Develop Reference

Apply ValidationAttributes from DataAnnotation to all elements of an IEnumerable

I am using Microsoft.Extension.Options in ASP.NET Core 3.1 and I want to validate entries in an configuration file.
For this I want that, e.g. a RangeAttribute is applied to each element of an IEnumerable.
class MyConfiguration
{
[ApplyToItems]
[Range(1, 10)]
publlic IList<int> MyConfigValues { get; set; }
}
Or something like that. How do I write the ApplyToItems method?
As far as I know there is no way to retrieve the other ValidationAttributes while a possible ApplyToItems is validated.
Alternatively I could imagine something like:
[Apply(Range(1, 10)]
public List<int> MyConfigValues { get; set; }
but is that even valid syntax? How would I write an Attribute like Apply that takes other Attributes as parameter without falling back on something like
[Apply(new RangeAttribute(1, 10)]
which does not look nice.

To create a custom data annotation validator follow these gudelines:
Your class has to inherit from System.ComponentModel.DataAnnotations.ValidationAttribute class.
Override bool IsValid(object value) method and implement validation logic inside it.
That's it.
(from How to create Custom Data Annotation Validators)
So in your case it could be something like this:
public class ApplyRangeAttribute : ValidationAttribute
{
public int Minimum { get; set; }
public int Maximum { get; set; }
public ApplyRangeAttribute()
{
this.Minimum = 0;
this.Maximum = int.MaxValue;
}
public override bool IsValid(object value)
{
if (value is IList<int> list)
{
if (list.Any(i => i < Minimum || i > Maximum))
{
return false;
}
return true;
}
return false;
}
}
Edit
Here's how you would use it:
class MyConfiguration
{
[ApplyRange(Minimum = 1, Maximum = 10)]
public IList<int> MyConfigValues { get; set; }
}

How to define an attribute to get the property name on which it is declared in c#?

In my DB, all the tables have the status filed, but every table has a different name for that column. For e.g. the user table has user_status and the branch table has branch_status. All these columns of different tables would be having the same value. I have created POCO entities for all and wanted to create a generic function that would perform a query on the status field of the specified POCO entity class. So I wanted to create an attribute stating that this would be the status field of the specified entity class.
So, attribute that will give me the property name on which it is declared. The attribute would be declared only on a single property in the class. Till now I have done the following and it is working but wanted to know the efficient way to achieve the same. Below is my code:
Custom Attribute:
[AttributeUsage(AttributeTargets.Property, Inherited = true, AllowMultiple = false)]
public class StatusFieldAttribute : Attribute
{
}
Declaration
public class UserTable
{
public int UserId{ get; set; }
[StatusField]
public int UserStatus { get; set; }
}
public class BranchTable
{
public int BranchId{ get; set; }
[StatusField]
public int BranchStatus { get; set; }
}
I want to get the property name from UserTable and BranchTable having the StatusField Attribute. I have achieve the same as:
Type type = typeof(UserTable);
string statusFieldName = type.GetProperties().Where(prop => Attribute.IsDefined(prop, typeof(StatusFieldAttribute))).FirstOrDefault().Name;
And the above code gives the proper output as UserStatus. But is there an efficient way to achieve the same using something like below:
[AttributeUsage(AttributeTargets.Property, Inherited = true, AllowMultiple = false)]
public class StatusFieldAttribute : Attribute
{
public StatusFieldAttribute([CallerMemberName] string propertyName = null)
{
StatusFieldName = propertyName;
}
public string StatusFieldName { get; }
}
And access the StatusFieldName to get the name of the property on which the StatusField attribute is declared in a particular class.

One approach is to define a marker interface (prior to C# 8), and define an extension method to retrieve the proper status field, like so:
interface IStatusFieldMarker
{
}
static class Extensions
{
public static string GetStatusFieldName(this IStatusFieldMarker t) =>
t.GetType()
.GetProperties()
.Single(p => Attribute.IsDefined(p, typeof(StatusFieldAttributeAttribute)))
.Name;
// optional getter/setter
public static int GetStatus(this IStatusFieldMarker t) =>
(int)(t.GetType()
.GetProperty(GetStatusFieldName(t))
.GetValue(t));
public static void SetStatus(this IStatusFieldMarker t, int value)
{
t.GetType()
.GetProperty(GetStatusFieldName(t))
.SetValue(t, value);
}
}
Then mark the POCO objects with the interface:
class Branch : IStatusFieldMarker
{
[StatusFieldAttribute]
public int BranchStatus { get; set; }
}
class User : IStatusFieldMarker
{
[StatusFieldAttribute]
public int UserStatus { get; set; }
}
You can then call the GetStatusFieldName extension method to get the field name:
Branch b = new Branch() { BranchStatus = 3 };
User u = new User() { UserStatus = 1 };
Console.WriteLine(b.GetStatusFieldName()); // prints BranchStatus
Console.WriteLine(u.GetStatusFieldName()); // prints UserStatus
Console.WriteLine(b.GetStatus()); // prints 3
Console.WriteLine(u.GetStatus()); // prints 1

