Is there a way to loop through all the properties from within a class's constructor so I can set all their default values instead of having to list each one like
this.prop1 = "?";
//repeat for each prop
For example:
public class thisClass()
{
library()
{
foreach (property as p in thisClass)
{
p.value = "?";
}
}
public string prop1 {get; set;}
public string prop2 {get; set;}
etc.
}
You could do this with Reflection (via Type.GetProperties and PropertyInfo.SetValue), but I wouldn't recommend it. It will reduce the readability and maintainability, as well as have a negative performance impact.
The advantage of listing out the properties and defining their initial values is that you see it, right up front, in your constructor. You can, alternatively, provide the backing field for your properties, and define them inline on the fields.
I wouldn't do it, really. Properties should be explicitly initialized by constructors, that's why they exist. Don't forget to initialize fields as well.
But I don't know why you need it, so here is some code.
It is not so easy to reliably set any property, including private properties. Usually I do this like this (out of my head, I will check with my real code tomorrow):
var properties = this.GetType().Properties(
BindingFlags.Instance
| BidningFlags.NonPublic
| BindingFlags.Public);
foreach(PropertyInfo property in properties)
{
// if a property is declared on a base type with a private setter,
// get the definition again from the declaring type,
// unless you can't call the setter.
// Probably it is even more reliable to get the properties setter
// from the declaring type.
if (property.DeclaringType != this)
{
property = property.DeclaringType.GetProperty(
property.PropertyName,
BindingFlags.Instance
| BidningFlags.NonPublic
| BindingFlags.Public);
}
if (property.CanWrite)
{
// assumed that you define a dictionary having the default values.
property.SetValue(this, defaultValues[property.PropertyType];
}
}
I would not recommend it, but since you are asking:
var props = GetType().GetProperties().Where(prop => prop.CanWrite && prop.PropertyType == typeof(string))
foreach(var prop in props)
prop.SetValue(this, "?", null);
Do something like this. Works great. Only problem you can't rely on order.
var properties = typeof(T).GetProperties();
foreach(var prop in properties ){
}
From the horses mouth: The GetProperties method does not return properties in a particular order, such as alphabetical or declaration order. Your code must not depend on the order in which properties are returned, because that order varies.
That said, your problem is better (as in software design better) addressed by just assigning all properties manually. If you find yourself in a situation with too many properties, you should use a container instead. A List<>, for example.
I probably wouldn't recommend setting all properties to a fixed value other than null... Particularly as it may be naive to assume that all of your properties are happy with that default state and even more so, that users of your class would most likely be expecting null (or more precisely default(T)) in place of an unknown value.
Just as a suggestion, if this is for the sakes of displaying the "?" in a UI when the specific values are not yet known then perhaps you could make use of the appropriate binding classes within the framework.
For example, winforms Binding class has "NullValue" property that will be passed to the bound control's property when the datasource has null or DbNull.Value in it.
But if you really want to go down the path that you've asked for then, as suggested above, the Type.GetProperties() should do the trick. Make sure you consider cases of inherited, abstract, overridden or virtual properties and whether setting the default value is appropriate - particularly in light of the fact that the norm is set/leave a value to null/default(T) when you don't actually have a known value.
Related
On my WinForm, I want to show each Property (as a label) and its value at run time depending on the type of the object. Something like this:
public void ShowDetails(object anyType)
{
// Generate label per property and show value of the property against a label.
}
How can I achieve this? There are more than 100 classes having different properties.
I am using C# 4.0.
You use reflection.
PropertyInfo[] properties = obj.GetType().GetProperties();
foreach (PropertyInfo property in properties)
{
object propertyValue = property.GetValue(obj, null);
}
That should be enough to get you started.
You can also get lots of other information out of the PropertyInfo such as the name of the property, the type, the accessibility, and so on. Note that it's possible (but very uncommon) to have a property without a getter, so you may want to check for that first. You also may want to only get public properties, rather than all properties. You also may want to check if the property is an indexer, as it will need a non-null value for the second parameter of GetValue. Oh, and you will also get static properties returned; you may or may not want those as well.
Use System.Reflection.PropertyInfo . You can loop through all properties (and sub-properties)
MSDN link
You can easily store the properties and their values in a dictionary
Dictionary<string,object> properties = anyType.GetType()
.GetProperties()
.ToDictionary(p=>p.Name,p=>p.GetValue(anyType,null));
I'd read up on Reflection. It will allow you to access the property names and values of class member at runtime.
