Sometimes it's useful to have more information about a DependencyProperty than metadata supplies. For example, in generalized tracing methods and event handlers, it's helpful to be able to find the name (and other characteristics) of a DependencyProperty on a DependencyObject.
In WinUI 3, System.Reflection reports DependencyProperties as MemberType=Properties so we use GetProperties() to get a PropertyInfo[] containing information on all properties on an object or type (WPF reports DependencyProperties as MemberType=Fields so we'd use GetFields() instead).
DependencyProperties are actually declared as static fields and the Get... methods in Reflection typically default to not listing static members. When calling GetProperties, be sure to include the BindingFlags.Static flag, as well as BindingFlags.Public to include public DependencyProperties in your list.
Next, we check each of the returned PropertyInfo[] items to see if they are actually about the DependencyProperty identifier of interest. If dp holds the identifier of your DependencyProperty of interest and pi is the PropertyInfo item you're checking, use the object.ReferenceEquals(pi.GetValue(sender), dp) method to see if they both refer to the same identifier (Note: this will be false if pi and dp are on different objects or identify different properties). There should be one, and only one, match in the returned array.
Here is the beginning of a PropertyChanged handler that demonstrates:
private static void BoundPropertyChanged(DependencyObject sender, DependencyProperty dp)
{
// sender is DependencyObject upon which the dp changed
PropertyInfo dpInfo = sender.GetType().GetProperties(BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy)
.Where(pi => pi.PropertyType == typeof(DependencyProperty) && object.ReferenceEquals(pi.GetValue(sender), dp)).Single();
// Trace logic, etc.
}
The first statement has four parts:
sender.GetType().GetProperties(BindingFlags.Public | BindingFlags.Static | BindingFlags.FlattenHierarchy) fetches information on all properties on sender's type. The BindingFlags.FlattenHierarchy flag means, essentially, include inherited properties, too.
.Where(pi => pi.PropertyType == typeof(DependencyProperty) selects only members that have a PropertyType of DependencyProperty (routed events might be in the list from 1), too).
&& object.ReferenceEquals(pi.GetValue(sender), dp)) further limits the selected items to those that refer to the same identifiers.
the .Single(); clause selects the sole member of the resulting list and throws an error if there is more (or less) than one.
One caveat is that this approach relies heavily on reflection, which is relatively slow.
I don't know of a better way to do this, but I'd welcome suggestions and improvements!
Related
In Java, it turns out that field accessors get cached, and using accessors has side-effects. For example:
class A {
private static final int FOO = 5;
}
Field f = A.class.getDeclaredField("FOO");
f.setAccessible(true);
f.getInt(null); // succeeds
Field mf = Field.class.getDeclaredField("modifiers" );
mf.setAccessible(true);
f = A.class.getDeclaredField("FOO");
f.setAccessible(true);
mf.setInt(f, f.getModifiers() & ~Modifier.FINAL);
f.setInt(null, 6); // fails
whereas
class A {
private static final int FOO = 5;
}
Field mf = Field.class.getDeclaredField("modifiers" );
mf.setAccessible(true);
f = A.class.getDeclaredField("FOO");
f.setAccessible(true);
mf.setInt(f, f.getModifiers() & ~Modifier.FINAL);
f.setInt(null, 6); // succeeds
Here's the relevant bit of the stack trace for the failure:
java.lang.IllegalAccessException: Can not set static final int field A.FOO to (int)6
at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:76)
at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:100)
at sun.reflect.UnsafeQualifiedStaticIntegerFieldAccessorImpl.setInt(UnsafeQualifiedStaticIntegerFieldAccessorImpl.java:129)
at java.lang.reflect.Field.setInt(Field.java:949)
These two reflective accesses are of course happening in very different parts of my code base, and I don't really want to change the first to fix the second. Is there any way to change the second reflective access to ensure it succeeds in both cases?
