How to use IDataErrorInfo.Error in a WPF program?

Question

I have an object like that:

public class Person : IDataErrorInfo
{
    public string PersonName{get;set;}
    public int Age{get;set;}

    string IDataErrorInfo.this[string propertyName]
    {
        get
        {
            if(propertyName=="PersonName")
            {
                if(PersonName.Length>30 || PersonName.Length<1)
                {
                    return "Name is required and less than 30 characters.";
                }
            }
            return null;
        }
    }

    string IDataErrorInfo.Error
    {
        get
        {
            if(PersonName=="Tom" && Age!=30)
            {
                return "Tom must be 30.";
            }
            return null;
        }
    }
}

Binding the PersonName and Age properties is easy:

<TextBox Text="{Binding PersonName, ValidatesOnDataErrors=True}" />
<TextBox Text="{Binding Age, ValidatesOnDataErrors=True}" />

However, how can I use the Error property and show it appropriately?

Portrait

I finally found a workaround and I made a post here. – Portrait

The Error property is not really used in WPF. You could even throw a NotImplementedException in there. IDataErrorInfo was used by WPF "because it was already there", but only for the this[] part. Not the prettiest corner of WPF, I think. – Savanna

@Robin, you could post an answer to this question. It's been like 3-4 years and there's no accepted answer ;) – Atharvaveda

Answer 1

You should modify the TextBox style so it shows what's wrong with the property. Here is a simple example that shows the error as tooltip:

<Style TargetType="TextBox">
        <Style.Triggers>
            <Trigger Property="Validation.HasError" Value="true">
                <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self},
                        Path=(Validation.Errors).CurrentItem.ErrorContent}" />
            </Trigger>
        </Style.Triggers>
</Style>

Just put it inside Application.Resources from your app.xaml file and it will be aplied for every textbox of your application:

<Application.Resources>
    <Style TargetType="TextBox">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self},
                            Path=(Validation.Errors).CurrentItem.ErrorContent}" />
                </Trigger>
            </Style.Triggers>
    </Style>
</Application.Resources>

Zenobia

Answer 2

Here is an example, adapted from this question, that shows how to display the error in Tooltip:

<TextBox>
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="true">
                    <Setter Property="ToolTip"
                Value="{Binding RelativeSource={RelativeSource Self}, 
                       Path=(Validation.Errors)[0].ErrorContent}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

Farcical

Hi, I set a break point at IDataErrorInfo.Error.get, but It is never triggered. What's the matter? – Portrait

@jgg - The Error get probably won't get called in this case. The IDataErrorInfo.this get would, and would be passed "PersonName" and "Age". So you would need to handle both properties, but right now you only handle the former. – Farcical