I am comparing two objects of the same type and returning the differences into a list of FieldChange objects. Right now I am listing out each field comparison like this which seems a bit suboptimal.
Is there a cleaner way of refactoring the code below to avoid the repetition? There are two sets of code below but in reality I have about 20 comparisons.
var changes = new List<FieldChange>();
if (proposedUpdatedProject.StatusId != existingProject.StatusId)
{
var previousStatusName = existingProject.StatusShortName;
existingProject.Status = ProjectModel.Repository.Fetch<ProjectStatus>(proposedUpdatedProject.StatusId);
changes.Add(new FieldChange { FieldName = "Status", PreviousValue = previousStatusName, NewValue = existingProject.StatusShortName });
}
if (proposedUpdatedProject.TechOwnerId != existingProject.TechOwnerId)
{
var previousTechOwnerName = existingProject.TechOwnerName;
existingProject.TechOwner = ProjectModel.Repository.Fetch<Person>(proposedUpdatedProject.TechOwnerId);
changes.Add(new FieldChange { FieldName = "Tech Owner", PreviousValue = previousTechOwnerName, NewValue = existingProject.TechOwnerName });
}
NOTE: that all objects are derived from the same object called BaseObj. Also note that I am not just putting the values of the comparable fields into FieldChange object (id versus Name property)
You could create a method attribute, for example called ComparableAttribute.
Then you can decorate all methods in these objects with this attribute.
You can use reflection in the method you are making the comparison and iterate through all Comparable properties. The code would be much shorter (one iteration, instead of 20 if statements).
If you need custom information for certain properties, you could specify it via the ComparableAttribute attribute, as parameters.
The compare method would still take as parameters two instances, but you'll end up with a much smaller implementation. You could even cache the PropertyInfos for your types, so you don't reflect at each comparison.
Why not reflect into the structures:
existingProject
And compare all fields. By writing the code long-hand this way, you are adding quite a bit of information to the problem like the association between the StatusID and the StatusName, but if you choose some proper naming conventions, you could potentially automate the whole thing.
Why not use INotifyPropertyChanged interface ? Look here for information on it. You simply have to implement it, and subscribe to the event. One more link
I'm doing some dynamic code generation using Reflection, and I've come across a situation where I need to get the backing field of a property (if it has one) in order to use its FieldInfo object.
Now, I know you can use
.IsDefined(typeof(CompilerGeneratedAttribute), false);
on a FieldInfo to discover whether it's autogenerated, so I assume there's a similar thing for Properties which auto-generate fields?
Cheers, Ed
The get_ and set_ methods for properties also get the CompilerGeneratedAttributed applied to them. While there is no strong coupling through attributes, there is a naming convention used for the backing fields of an auto property:
public string Foo { get; set;}
Produces a private string <Foo>k__BackingField member (the < and > here are part of the name, as they're legal in IL but not in C#; they have nothing to do with generics).
As an example, this will get a list of all of the auto properties in a class, along with their backing fields:
t.GetProperties().Where(p =>
(p.GetGetMethod() ?? p.GetSetMethod()).IsDefined(typeof(CompilerGeneratedAttribute), false))
.Select(p => new
{
Property = p,
Field = t.GetField(string.Format("<{0}>k__BackingField", p.Name),
System.Reflection.BindingFlags.NonPublic |
System.Reflection.BindingFlags.Instance)
});
There is no built-in method for doing this since the presence of a property does not necessarily guarantee the presence of a backing field.
I found this article which explains one way of doing it. It involves getting the IL of the property's setter and parsing it looking for evidence of a field being set.
Andrew is right.
Actually, a property is just a "pointer" to methods, usually getters/setters when they are generated by Visual Studio or other high level language (most of the time).
Parsing the setter is not easy, though. And, since internally setters are just another vanilla methods, they can use more than one fields, or none at all, or even call another methods.
Perhaps you can come up with a solution for the common scenarios, but you have to parse the IL bytecode.
I have a name of a property and need to find its value within a Class, what is the fastest way of getting to this value?
I am making the assumption that you have the name of the property in runtime; not while coding...
Let's assume your class is called TheClass and it has a property called TheProperty:
object GetThePropertyValue(object instance)
{
Type type = instance.GetType();
PropertyInfo propertyInfo = type.GetProperty("TheProperty");
return propertyInfo.GetValue(instance, null);
}
I assume you mean you have the name of the property as a string. In this case, you need to use a bit of reflection to fetch the property value. In the example below the object containing the property is called obj.
var prop = obj.GetType().GetProperty("PropertyName");
var propValue = prop.GetValue(obj, null);
Hope that helps.