I tried looking at the Field object, and it doesn't have any methods that seem like they would help. In the debugger, I noticed overrideFieldAccessor is set on the second Field returned in the first example and doesn't see the changes to the modifiers. I'm not sure what to do about it, though.
If it makes a difference, I'm using openjdk-8.
If you want the modifier hack (don't forget it is a total hack) to work, you need to change the modifiers private field before the first time you access the field.
So, before you do f.getInt(null);, you need to do:
mf.setInt(f, f.getModifiers() & ~Modifier.FINAL);
The reason is that only one internal FieldAccessor object is created for each field of a class (*), no matter how many different actual java.lang.reflect.Field objects you have. And the check for the final modifier is done once when it constructs the FieldAccessor implementation in the UnsafeFieldAccessorFactory.
When it is determined you can't access final static fields (because, the setAccessible override doesn't works but non-static final fields, but not for static final fields), it will keep failing for every subsequent reflection, even through a different Field object, because it keeps using the same FieldAccessor.
(*) barring synchronization issues; as the source code for Field mentions in a comment:
// NOTE that there is no synchronization used here. It is correct
(though not efficient) to generate more than one FieldAccessor for a
given Field.
Using EventAggregator with Unity and PRISM, is there a way to get the list of subscribers to a specific event?
I know that some people will say that "Why do you want to know this, the whole point of using an EventAggregator is so that we don't care who or how many subscribers".
This is purely an exercise that I want to carry out it is not for any specific purpose.
EventAggregator must store a list of subscribers is there a way to expose this list?
Yes, the subscriber information is contained within the EventAggregator object graph. Unfortunately (for your request), this information is stored within private fields.
Events are stored in a private Dictionary<Type, EventBase> events. The subscriptions are stored within the event itself (in EventBase) as a
private readonly List<IEventSubscription> _subscriptions = new List<IEventSubscription>();
One option to retrieve the subscription information is to use reflection to access the private member variables (assuming reflection permission exists).
If you can modify the events then another option is to expose this information via the specific PubSub<EventBase> since EventBase allows access to subscriptions for subclasses via the protected ICollection<IEventSubscription> Subscriptions property. For example:
public class MyEvent : PubSubEvent<MyEventData>
{
public new ICollection<IEventSubscription> Subscriptions => base.Subscriptions;
}
Then you can query the event:
var myEvent = eventAggregator.GetEvent<MyEvent>();
var sub = (EventSubscription<MyEventData>) myEvent.Subscriptions.FirstOrDefault();
MethodInfo method = sub.Action.Method;
object target = sub.Action.Target;
If you don't need the specific EventSubscription<T> data (e.g. just want to know subscription counts) then you could avoid the specific cast and use the IEventSubscription interface.
I have been trying to use one View for updating an object and all its child collections (based on one-to-many relationships in an SQL Server database with an Entity Framework model).
It was suggested I should use AutoMapper, and I tried that and got it to work. (see Trying to use AutoMapper for model with child collections, getting null error in Asp.Net MVC 3 ).
