}

For the Error property we can just return null because we don’t want to return a single error message for the entire object.

public string Error
{
    get { return null; }
}

For the property which returns an error for a text box we could do the validation like this

public string this[string columnName]
{
    get
    {
        string error = null;
        switch (columnName)
        {
            case "FirstValue":
                try
                {
                    int.Parse(_firstValue);
                }
                catch (Exception)
                {
                    error = "That is not a number";
                }
                break;
            case "SecondValue":
                try
                {
                    int.Parse(_firstValue);
                }
                catch (Exception)
                {
                    error = "That is not a number";
                }
                break;
        }
        return error;
    }
}

But as we saw in a previous post about using ValidatesOnExceptions to do validation it’s much easier to write unit tests when there are separation of concerns and the ViewModel is not responsible for validation.

So this means we need to create a class for storing the validation error for each textbox

public class ValidationBase
{
    public readonly Dictionary<string, string> Errors;

    public ValidationBase()
    {
        Errors = new Dictionary<string, string>();
    }

    public void AddError(string propertyName, string message)
    {
        if (!Errors.ContainsKey(propertyName))
        {
            Errors[propertyName] = message;
        }
    }

    public void RemoveErrors(string propertyName)
    {
        Errors.Remove(propertyName);
    }

    public string GetErrorMessageForProperty(string propertyName)
    {
        string message;
        Errors.TryGetValue(propertyName, out message);
        return message;
    }

    public bool HasErrors()
    {
        return Errors.Count != 0;
    }
}

which is inherited by a CalculatorValidator class that returns a boolean value if the property value is not valid

[Export(typeof(ICalculatorValidator))]
public class CalculatorValidator : ValidationBase, ICalculatorValidator
{
    [ImportMany]
    public IEnumerable<ICalculatorValidationRule> CalculatorValidationRules { get; set; }

    public bool IsPropertyValid(string propertyName, string value)
    {
        RemoveErrors(propertyName);
        foreach (var calculatorValidationRule in CalculatorValidationRules)
        {
            if (!calculatorValidationRule.IsValid(value))
            {
                AddError(propertyName, calculatorValidationRule.ErrorMessage);
                return false;
            }
        }
        return true;
    }
}

If you’re new to the series the validation rules for calculator are being imported by MEF. Read how and why we are doing this in the ‘Applying the Open Closed Principle in Silverlight and WPF using MEF‘ post.

Now we can inject/import the CalculatorValidator into the ViewModel and use it to validate the users input.

As we only want the user to be able to click the calculate button when the form is valid we call we the CheckIfCalculteButtonShouldBeEnabled method when a text box value has changed.

[Export]
public class CalculatorViewModel : INotifyPropertyChanged, IDataErrorInfo
{
    private string _firstValue;
    private string _secondValue;
    private string _result;

    private readonly ICalculator _calculator;
    private readonly RelayCommand _calculateCommand;

    public event PropertyChangedEventHandler PropertyChanged;
    private readonly ICalculatorValidator _calculatorValidator;

    [ImportingConstructor]
    public CalculatorViewModel(ICalculator calculator, ICalculatorValidator calculatorValidator)
    {
        _calculator = calculator;
        _calculatorValidator = calculatorValidator;
        _calculateCommand = new RelayCommand(Calculate) { IsEnabled = true };

        _firstValue = "0";
        _secondValue = "0";
    }
    public void Calculate()
    {
        Result = _calculator.Add(Convert.ToInt32(FirstValue), Convert.ToInt32(SecondValue)).ToString();
    }

    public string FirstValue
    {
        get { return _firstValue; }
        set
        {
            _firstValue = value;
            OnPropertyChanged("FirstValue");
        }
    }

    public string SecondValue
    {
        get { return _secondValue; }
        set
        {
            _secondValue = value;
            OnPropertyChanged("SecondValue");
        }
    }

    public void CheckIfCalculteButtonShouldBeEnabled()
    {
        _calculateCommand.IsEnabled = _calculatorValidator.HasErrors() == false;
    }

    public string Result
    {
        get { return _result; }
        private set
        {
            _result = value;
            OnPropertyChanged("Result");
        }
    }

    public RelayCommand CalculateCommand
    {
        get { return _calculateCommand; }
    }

