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Difference between Display and Description attribute

I'm trying to enhance my enum so I've tried a suggestion on Display and another one on Description.
I'm annoyed because I don't understand the difference between them. Both Description class and Display class are from framework 4.5.
It's additionally annoying since neither of them work in the code. I'm testing the following but I only get to see the donkeys...
[Flags]
public enum Donkeys
{
[Display(Name = "Monkey 1")]
Donkey1 = 0,
[Description("Monkey 2")]
Donkey2 = 1
}

Neither of these attributes have any effect on the enum's ToString() method, which is what gets called if you just try to insert it into a Razor template. ToString() always uses the name declared in code -- Donkey1 and Donkey2 in your case. To my knowledge, there's no built-in way to specify an alternate string representation for the enum to use automatically.
I assume there are (at least) two reasons for that:
Serialization. ToString() uses the name so that Enum.Parse() can parse it back into the enum.
Localization. .NET was designed with global audiences firmly in mind, and if you want a human-readable string representation of an enum, it's extremely unlikely that there will be just one string representation, at which point it's going to be up to your application to figure out how to do it.
If you know your app will never be translated to other languages, or if you just want a string representation you can use in debug output, you're welcome to use an attribute (either one from the Framework, or one you declare yourself) to define a string representation for each enum value, and write some utility functions to do the string conversion. But you can't make the enum's ToString() do it for you (since that would break serialization); you'd have to write your own code to do it.
However, since you're writing a Web app, there's a fair chance that you will have a global audience -- in which case you'll need to localize your enum strings the same way you localize all your other text.

How can I internationalize strings representing C# enum values?

I've seen many questions and answers about mapping strings to enums and vice-versa, but how can I map a series of localized strings to enums?
Should I just create an extension method like this that returns the proper string from a resource file? Is there a way to localize attributes (like "Description") that are used in solutions like this?
Which is the preferred solution - extension method or attributes. It seems to me that this isn't the intended purpose of attributes. In fact, now that I think about it, if I were to use an extension method an attribute seems like something I'd use to specify a key in a resource file for the localized string I want to use in place of the enum value.
EDIT - example:
Given the following enum,
public enum TransactionTypes {
Cheque = 1,
BankTransfer = 2,
CreditCard = 3
}
I would like a way to map each type to a localized string. I started off with an extension method for the enum that uses a switch statement and strongly typed references to the resource file.
However, an extension method for every enum doesn't seem like a great solution. I've started following this to create a custom attribute for each enumerated value. The attribute has a base name and key for the resource file containing localized strings. In the above enum, for example, I have this:
...
[EnumResourceAttribute("FinancialTransaction", "Cheque")]
Cheque = 1,
...
Where "FinanacialTransaction" is the resx file and "Cheque" is the string key. I'm trying to create a utility method to which I could pass any value from any enumeration and have it return the localized string representation of that value, assuming the custom attribute is specified. I just can't figure out how to dynamically access a resource file and a key within it.

I would definitely suggest using a resource file, probably with a method (extension or otherwise) to make it simple to get hold of the relevant resource. As the number of languages you support grows, you don't really want the code to be full of text, distracting you from the values themselves.
Likewise translation companies are likely to be geared up to handle resx files - they're not going to want to mess around in your source code, and you shouldn't let them do so anyway :)
Just use resources which are keyed on the name of the enum and the value within it. Straightforward, scales to multiple enums and multiple languages, doesn't clutter up your source code, works well with translation tools, and is basically going along with the flow of i18n within .NET.
EDIT: For mapping the enum values to the resource names, I'd just do something like:
public static string ToResourceName<T>(this T value) where T : struct
{
return typeof(T).Name + "." + value;
}
Then you could do:
string resource = MyEnum.SomeValue.ToResourceName();
Obviously that's performing string concatenation every time - you could cache that if you wanted to, but I wouldn't bother unless you had some indication that it was actually a problem.
That doesn't stop you using the extension method for non-enums, of course. If you want to do that, you need something like Unconstrained Melody.

I continued with the custom attributes and created this utility method:
public static string getEnumResourceString(Enum value)
{
System.Reflection.FieldInfo fi = value.GetType().GetField(value.ToString());
EnumResourceAttribute attr = (EnumResourceAttribute)System.Attribute.GetCustomAttribute(fi, typeof(EnumResourceAttribute));
return (string)HttpContext.GetGlobalResourceObject(attr.BaseName, attr.Key);
}

Set or change Attribute's properties or fields at runtime in C#. Possible?

