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Retreive Arguments of a Workflow (with default values)?
// Just an holder for InArgument informations
class InArgumentInfo
{
    public string InArgumentName { get; set; }
    public string InArgumentDescription { get; set; }
    public bool InArgumentIsRequired { get; set; }
}

static ICollection<InArgumentInfo> GetInArgumentsInfos(Activity activity)
{
    var properties = activity.GetType()
        .GetProperties()
        .Where(p => typeof(InArgument).IsAssignableFrom(p.PropertyType))
        .ToList();

    var argumentsCollection = new Collection<InArgumentInfo>();

    foreach (var property in properties)
    {
        var descAttribute = property
            .GetCustomAttributes(false)
            .OfType<DescriptionAttribute>()
            .FirstOrDefault();

        string description = descAttribute != null && !string.IsNullOrEmpty(descAttribute.Description) ?
            descAttribute.Description :
            string.Empty;

        bool isRequired = property
            .GetCustomAttributes(false)
            .OfType<RequiredArgumentAttribute>()
            .Any();

        argumentsCollection.Add(new InArgumentInfo
        {
            InArgumentName = property.Name,
            InArgumentDescription = description,
            InArgumentIsRequired = isRequired
        });
    }

    return argumentsCollection;
}

var dynamicActivity = ActivityXamlServices.Load(foo) as DynamicActivity
foreach(DynamicActivityProperty prop in dynamicActivity.Properties)
{
   // ...
}

foreach (var prop in dynamicActivity .Properties)
{
    object defaultValue;
    if (prop.Value == null)
    {
        defaultValue = null;
    }
    else
    {
        Type genericTypeDefinition = prop.Type.GetGenericTypeDefinition();
        if (genericTypeDefinition == typeof(InArgument<>) || genericTypeDefinition == typeof(InOutArgument<>))
        {
            var valueProp = prop.Value.GetType().GetProperty("Expression", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.DeclaredOnly);
            var expression = valueProp.GetValue(prop.Value, null);

            var expressionValueProp = expression.GetType().GetProperty("Value");
            defaultValue = expressionValueProp.GetValue(expression, null);
        }
    }
}