Question

Binding is so powerful in WPF. Supposed that we have a Number property (nullable int) and is bound to a textbox.

I realized when it throws an error, the property has the last correct value.

I mean these are the processes:

TEXTBOX: ""     PROPERTY: null
TEXTBOX: "2"    PROPERTY: 2
TEXTBOX: "2b"   PROPERTY: 2   <-- here is the problem, should be null instead 2(by the error)

Is there a way which the binding set a null value when it produce an error?

Some persons told me I need to implement IDataErrorInfo, but I guess that interface is to validate business rules. So I wouldn’t prefer user it.

UPDATE:

    <TextBox Text="{Binding Number, UpdateSourceTrigger=PropertyChanged,
        ValidatesOnExceptions=True, ValidatesOnDataErrors=True,
        NotifyOnValidationError=True, TargetNullValue={x:Static sys:String.Empty}}"

Answer 1

You are using UpdateSourceTrigger=PropertyChanged, which means that anytime the user hits a key, it is storing the data in your data context

For example, user types 2, then your property is equal to "2". User types b and it will attempt to replace "2" with "2b", which fails, so the original property of "2" remains.

Remove the UpdateSourceTrigger and it will revert to the default of LostFocus, which means it will only update the property when the TextBox loses focus.

You could set the property to null when an error is produced, but I would not recommend doing that because then if a user accidently hits the wrong key, the TextBox would get cleared.

As a side note, use IDataErrorInfo for all validation, not just business rule validation. WPF is built to work with it. My Models use it to verify their data is the correct length, type, etc, and my ViewModels use it to verify that business rules are being followed

Edit

The alternative suggestion I would have would be to bind to a string value, not a number field. This way when the value changes, you can try and cast it to your Int and return an error if it can’t be cast.

public class SomeObject : IDataErrorInfo
{
    public string SomeString { get; set; }
    public Int32? SomeNumber { get; set; }

    #region IDataErrorInfo Members

    public string Error
    {
        get { throw new NotImplementedException(); }
    }

    public string this[string columnName]
    {
        get
        {
            if (columnName == "SomeString")
            {
                int i;
                if (int.TryParse(SomeString, i))
                {
                    SomeNumber = i;
                }
                else
                {
                    SomeNumber = null;
                    return "Value is not a valid number";
                }
            }
            return null;
        }
    }

    #endregion
}