    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,
                new PropertyChangedEventArgs(propertyName));
        }
    }

    public string this[string columnName]
    {
        get
        {
            string error = null;
            switch (columnName)
            {
                case "FirstValue":
                    error = ValidateNumber("FirstValue", _firstValue);
                    break;
                case "SecondValue":
                    error = ValidateNumber("SecondValue", _secondValue);
                    break;
            }

            CheckIfCalculteButtonShouldBeEnabled();
            return error;
        }
    }

    public string ValidateNumber(string propertyName, string value)
    {
        if (!_calculatorValidator.IsPropertyValid(propertyName, value))
        {
            return _calculatorValidator.GetErrorMessageForProperty(propertyName);
        }
        return null;
    }

    public string Error
    {
        get { return null; }
    }
}

The code below shows how we can unit test the ViewModel which implements the IDataErrorInfo interface.

[TestFixture]
public class When_using_the_CalculatorViewModel
{
    private Mock<ICalculator> _calculator;
    private Mock<ICalculatorValidator> _calculatorValidator;
    private CalculatorViewModel _calculatorViewModel;

    [SetUp]
    public void SetUp()
    {
        _calculator = new Mock<ICalculator>();
        _calculatorValidator = new Mock<ICalculatorValidator>();
        _calculatorViewModel = new CalculatorViewModel(_calculator.Object, _calculatorValidator.Object);
    }

    [Test]
    public void Initial_value_of_first_number_is_0()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.FirstValue;

        // Assert
        result.ShouldEqual("0");
    }

    [Test]
    public void Initial_value_of_second_number_is_0()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.SecondValue;

        // Assert
        result.ShouldEqual("0");
    }

    [Test]
    public void Initial_value_of_calculate_button_is_enabled()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.CalculateCommand.IsEnabled;

        // Assert
        result.ShouldBeTrue();
    }

    [Test]
    public void ValidateNumber_returns_null_if_value_is_valid()
    {
        // Arrange
        _calculatorValidator.Setup(c => c.IsPropertyValid("X","X")).Returns(true);

        // Act
        var result = _calculatorViewModel.ValidateNumber("X","X");

        // Assert
        result.ShouldBeNull();
    }

    [Test]
    public void ValidateNumber_returns_error_message_if_value_is_not_valid()
    {
        // Arrange
        const string errorMessage = "ErrorMessageText";
        _calculatorValidator.Setup(c => c.IsPropertyValid("X", "X")).Returns(false);
        _calculatorValidator.Setup(c => c.GetErrorMessageForProperty("X")).Returns(errorMessage);

        // Act
        var result = _calculatorViewModel.ValidateNumber("X", "X");

        // Assert
        result.ShouldEqual(errorMessage);
    }

    [Test]
    public void Calculate_command_should_not_be_enabled_if_ViewModel_is_not_valid()
    {
        // Arrange
        _calculatorValidator.Setup(c => c.HasErrors()).Returns(true);

        // Act
        _calculatorViewModel.CheckIfCalculteButtonShouldBeEnabled();

        // Assert
        _calculatorViewModel.CalculateCommand.IsEnabled.ShouldBeFalse();
    }

    [Test]
    public void Calculate_command_should_be_enabled_if_ViewModel_is_valid()
    {
        // Arrange
        _calculatorValidator.Setup(c => c.HasErrors()).Returns(false);

        // Act
        _calculatorViewModel.CheckIfCalculteButtonShouldBeEnabled();

        // Assert
        _calculatorViewModel.CalculateCommand.IsEnabled.ShouldBeTrue();
    }
}

The final change we need to make is to change the text box binding to ValidatesOnDataErrors=True in the XAML file.

<TextBox Text="{Binding FirstValue, Mode=TwoWay, ValidatesOnDataErrors=True}"/>
<TextBox Text="{Binding SecondValue, Mode=TwoWay, ValidatesOnDataErrors=True}"/>

So what have we achieved?

In this post we have refactored the code to use the IDataErrorInfo interface and are no longer throwing exceptions to do validation.

Whether you choose to use it or not depends if you want multiple errors for a single property to be combined into a single error message.

Category: MVVM, Silverlight, Unit tests, WPF

Comments

One Response to “Using IDataErrorInfo for validation in MVVM with Silverlight and WPF”

  1. Dew Drop – July 7, 2010 | Alvin Ashcraft's Morning Dew
    July 7th, 2010 @ 1:48 pm

    [...] Using IDataErrorInfo for validation in MVVM with Silverlight and WPF (Jag Reehal) [...]

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