I believe there is no human way to change any attribute or field inside an Attribute apart from doing it in the constructor. That is, short of redesigning and recompiling Visual Studio yourself. There is already a similar question posted here:
Change Attribute's parameter at runtime
but I believe the peculiarities of my problem are different enough to require a new post.
I use an enumeration to keep track of the different columns of a DataTable. I use attributes in each enumeration element to indicate the underlying type and the description -in case the .ToString() would give an "ugly" result due to the rigid set of characters that are allowed to name an enumeration element, such as "Tomato_Field" when you want "Tomato Field", and the like. This allows me to place all the related information in the same object, which is, I believe, what it should be. This way I can later create all the columns with a simple and clean foreach that cycles through the elements of the enumeration and extracts the metedata (description and type) to create each column.
Now, some of the columns are autocalculated, which means that during their creation -via DataTable Identifier.Columns.Add.(NameOfColumn,underlyingType,optional: autocalculatedString)- I need to specify a string that determines how it should be calculated. That string must use the names of other columns, which might be in the Description Attribute. The approach that looks logical is to use another attribute that holds the string, which should be built using the names of the other columns, requiring access to the metadata. Now that seems impossible in the constructor: you are forced to provide a constant string. You can't use a method or anything.
This problem could be solved if there were a way to change a property inside the attribute (lets call it AutocalculatedStringAttribute) at runtime. If you access the metadata you can retrieve the string you used at the constructor of the Attribute, and you can of course change that string. However, if you later access the metadata again that change is ignored, I believe the constructor is called every time the metadata is accessed at runtime, thus ignoring any changes.
There are, of course, dirty ways to achive what I am trying to do, but my question is specifically if there is a way to properly use attributes for this. Short of resorting to CodeDOM to recompile the whole assembly with the constructor of the AutocalculatedStringAttribute changed, a certain overkill.

Right, the metadata that's used to initialize the attribute is immutable. But you can add properties and methods to an attribute class that can run code and return relevant info after the attribute object is constructed. The data they rely on doesn't have to be stored in metadata, it can be persisted anywhere.
Of course, such code wouldn't have to be part of the attribute class implementation, it could just as well be part of the code that instantiates the attribute. Which is where it belongs.

It isn't entirely clear to me what code is consuming this attribute, and it matters...
You cannot change an attribute that is burned into the code - you can query it with reflection, but that is about it. However, in many cases you can still do interesting things - I don't know if they apply to your scenario, though:
you can subclass many attributes like [Description], [DisplayName], etc - and while you pass in a constant string (typically a key) to the .ctor, it can return (through regular C#) more flexible values - perhaps looking up the description from a resx to implement i18n
if the caller respects System.ComponentModel, you can attach attributes at runtime to types etc very easily - but much harder on individual properties, especially in the case of DataTable etc (since that has a custom descriptor model via DataView)
you can wrap things and provide your own model via ICustomTypeDescriptor / TypeDescriptionProvider / PropertyDescriptor - lots of work, but provides access to set your own attributes, or return a description (etc) outside of attributes
I don't know how much of this is suitable for your environment (perhaps show some code of what you have and what you want), but it highlights that (re the question title) yes: there are things you can do to tweak how attributes are perceived at runtime.

I wanted to post this as a comment but since I wanted to include some code I couldn't, given the 600 characters limit. This is the cleanest solution I have managed to find, although it does not include all the info to create the columns on the enum, which is my goal. I have translated every field to make it easier to follow. I am not showing some code which has an obvious use (in particular the implementations of the other custom attributes and their static methods to retrieve the metadata, assume that it works).
This gets the job done, but I would ideally like to include the information stored in the strings "instancesXExpString " and "totalInstancesString" in the Autocalculated attribute, which currently only marks the columns that have such a string. This is what I have been unable to do and what, I believe, cannot be easily accomplished via subclassing -although it is an ingenious approach, I must say.
Thanks for the two prompt replies, btw.
And without any further ado, lets get to the code:
// Form in which the DataGridView, its underlying DataTable and hence the enumeration are:
public partial class MainMenu : Form {
(...)
DataTable dt_expTable;
//Enum that should have all the info on its own... but does not:
public enum e_columns {
[TypeAttribute(typeof(int))]
Experiments = 0,
[TypeAttribute(typeof(decimal))]
Probability,
[DescriptionAttribute("Samples / Exp.")]
[TypeAttribute(typeof(int))]
SamplesXExperiment,
[DescriptionAttribute("Instances / Sample")]
[TypeAttribute(typeof(int))]
InstancesXSample,
[DescriptionAttribute("Instances / Exp.")]
[TypeAttribute(typeof(int))]
[Autocalculated()]
InstancesXExp,
[DescriptionAttribute("Total Instances")]
[TypeAttribute(typeof(long))]
[Autocalculated()]
Total_Instances
};
//These are the two strings
string instancesXExpString = "[" + DescriptionAttribute.obtain(e_columns.SamplesXExperiment) + "] * [" + DescriptionAttribute.obtain(e_columns.InstancesXMuestra) + "]";
string totalInstancesString = "[" + DescriptionAttribute.obtain(e_columns.InstancesXExp) + "] * [" + DescriptionAttribute.obtain(e_columns.Experiments) + "]";
public MainMenu() {
InitializeComponent();
(...)
}
private void MainMenu_Load(object sender, EventArgs e) {
(...)
// This is the neat foreach I refered to:
foreach (e_columns en in Enum.GetValues(typeof(e_columnas))) {
addColumnDT(en);
}
}
private void addColumnDT(Enum en) {
//*This is a custom static method for a custom attrib. that simply retrieves the description string or
//the standard .ToString() if there is no such attribute.*/
string s_columnName = DescriptionAttribute.obtain(en);
bool b_typeExists;
string s_calculusString;
Type TypeAttribute = TypeAttribute.obtain(en, out b_typeExists);
if (!b_typeExists) throw (new ArgumentNullException("Type has not been defined for one of the columns."));
if (isCalculatedColumn(DescriptionAttribute.obtain(en))) {
s_calculusString = calcString(en);
dt_expTable.Columns.Add(s_columnName, TypeAttribute, s_calculusString);
} else {
dt_expTable.Columns.Add(s_columnName, TypeAttribute);
}
}
private string calcString(Enum en) {
if (en.ToString() == e_columns.InstancessXExp.ToString()) {
return instancesXExpString;
} else if (en.ToString() == e_columns.Total_Samples.ToString()) {
return totalInstancesString;
} else throw (new ArgumentException("There is a column with the autocalculated attribute whose calculus string has not been considered."));
}
(...)
}
I hope this piece of code clarifies the situation and what I am trying to do.

