Fix property grid search exception

[WilliamBZA]

Connects to #733

• Updates to the latest version of DevExpress that we are licensed for
• Includes a value converter and behavior to wrap search strings in quotes - this prevents the exception from occurring

[adamralph]

This could be more for my own education than anything else, but looking purely at the method signatures, I would naively write these methods like so:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (targetType != typeof(string))
    {
        throw new InvalidOperationException($"Cannot convert value to target type '{targetType}'.");
    }

    if (value == null)
    {
        return null;
    }

    var stringValue = value as string;

    if (stringValue == null)
    {
        throw new InvalidOperationException($"The value is not a string.");
    }

    return stringValue.StartsWith("\"", true, culture) && stringValue.EndsWith("\"", true, culture)
        ? stringValue.Substring(1, stringValue.Length - 2)
        : stringValue;
}

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) =>
    string.Format(culture, "\"{0}\"", value);

However, I don't know what assumptions can be made wrt the WPF execution context, and what does or does not need to be done. Things to note about the above snippet:

• Respecting the targetType argument and throwing if it's not a string
• Short circuiting the case where value is null.
• Avoiding the string allocation by safe casting value to a string.
• Throwing if value is not a string
• using the supplied culture argument where appropriate (in both methods)

[WilliamBZA]

I'm not a real fan of the exceptions. The converters are invoked by the databinding pipeline so any exceptions are treated as application exceptions (could crash?)

What about:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (targetType != typeof(string))
    {
        return Binding.DoNothing;
    }

    var stringValue = value as string;
    if (stringValue == null)
    {
        return DependencyProperty.UnsetValue;
    }

    return stringValue.StartsWith("\"", true, culture) && stringValue.EndsWith("\"", true, culture)
        ? stringValue.Substring(1, stringValue.Length - 2)
        : stringValue;
}

[HEskandari]

You're both right! You could use exceptions in a converter and they show up as warnings, but the right approach is to use the DependencyProperty.UnsetValue as @WilliamBZA highlighted.

[adamralph]

@WilliamBZA your suggestion LGTM.

[WilliamBZA]

Updated based on feedback and rebased on master.

I think this is good to go now

[HEskandari]

I suppose this is the 'fix'? Should we put a comment there or somehow make it more obvious that this is not a generic behavior? Maybe a link to the bug report?