WPF TextBox Validation

Question

I have validation hooked up to a model that is bound to the TextBox container. When the window is first opened validation errors appear as the model is empty, I do

Answer 1

This really depends on your implementation of IDataErrorInfo. If you base it around a Dictionary of error messages you can control when validation runs that adds to that list. You would normally want to do that from your property setters (like whenever you call PropertyChange), here calling CheckValidationState:

    public string this[string columnName]
    {
        get
        {
            return ValidateProperty(columnName);
        }
    }

    public Dictionary Errors { get; private set; }

    protected void SetError(string propertyName, string errorMessage)
    {
        Debug.Assert(!String.IsNullOrEmpty(propertyName), "propertyName is null or empty.");
        if (String.IsNullOrEmpty(propertyName))
            return;

        if (!String.IsNullOrEmpty(errorMessage))
        {
            if (Errors.ContainsKey(propertyName))
                Errors[propertyName] = errorMessage;
            else
                Errors.Add(propertyName, errorMessage);
        }
        else if (Errors.ContainsKey(propertyName))
            Errors.Remove(propertyName);

        NotifyPropertyChanged("Errors");
        NotifyPropertyChanged("Error");
        NotifyPropertyChanged("Item[]");
    }

    protected virtual string ValidateProperty(string propertyName)
    {
        return Errors.ContainsKey(propertyName) ? Errors[propertyName] : null;
    }

    protected virtual bool CheckValidationState(string propertyName, T proposedValue)
    {
        // your validation logic here
    }

You can then also include a method that validates all of your properties (like during a save):

    protected bool Validate()
    {
        if (Errors.Count > 0)
            return false;

        bool result = true;
        foreach (PropertyInfo propertyInfo in GetType().GetProperties())
        {
            if (!CheckValidationState(propertyInfo.Name, propertyInfo.GetValue(this, null)))
                result = false;
            NotifyPropertyChanged(propertyInfo.Name);
        }
        return result;
    }

UPDATE:

I would recommend putting the above code into a base ViewModel class so you can reuse it. You could then create a derived class like this:

public class SampleViewModel : ViewModelBase
{
    private string _firstName;

    public SampleViewModel()
    {
        Save = new DelegateCommand