C# Custom Attribute Alternatives

Currently, I've created a class with ~30 properties to be set. This is done to build up a URL request later on(ie, "http://www.domain.com/test.htm?var1=a&var2=b...&var30=dd").
The issue I'm facing is the property names don't necessarily match the query variable names(this is intended to be different). For example, I may have a variable titled "BillAddress", whereas the query variable will need to be "as_billaddress".
I have no control over the query variable naming scheme as these are set at an external source.
One possible solution I've used is creating a custom attribute and decorating the properties with their respective query counterparts:
[CustomQueryAttribute("as_billaddress")]
string BillAddress{get;set;}
To retrieve the attribute though, requires a little reflection and due to the larger number of properties, I was curious if there is a neater way to accomplish this functionality. Not so much as setting/retrieving custom attributes without reflection, but being able to tie an alternate string variable to any property.
I've also pondered about setting each variable up as a sort of KeyValuePair, with each key representing the query counterpart, but I didn't get too far in that thought.
To summarize/clarify my above backstory, what would you do to associate a string with a property(not the value of the property)?
As always, any comments are greatly appreciated.

I would probably stick with a custom attribute, but the other potential option would be to do something like hold a static Dictionary that had string and property info (or property name), so you could get/set the property directly via this.
Something like:
static Dictionary<string, PropertyInfo> propertyMap = new Dictionary<string, PropertyInfo>();
static MyClass()
{
Type myClass = typeof(MyClass);
// For each property you want to support:
propertyMap.Add("as_billaddress", MyClass.GetProperty("BillAddress"));
// ...
}
You could then just do a dictionary lookup instead of using reflection in each call... This could also be setup fairly easy using configuration, so you could reconfigure the mappings at runtime.

A custom attribute seems like the best option to me - the framework seems to do this a lot as well (specifically with serialization).

If you look at popular ORM mappers then nearly all either use custom attributes or some kind of XML mapping file. The advantage of the latter is that you can modify the mapping without recompiling your application - the downside is that it hurts performance. However, I'd say your choice seems perfectly reasonable.

How do I use ValidateDecimal attribute in Castle.ActiveRecord?

I have a class that inherits ActiveRecordValidationBase that contains the following property:
[Property]
[ValidateDecimal]
public Decimal UnitCost { get; set; }
I also have a UnitCostTextBox that accepts input for said UnitCost.
What I would like to do is perform validation once using Castle's validators. However, it seems that before I can pass UnitCodeTextBox.Text to my object, I will need to first convert it to a decimal first.
If I have an erroneous input, an exception will be thrown. So this means I still have to perform regex validations and converting the string to a decimal type before handing it over to Castle.ActiveRecord.
Doesn't this mean it's redundant to have a [ValidateDecimal] since I've already sanitized UnitCost?
I'm wondering how do you guys do it? I have googled for examples, but most of them only handle [ValidateNonEmpty] or [ValidateEmail] which are all strings anyway, not different data types

What you're missing is the binding part (actually you're doing it manually). Castle.Components.Binder and Castle.Components.Validator are used together to automatically bind and validate string input (e.g. an HTML form) into strongly typed objects.
MonoRail does this with the [DataBind] attribute and the AR-specific [ARDataBind]
In a WebForms application you'll have to implement binding+validation yourself (you can of course use Castle.Components.Binder and Castle.Components.Validator)