Develop Reference

Add custom attribute to a class generated by Entity Framework

I am trying to use a custom attribute on a Entity class generated automatically by the Entity Framework.
The problem is how to add an property attribute on an existing field?
Here the point where I am right now:
// the custom attribute class
public class MyCustomAttribute : Attribute
{
public String Key { get; set; }
}
// Entity Framework class generated automatically
public partial class EntityClass
{
public String Existent { get; set; }
//...
}
// set a metadata class for my entity
[MetadataType(typeof(EntityClassMetaData))]
public partial class EntityClass
{
// if I add a new property to the entity, it works. This attribute will be read
[MyCustomAttribute(Key = "KeyOne" )]
public int newProp { get; set; }
}
public class EntityClassMetaData
{
// adding the custom attribute to the existing property
[MyCustomAttribute(Key = "keyMeta") ]
public String Existent { get; set; }
}
Running this test:
[TestMethod]
public void test1()
{
foreach (var prop in typeof(EntityClass).GetProperties())
{
var att = prop.GetCustomAttribute<MyCustomAttribute>();
if (att != null)
{
Console.WriteLine($"Found {att.Key}");
}
}
}
will produce:
Found KeyOne
Or the Metadata class store the attribute in a different way or only works for data annotations.
I am stuck here, how can I set and read custom attributes of the generated class without having to edit the generated file?

I came across this same problem today. I figured EF magic would do the trick and map the attribute across to each model property. Turns out it does, but only for EF data annotations and I couldn't find an answered solution to pull out custom attributes so made this function. Hope it helps dude.
private object[] GetMetadataCustomAttributes(Type T, string propName)
{
if (Attribute.IsDefined(T, typeof(MetadataTypeAttribute)))
{
var metadataClassType =
(T.GetCustomAttributes(typeof(MetadataTypeAttribute), true).FirstOrDefault() as
MetadataTypeAttribute).MetadataClassType;
var metaDataClassProperty = metadataClassType.GetProperty(propName);
if (metaDataClassProperty != null)
{
return metaDataClassProperty.GetCustomAttributes(true);
}
}
return null;
}

I believe if you want to set an attribute in the metadata class, you have to use this syntax:
public class EntityClassMetaData
{
// adding the custom attribute to the existing property
[MyCustomAttribute(Key = "keyMeta") ]
public String Existent;
}
You must not have { get; set; } on your pre-existing property - just the property with the correct name and datatype.

How can I use an app setting in a custom resolver?

I'll try and explain what I want to do. I have a Server object, that contains a parameter with a date of when it was last "synced"
public class Server
{
public Guid Id { get; set; }
public DateTime LastSyncedAt { get; set; }
}
I want to map this to a ServerSummary object, which has a Status parameter.
public class ServerSummary
{
public Guid Id { get; set; }
public string Status { get; set; }
}
This status will be set by checking to see if the Server has been synced in the last X minutes, with X being stored in my appsettings.json file:
{
"SyncOffsetMinutes": "5"
}
I have a model class for this:
public class AppSettings
{
public int SyncOffsetMinutes { get; set; }
}
which is configured in my Startup.cs class:
public IConfiguration Configuration { get; }
public void ConfigureServices(IServiceCollection services)
{
...
services.Configure<AppSettings>(Configuration);
}
To resolve this Status property, I have this in my AutoMapper profile configuration:
public class MyAutoMapperProfileConfiguration : Profile
{
public MyAutoMapperProfileConfiguration()
{
CreateMap<Server, ServerSummary>()
.ForMember(d => d.Status, o => o.ResolveUsing<ServerSyncStatusResolver>());
}
}
In my custom resolver, I'm trying to use DI to inject my AppSettings so that I can use the app setting in my Resolve method:
public class ServerSyncStatusResolver : IValueResolver<Server, ServerSummary, string>
{
private AppSettings _appSettings;
public ServerSyncStatusResolver(AppSettings appSettings)
{
_appSettings = appSettings;
}
public string Resolve(Server source, ServerSummary destination, string member, ResolutionContext context)
{
return source.LastSyncedAt.AddMinutes(_appSettings.SyncOffsetMinutes) < DateTime.UtcNow ? "Offline" : "Online";
}
}
But when I actually do my mapping:
var servers = _dbContext.Servers.ToList();
var serverSummaries = Mapper.Map<List<Server>, List<ServerSummary>>(servers);
I get an error saying
No parameterless constructor defined for this object
Is there a way to inject my AppSettings file into the resolver? Am I doing it wrong?

I was very much wrong, see the update below
You can not use injections at this point, because the resolver instance is created at the time the mapper configuration is initialized.
The only possibility is to put AppSettings as a part of one of the objects to be mapped.
The only thing you can use is a static class and update its members from another places, but this is a very, very, very bad solution. I did not tell you, it's almost illegal =)
Update by #Lucian Bargaoanu from comments
You can find answer in documentation here

How to ignore a property based on a runtime condition?

