//Arrange Act Assert

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.

When you’re using the MVVM pattern with WPF or Silverlight it’s very easy to a have ViewModels that do too much.

In this part of the Silverlight Refactoring series we will convert a ViewModel with multiple responsibilities so that it adheres to the Single Responsibility Principle (SRP).

The code used in this post can be downloaded here.

Why should you care?

Currently the ViewModel looks like this

using System;
using System.ComponentModel;
using System.Windows.Input;

namespace SilverlightCalculator
{
    public class CalculatorViewModel : INotifyPropertyChanged
    {
        private string _firstValue;
        private string _secondValue;
        private string _result;

        private readonly ICommand _calculateCommand;       

        public event PropertyChangedEventHandler PropertyChanged;

        public CalculatorViewModel()
        {
            _calculateCommand = new RelayCommand(Calculate){IsEnabled = true};
        }

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

        public ICommand CalculateCommand
        {
            get { return _calculateCommand; }
        }

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

As you can see the logic to add two numbers is in the Calculate method of the ViewModel.

Therefore the ViewModel is responsible for calculating two numbers.

A ViewModel without separation of concerns is difficult to test, maintain and contains code you can’t reuse.

Although we’re only adding two numbers here, brushing the issue under the carpet will have consequences later down the line.

For example in addition to adding numbers what if another developer was given the task of storing the result in a database.

If the ViewModel was as it is now, with no separation of concerns, there is a chance they would put the code to store the value in the calculate method like this

public void Calculate()
{   
    // Calculate result
    Result = (Convert.ToInt32(FirstValue) + Convert.ToInt32(SecondValue)).ToString();

    // Store value in database
    DatabaseConnection connection = new DatabaseConnection();
    DatabaseCommand command = new DatabaseCommand("TSQL TO STORE RESULT", connection);
    command.ExecuteNonQuery();
}

Now the ViewModel would be responsible for two things. Next it would be three and this would continue until someone decides to separate the concerns or it gets to a stage where it’s impossible to work with let alone read.

Refactoring the ViewModel to follow the single responsibility principle

Let’s start by creating a class for calculating numbers.

public class Calculator 
{
    public int Add(int firstValue, int secondValue)
    {
        return firstValue + secondValue;
    }
}

Notice how the parameters passed to the method are integers, the calculator doesn’t accept string values.

What we are saying here is while it’s acceptable for the ViewModel to use strings for storing numbers the domain layer (or business logic if you prefer) does not.

In other words the add method does nothing more than add two numbers together. Validation should be handled elsewhere.

This makes the unit tests for the calculator very easy to write

[TestFixture]
public class When_calculating_numbers
{
    [Test]
    public void Should_be_able_to_add_two_numbers_together()
    {
        // Arrange
        Calculator calculator = new Calculator(); 

        // Act
        var result = calculator.Add(5, 5);

        //Assert            
        Assert.That(result, Is.EqualTo(10));            
    }
}

The code below shows how the ViewModel could create an instance of the Calculator class and call the Add Method. Notice how I say could and not should. We’ll see why later in the series.

public void Calculate()
{
    Calculator calculator = new Calculator();
    Result = calculator.Add(Convert.ToInt32(FirstValue), Convert.ToInt32(SecondValue)).ToString();
}

So what have we achieved?

As good boy scouts we have ‘left the ViewModel cleaner than we found it’.

By following the single responsibility principle we have created a ViewModel which is a better canvas for other developers to work with.

If you have arrived here from a search engine, this post is part of series about refactoring Silverlight applications. So if you’re thinking why is the calculator class tightly coupled to the ViewModel? Find out how this can be resolved by applying SOLID design principles using MEF in Silverlight and WPF.