But the solution is really hard to maintain. And when I try the simple one I had to begin with, using an entity object directly as the model (a "Consultant" object, the parent of all the child collections), I am able to get all the correct changed data back in the POST, and I can use UpdateModel to get them, including child collections. Simple. Granted, UpdateModel only worked after creating a custom model binder from a tip here at SO:
From my custom model binder:
public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
bindingContext.ModelMetadata.ConvertEmptyStringToNull = false;
return base.BindModel(controllerContext, bindingContext);
}
protected override void SetProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value)
{
ModelMetadata propertyMetadata = bindingContext.PropertyMetadata[propertyDescriptor.Name];
propertyMetadata.Model = value;
string modelStateKey = CreateSubPropertyName(bindingContext.ModelName, propertyMetadata.PropertyName);
// Try to set a value into the property unless we know it will fail (read-only
// properties and null values with non-nullable types)
if (!propertyDescriptor.IsReadOnly)
{
try
{
if (value == null)
{
propertyDescriptor.SetValue(bindingContext.Model, value);
}
else
{
Type valueType = value.GetType();
if (valueType.IsGenericType && valueType.GetGenericTypeDefinition() == typeof(EntityCollection<>))
{
IListSource ls = (IListSource)propertyDescriptor.GetValue(bindingContext.Model);
IList list = ls.GetList();
foreach (var item in (IEnumerable)value)
{
list.Add(item);
}
}
else
{
propertyDescriptor.SetValue(bindingContext.Model, value);
}
}
}
catch (Exception ex)
{
// Only add if we're not already invalid
if (bindingContext.ModelState.IsValidField(modelStateKey))
{
bindingContext.ModelState.AddModelError(modelStateKey, ex);
}
}
}
}
Here's my simple Edit POST method:
[HttpPost]
[ValidateInput(false)] //To allow HTML in description box
public ActionResult Edit(int id, FormCollection collection)
{
Consultant consultant = _repository.GetConsultant(id);
UpdateModel(consultant);
_repository.Save();
return RedirectToAction("Index");
}
But after that UpdateModel worked. The problem is, at the next stage, when trying to call SaveChanges on the context, that fails. I'm getting this error:
The operation failed: The relationship
could not be changed because one or
more of the foreign-key properties is
non-nullable. When a change is made to
a relationship, the related
foreign-key property is set to a null
value. If the foreign-key does not
support null values, a new
relationship must be defined, the
foreign-key property must be assigned
another non-null value, or the
unrelated object must be deleted.
I don't understand what is wrong. I'm seeing all the correct values in the Consultant object posted, I just can't save it to database. The route of AutoMapper in this case (although an interesting tool) is not working well, it's complicating my code immensely and making the application, which should be rather simple, a nightmare to maintain.
Can anyone offer any insight into why I'm getting this error and how to overcome it?
UPDATE:
Reading some posts here, I found one that seemed slightly related: How to update model in the database, from asp.net MVC2, using Entity Framework? . I don't know if it relates to this, but when I inspected the Consultant object after POST it seems this object itself has entitykey, but the individual items in a collection do not (EntityKeySet = null). Each item however does have the correct id. I don't pretend to understand any of this with the EntityKey, so please explain if it has any bearings on my issue, and if so, how to resolve it...
UPDATE 2:
I thought of something that might have something to do with my problems: The View is using a technique described by Steven Sanderson (see http://blog.stevensanderson.com/2010/01/28/editing-a-variable-length-list-aspnet-mvc-2-style/ ), and when debugging it seems to me as if UpdateModel has trouble matching the items in a collection in the View with the ones in the actual Consultant object. I'm wondering if this has to do with the indexing in this technique. Here's the helper from that code (I can't follow it very well myself, but it uses a Guid to create indexes, which might be the problem):
public static class HtmlPrefixScopeExtensions
{
private const string idsToReuseKey = "__htmlPrefixScopeExtensions_IdsToReuse_";
public static IDisposable BeginCollectionItem(this HtmlHelper html, string collectionName)
{
var idsToReuse = GetIdsToReuse(html.ViewContext.HttpContext, collectionName);
string itemIndex = idsToReuse.Count > 0 ? idsToReuse.Dequeue() : Guid.NewGuid().ToString();
// autocomplete="off" is needed to work around a very annoying Chrome behaviour whereby it reuses old values after the user clicks "Back", which causes the xyz.index and xyz[...] values to get out of sync.
html.ViewContext.Writer.WriteLine(string.Format("<input type=\"hidden\" name=\"{0}.index\" autocomplete=\"off\" value=\"{1}\" />", collectionName, html.Encode(itemIndex)));
return BeginHtmlFieldPrefixScope(html, string.Format("{0}[{1}]", collectionName, itemIndex));
}
public static IDisposable BeginHtmlFieldPrefixScope(this HtmlHelper html, string htmlFieldPrefix)
{
return new HtmlFieldPrefixScope(html.ViewData.TemplateInfo, htmlFieldPrefix);
}
private static Queue<string> GetIdsToReuse(HttpContextBase httpContext, string collectionName)
{
// We need to use the same sequence of IDs following a server-side validation failure,
// otherwise the framework won't render the validation error messages next to each item.