I have a simple pair of classes which for I've set up a mapping at initialization time.
public class Order {
public int ID { get; set; }
public string Foo { get; set; }
}
public class OrderDTO {
public int ID { get; set; }
public string Foo { get; set; }
}
...
Mapper.CreateMap<Order, OrderDTO>();
Now at a certain point I need to map an Order to an OrderDTO. BUT depending on some circumstances, I might need to ignore Foo during mapping. Let's also assume that I cannot "store" the condition in the source or destination object.
I know how I can configure the ignored properties at initialization time, but I have no idea how I could achieve such a dynamic runtime behavior.
Any help would be appreciated.
UPDATE
My use case for this behaviour is something like this. I have an ASP.NET MVC web grid view which displays a list of OrderDTOs. The users can edit the cell values individually. The grid view sends the edited data back to the server like a collection of OrderDTOs, BUT only the edited field values are set, the others are left as default. It also sends data about which fields are edited for each primary key. Now from this special scenario I need to map these "half-empty" objects to Orders, but of course, skip those properties which were not edited for each object.
The other way would be to do the manual mapping, or use Reflection somehow, but I was just thinking about if I could use AutoMapper in some way.

I've digged into the AutoMapper source code and samples, and found that there is a way to pass runtime parameters at mapping time.
A quick example setup and usage looks like this.
public class Order {
public int ID { get; set; }
public string Foo { get; set; }
}
public class OrderDTO {
public int ID { get; set; }
public string Foo { get; set; }
}
...
Mapper.CreateMap<Order, OrderDTO>()
.ForMember(e => e.Foo, o => o.Condition((ResolutionContext c) => !c.Options.Items.ContainsKey("IWantToSkipFoo")));
...
var target = new Order();
target.ID = 2;
target.Foo = "This should not change";
var source = new OrderDTO();
source.ID = 10;
source.Foo = "This won't be mapped";
Mapper.Map(source, target, opts => { opts.Items["IWantToSkipFoo"] = true; });
Assert.AreEqual(target.ID, 10);
Assert.AreEqual(target.Foo, "This should not change");
In fact this looks quite "technical", but I still think there are quite many use cases when this is really helpful. If this logic is generalized according to application needs, and wrapped into some extension methods for example, then it could be much cleaner.

Expanding on BlackjacketMack's comment for others:
In your MappingProfile, add a ForAllMaps(...) call to your constructor.
using AutoMapper;
using System.Collections.Generic;
using System.Linq;
public class MappingProfile : Profile
{
public MappingProfile()
{
ForAllMaps((typeMap, mappingExpression) =>
{
mappingExpression.ForAllMembers(memberOptions =>
{
memberOptions.Condition((o1, o2, o3, o4, resolutionContext) =>
{
var name = memberOptions.DestinationMember.Name;
if (resolutionContext.Items.TryGetValue(MemberExclusionKey, out object exclusions))
{
if (((IEnumerable<string>)exclusions).Contains(name))
{
return false;
}
}
return true;
});
});
});
}
public static string MemberExclusionKey { get; } = "exclude";
}
Then, for ease of use, add the following class to create an extension method for yourself.
public static class IMappingOperationOptionsExtensions
{
public static void ExcludeMembers(this AutoMapper.IMappingOperationOptions options, params string[] members)
{
options.Items[MappingProfile.MemberExclusionKey] = members;
}
}
Finally, tie it all together: var target = mapper.Map<Order>(source, opts => opts.ExcludeMembers("Foo"));

How to extend mef using custom attributes?

Is it possible to add some additional attributes to my components which are then set/hydrated using some custom logic/perhaps from a data store? Similar to adding some custom builder strategy in cab/unity ?
UPDATE
e.g.
assuming a class has these properties
[MyImport] string name1 { get; set }
[MyImport] MyType name2 { get; set }
[MyGuid] Guid { get; set; }
with custom attributes MyImport and MyGuid which are resolved by an "extension" to MEF ( which gets executed after the [imports] are resolved ) and has code along these lines
// property SET
var valu = myDBStore.GetValue( instanceGUID, propertyInfo.Name);
propertyInfo.SetValue( instance, TypeDescripter.GetConverter(valu).ConvertTo(propertyType), null);
// property GET - for example only, used during dehydration outside of MEF !
var valu = propertyInfo.GetValue( instance, null);
myDBStore.SetValue( instanceGUID, propertyInfo.Name, TypeDescripter.GetConverter(valu).ConvertTo(typeof(string));
// the above is pseudo code only, pls no comments on correct args/syntax :)