If you're interested in speed at runtime rather than development, have a look at Jon Skeet's Making reflection fly and exploring delegates blog post.
Just use the name of the property. If it is a nullable property (e.g. int ? property) use property.Value.
At runtime you can use reflection to get the value of the property.
Two caveats:
Obfuscation: An obfuscator may change
the name of the property, which
will break this functionality.
Refactoring: Using reflection in this
manner makes the code more difficult
to refactor. If you change the name
of the property, you may have to
search for instances where you use
reflection to get the property value
based upon name.
Is it possible to add attributes at runtime or to change the value of an attribute at runtime?
This really depends on what exactly you're trying to accomplish.
The System.ComponentModel.TypeDescriptor stuff can be used to add attributes to types, properties and object instances, and it has the limitation that you have to use it to retrieve those properties as well. If you're writing the code that consumes those attributes, and you can live within those limitations, then I'd definitely suggest it.
As far as I know, the PropertyGrid control and the visual studio design surface are the only things in the BCL that consume the TypeDescriptor stuff. In fact, that's how they do about half the things they really need to do.
Attributes are static metadata. Assemblies, modules, types, members, parameters, and return values aren't first-class objects in C# (e.g., the System.Type class is merely a reflected representation of a type). You can get an instance of an attribute for a type and change the properties if they're writable but that won't affect the attribute as it is applied to the type.
You can't. One workaround might be to generate a derived class at runtime and adding the attribute, although this is probably bit of an overkill.
Well, just to be different, I found an article that references using Reflection.Emit to do so.
Here's the link: http://www.codeproject.com/KB/cs/dotnetattributes.aspx , you will also want to look into some of the comments at the bottom of the article, because possible approaches are discussed.
No, it's not.
Attributes are meta-data and stored in binary-form in the compiled assembly (that's also why you can only use simple types in them).
I don't believe so. Even if I'm wrong, the best you can hope for is adding them to an entire Type, never an instance of a Type.
If you need something to be able to added dynamically, c# attributes aren't the way. Look into storing the data in xml. I recently did a project that i started w/ attributes, but eventually moved to serialization w/ xml.
Why do you need to? Attributes give extra information for reflection, but if you externally know which properties you want you don't need them.
You could store meta data externally relatively easily in a database or resource file.
Like mentionned in a comment below by Deczaloth, I think that metadata is fixed at compile time. I achieve it by creating a dynamic object where I override GetType() or use GetCustomType() and writing my own type. Using this then you could...
I tried very hard with System.ComponentModel.TypeDescriptor without success. That does not means it can't work but I would like to see code for that.
In counter part, I wanted to change some Attribute values.
I did 2 functions which work fine for that purpose.
// ************************************************************************
public static void SetObjectPropertyDescription(this Type typeOfObject, string propertyName, string description)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(DescriptionAttribute)] as DescriptionAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("description", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, description);
}
}
}
// ************************************************************************
public static void SetPropertyAttributReadOnly(this Type typeOfObject, string propertyName, bool isReadOnly)
{
PropertyDescriptor pd = TypeDescriptor.GetProperties(typeOfObject)[propertyName];
var att = pd.Attributes[typeof(ReadOnlyAttribute)] as ReadOnlyAttribute;
if (att != null)
{
var fieldDescription = att.GetType().GetField("isReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
if (fieldDescription != null)
{
fieldDescription.SetValue(att, isReadOnly);
}
}
}
When faced with this situation, yet another solution might be questioning you code design and search for a more object-oriented way. For me, struggling with unpleasant reflection work arounds is the last resort. And my first reaction to this situation would be re-designing the code. Think of the following code, which tries to solve the problem that you have to add an attribute to a third-party class you are using.
class Employee {} // This one is third-party.
And you have code like
var specialEmployee = new Employee();
// Here you need an employee with a special behaviour and want to add an attribute to the employee but you cannot.
The solution might be extracting a class inheriting from the Employee class and decorating it with your attribute:
[SpecialAttribute]
class SpecialEmployee : Employee
{
}
When you create an instance of this new class
var specialEmployee = new SpecialEmployee();
you can distinguish this specialEmployee object from other employee objects. If appropriate, you may want to make this SpecialEmployee a private nested class.