string key = idsToReuseKey + collectionName;
var queue = (Queue<string>)httpContext.Items[key];
if (queue == null)
{
httpContext.Items[key] = queue = new Queue<string>();
var previouslyUsedIds = httpContext.Request[collectionName + ".index"];
if (!string.IsNullOrEmpty(previouslyUsedIds))
foreach (string previouslyUsedId in previouslyUsedIds.Split(','))
queue.Enqueue(previouslyUsedId);
}
return queue;
}
private class HtmlFieldPrefixScope : IDisposable
{
private readonly TemplateInfo templateInfo;
private readonly string previousHtmlFieldPrefix;
public HtmlFieldPrefixScope(TemplateInfo templateInfo, string htmlFieldPrefix)
{
this.templateInfo = templateInfo;
previousHtmlFieldPrefix = templateInfo.HtmlFieldPrefix;
templateInfo.HtmlFieldPrefix = htmlFieldPrefix;
}
public void Dispose()
{
templateInfo.HtmlFieldPrefix = previousHtmlFieldPrefix;
}
}
}
But then again, I wouldn't have thought this should be the problem since the hidden input contains the id in the value attribute, and I thought UpdateModel just looked at the name of the field to get Programs (the collection) and Name (the property), and then the value to the the id...? And then again there's seems to be some mismatch during update. Anyway, here's the generated html from FireBug also:
<td>
<input type="hidden" value="1" name="Programs[cabac7d3-855f-45d8-81b8-c31fcaa8bd3d].Id" id="Programs_cabac7d3-855f-45d8-81b8-c31fcaa8bd3d__Id" data-val-required="The Id field is required." data-val-number="The field Id must be a number." data-val="true">
<input type="text" value="Visual Studio" name="Programs[cabac7d3-855f-45d8-81b8-c31fcaa8bd3d].Name" id="Programs_cabac7d3-855f-45d8-81b8-c31fcaa8bd3d__Name">
<span data-valmsg-replace="true" data-valmsg-for="Programs[cabac7d3-855f-45d8-81b8-c31fcaa8bd3d].Name" class="field-validation-valid"></span>
</td>
Anyone know if this is the problem? And if so, how can I work around it to be able to easily update the collections with UpdateModel? (While still being able to add or remove items in the View before POST, which was the purpose of this technique to begin with).
It looks like there is a Parent entity that has a one to many relationship with your Consultant entity. When you change an attribute of the Consultant entity that is used as the ForeignKey for that relationship, Entity Framework sets the relevant field in the Parent entity to null to decouple the relationship. When that field is not nullable you'll get this error. Actually that error definition is surprisingly good, I've seen this problem with far more cryptic errors.
So, I recommend that you check the parent entity in the database, and proceed to a remedy from there (if you can change it to nullable all is well, if it is part of a different constraint -pk or suchlike- you'll have to fiddle with your object models). I'd ask you to post your entity models, but the chunk of text is intimidating as it is.
I think the error you are getting is related to: EF 4: Removing child object from collection does not delete it - why? You have created an orphan somewhere.
Yes it is related to HtmlPrefixScopeExtensions, but only because you are using Mvc Futures model binders.
In global.asax.cs comment out the line
Microsoft.Web.Mvc.ModelBinding.ModelBinderConfig.Initialize();
and retry: it will work ok !
The problem happens because the MVC futures model binder does not handle correctly this case. It converts ok the form data into your model when you submit the form, but it has a problem when filling the ModelState object when you use HtmlPrefixScopeExtensions to generate non incremental ids.
The model itself is correctly created from the form data. The problem lies inside ModelState which contains only the last value of the collection instead of all elements of the collection.