EDIT
components which are then set/hydrated using some custom logic/perhaps from a data store
One can do this via an "ExportFactory".
// "ExportFactory"
public sealed class DataStoreProvider
{
[Export(typeof(Model))]
public Model Item
{
get
{
return [custom logic];
}
}
}
public class NeedsModel
{
[Import(typeof(Model))]
public Model Item { get; set; }
}
Initial Answer
This is possible through MEF's Lazy<T, TMetadata>.
public interface ISomeMetadata
{
string UsefulInfo { get; }
}
[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class, AllowMultiple=false)]
public class ExportBaseAttribute : ExportAttribute, ISomeMetadata
{
public ExportBaseAttribute(string usefulInfo)
:base(typeof(BaseExport))
{
UsefulInfo = usefulInfo;
}
public string UsefulInfo { get; private set; }
}
// BaseExport class is not needed.. just showing advanced attribute usage.
public abstract class BaseExport { }
[ExportBase("Useful Filter Information")]
public class SomeExport : BaseExport
{
}
Then, in your host (composer), you can
[ImportMany(typeof(BaseExport))]
Lazy<BaseExport, ISomeMetadata>[] _baseExports
After you compose, you can run a LINQ filter using .Metadata
var goodExports = from export in _baseExports
where export.Metadata.UsefulInfo ...
select export;

Using Automappper to map to methods

I'm building an ASP.NET MVC 2 application in C# and i am successfully using Automapper to map values back and forth between ViewModels and business objects.
In addition to several explicit properties, my business objects wrap a dictionary as a catch all for properties that aren't explicitly defined. Something similar to:
public class MyBusinessObject {
public void SetExtraPropertyValue<T>(string key, T value) {
// ...
}
public T GetExtraPropertyValue<T>(string key, T defaultValue) {
// ...
}
}
In my ViewModel, I have the freedom to create any properties I want, but I cannot modify the business objects.
So let's say I create a view model like this:
class MyViewModel {
public string CustomProp { get; set; }
}
and the value I want to store and retrieve will need to use
businessModelInstance.SetExtraPropertyValue("CustomProp", newVal);
and
businessModelInstance.GetExtraPropertyValue("CustomProp");
I have problems going both directions.
First, when going from the MyBusinessObject to the MyViewModel, I thought it should be simple to do in my custom Automapper profile:
CreateMap<MyBusinessObject, MyViewModel>()
.ForMember(dest => dest.CustomProp,
opt => opt.MapFrom(s => s.GetExtraPropertyValue("CustomProp", "")));
However, MyBusinessObject.CustomProp is never populated, though other properties are.
Secondly, I don't know how to configure getting a value from MyViewModel.CustomProp to calling MyBusinessObject.SetExtraPropertyValue.
Is there a way to accomplish this
mapping with Automapper?
Is there a completely different attack that I
should be trying?
Do I have to resort to manual mapping in my controller? For example, MyBusinessObject.SetExtraPropertyValue("CustomProp",
MyViewModel.CustomProp)
UPDATE: Here is my solution based on #Patrick Steele's suggestions:
I added a custom attribute to the view model properties that i wanted to map to extra property keys. A custom TypeConverter uses reflection to find these attributes and map properties appropriately.
public class ItemExtraPropertyConverter : ITypeConverter<MyViewModel, MyBusinessObject>
{
public MyBusinessObject Convert(ResolutionContext context)
{
var destination = context.DestinationValue as MyBusinessObject;
if (destination == null )
throw new InvalidOperationException("Destination type is not of type MyBusinessObject");
foreach (var property in context.SourceType.GetProperties())
foreach (var attribute in property.GetCustomAttributes(true).OfType<ExtraPropertyAttribute>())
{
var key = attribute.Key;
if (string.IsNullOrEmpty(key))
key = property.Name;
destination.SetExtraPropertyValue(key, property.GetValue(context.SourceValue, null));
}
return destination;
}
}
public class ExtraPropertyAttribute : Attribute
{
public ExtraPropertyAttribute()
{
}
public ExtraPropertyAttribute(string key)
{
Key = key;
}
public string Key { get; set; }
}
public class MyViewModel
{
[ExtraProperty]
public string CustomProp { get; set; }
[ExtraProperty("OtherPropertyValue")]
public string CustomProp2 { get; set; }
}
In the custom profile class's configure method:
CreateMap<MyViewModel, MyBusinessObject>()
.ConvertUsing<ItemExtraPropertyConverter>();

My guess is that something is wrong with your GetExtraPropertyValue and SetExtraPropertyValue implementations. I threw together a quick test and the mapping you provided above worked as expected. Here's the implementation I used for the test:
public class MyBusinessObject
{
private readonly Dictionary<string, object> extraProperties = new Dictionary<string, object>();
public int Id { get; set; }
public string Name { get; set; }
public void SetExtraPropertyValue<T>(string key, T value)
{
extraProperties.Add(key, value);
}
public T GetExtraPropertyValue<T>(string key, T defaultValue)
{
if (extraProperties.ContainsKey(key))
{
return (T)extraProperties[key];
}
return defaultValue;
}
}