The strongly typed helper method - which renders the list - only select items which are in your Model property list AND in the matching ModelState entry which is converted into a list. So because there is only one item in the matching ModelState entry other list items get deselected.
This method called by the strongly typed helper code:
htmlHelper.GetModelStateValue(fullName, typeof(string[]))
returns only the last element of the list, because ModelState["Programs[cabac7d3-855f-45d8-81b8-c31fcaa8bd3d].List"].Value contains only the last element of the list.
This is a bug (or non supported scenario) in MVC3 Futures extensible model binders.
I want to retrieve the private (implementation and other) methods of a class which implements an interface and also is derived from (inherits) a base class.
How can I achieve this using reflection?
Is there another way to achieve this?
This is what I'm trying to do. I need to view these private methods and their contents. I don't want to invoke them.
Dim assembly As System.Reflection.Assembly
Dim assemblyName As String assemblyName = System.IO.Path.GetFullPath("xyz.dll")
assembly = System.Reflection.Assembly.LoadFile(assemblyName)
assembly.GetType("myClass").Getmethods(Bindings.NonPublic)
assembly.GetType("myClass").GetMethods(BindingFlags.NonPublic) isn't working.
Making the assumption that you're trying to use the private methods of a 3rd party assembly:
I'd urge caution in using private methods, they have been made private for a reason and
might rely on some state that you won't be able to anticipate; Also, if the assembly gets updated in the future, the private behavior may be changed or removed causing you to have to re-write your code to compensate.
However, there are two methods at your disposal, if you want to continue.
One way us using the new Dynamic keyword in .net 4:
http://igoro.com/archive/use-c-dynamic-typing-to-conveniently-access-internals-of-an-object/
To use reflection, this question may help you:
How do I use reflection to invoke a private method?
Hope this helps.
BindingFlags
You must specify Instance or Static along with Public or NonPublic or no members will be returned.
Assembly myAssembly;
myAssembly = Assembly.LoadFrom(#"c:\xyz.dll");
Type myType = myAssembly.GetTypes()[0]; //change 0 according to your class type.
BindingFlags eFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
MethodInfo myMethod = myType.GetMethod("Methodname", eFlags);
object obj1 = myAssembly.CreateInstance(myType.FullName);
object obj2 = myMethod.Invoke(obj1, new object[2] { parameters1, parameters1 });
I would like to serialize the properties of the HttpBrowserCapibilities object so that it may be returned via a web method call. Currently the object cannot be serialized:
Cannot serialize member System.Web.Configuration.HttpCapabilitiesBase.Capabilities of type System.Collections.IDictionary, because it implements IDictionary.
...which is understandable. However, I would like to simply copy out the properties and their values to a hierarchy, i.e.
<HttpBrowserCapabilities>
<IsMobile>true</IsMobile>
</HttpBrowserCapabilities>
I'm starting to think I would need to use reflection to copy this object, but I haven't reached a conclusion. Does anyone have any suggestions to keep this simple?
Thanks,
George
Originally I posted an answer using XmlDocument, but I glossed over some of the web method stuff and didn't realize you were really trying to map a DTO.
Reflection sounds complicated but it really isn't. The following snippet will do what you want:
public static void Populate(object dest, IDictionary dictionary)
{
Type t = dest.GetType();
foreach (object key in dictionary)
{
PropertyInfo prop = t.GetProperty(key.ToString(),
BindingFlags.Instance | BindingFlags.Public);
if ((prop != null) && prop.CanWrite)
{
object value = dictionary[key];
prop.SetValue(dest, value, null);
}
}
}
Then invoke this as:
BrowserCapsDto dto = new BrowserCapsDto();
Populate(dto, Capabilities); // Capabilities is the real BrowserCaps
It's pretty easy because you already have an IDictionary and thus you already know all of the possible names you can map; you don't actually need to use any reflection on the source, just the destination.