I simply created below function to get the status name
public static string GetStatusField<T>(this T type) where T : Type
{
PropertyInfo propertyInfo = type.GetProperties().FirstOrDefault(prop => Attribute.IsDefined(prop, typeof(StatusFieldAttribute)));
if (propertyInfo == null)
{
Log.Error($"StatusFieldAttribute not found in {type.Name} ");
throw new Exception($"StatusFieldAttribute not found in { type.Name } ");
}
else
return propertyInfo.Name;
}
And invoked the function as
Type type = typeof(UserTable);
string statusFieldName = type.GetStatusField();

Protobuf-net Skip data during serialization

I use surrogates and I'd like to perform a check to skip wrong items during the serialization process, but I can't find a way to do it, any idea?
BeforeSerialize method is called after the creation of the surrogate and I'd like to know how to skip items that have no the requirements specified in the protoBuf serialization context.
Here following, a sample code to reproduce my scenario.
public class Person
{
public Person(string name, GenderType gender)
{
Name = name;
Gender = gender;
}
public string Name { get; set; }
public GenderType Gender { get; set; }
}
public class PersonSurrogate
{
public string Name { get; set; }
public byte Gender { get; set; }
public PersonSurrogate(string name, byte gender)
{
Name = name;
Gender = gender;
}
protected virtual bool CheckSurrogateData(GenderType gender)
{
return gender == GenderType.Both || (GenderType)Gender == gender;
}
#region Static Methods
public static implicit operator Person(PersonSurrogate surrogate)
{
if (surrogate == null) return null;
return new Person(surrogate.Name, (GenderType)surrogate.Gender);
}
public static implicit operator PersonSurrogate(Person source)
{
return source == null ? null : new PersonSurrogate(source.Name, (byte)source.Gender);
}
#endregion
protected virtual void BeforeSerialize(ProtoBuf.SerializationContext serializationContext)
{
var serializer = serializationContext.Context as FamilySerializer;
if (serializer == null)
throw new ArgumentException("Serialization context does not contain a valid Serializer object.");
if (!CheckSurrogateData(serializer.GenderToInclude))
{
// ** How can I exclude this item from the serialization ? **//
}
}
}
[Flags]
public enum GenderType : byte
{
Male = 1,
Female = 2,
Both = Male | Female
}
/// <summary>
/// Class with model for protobuf serialization
/// </summary>
public class FamilySerializer
{
public GenderType GenderToInclude;
public RuntimeTypeModel Model { get; protected set; }
protected virtual void FillModel()
{
Model = RuntimeTypeModel.Create();
Model.Add(typeof(Family), false)
.SetSurrogate(typeof(FamilySurrogate));
Model[typeof(FamilySurrogate)]
.Add(1, "People") // This is a list of Person of course
.UseConstructor = false;
Model.Add(typeof(Person), false)
.SetSurrogate(typeof(PersonSurrogate));
MetaType mt = Model[typeof(PersonSurrogate)]
.Add(1, "Name")
.Add(2, "Gender");
mt.SetCallbacks("BeforeSerialize", null, null, null); // I'd like to check surrogate data and skip some items - how can I do?
mt.UseConstructor = false; // Avoids to use the parameterless constructor.
}
}

What you describe is not a scenario that the serializer currently makes any attempt to target. Conditional serialization is supported on a per-property/field basis, but not on a per object basis.
There might still be ways to get it to work, though - but it depends on what the context is, i.e. what is it that has a Person object in your model? And can you change that model at all? (possibly not, since you're using surrogates).
My default answer, as soon as things stop working cleanly, is to say: create a separate DTO model for your serialization work, and populate that with the data that you intend to serialize - rather than fighting to get your regular domain model to play nicely with complex serialization requirements.

Validation strategy

I'm trying to build a series of attribute classes to make it easier for our development team to validate objects. The objects are POCO classes like this.
public class User
{
public string Name { get; set; }
public string Company { get; set; }
}
I want to decorate this model with a custom attribute.
public class User
{
[MustHaveValue]
public string Name { get; set; }
public string Company { get; set; }
}
Then I would create my own class implementing ValidationAttribute, the base class in .NET Framework, which belongs to System.ComponentModel.DataAnnotations.
public class MustHaveValueAttribute : ValidationAttribute
{
.
.
public override IsValid(object value)
{
// validation logic.
}
}
And then I can validate the User model whenever I want by making the set of instances like ValidationContext, List<ValidationResult>.
But in an enterprise environment, problems just can't be solved by a specific class. My validation scenario requires more complex and more flexible ways. Imagine that one of the required validation scenarios would something like this.
public class User
{
public string Name { get; set; }
public string Company { get; set; }
// Check if an item exists in this list.
[MustHaveMoreThanOneItem]
public IList<Client> Clients { get; set; }
}
Then I would need to make another attribute class
public class MustHaveMoreThanOneItemAttribute : ValidationAttribute
{
.
.
public override IsValid(object value)
{
// Let's assume this value is List<Client> for now.
// I know the exact type, so I'm going to cast it to List<Client> without further considerations
List<Client> clients = value as List<Client>;
if(clients.Count > 0) {
return true;
} else {
return false;
}
}
}
But the problem is that there are a lot of other models that have a nested list items. Try to imagine the time when I want to reuse the MustHaveMoreThanOneItem in one of the other models like...
public class Department
{
public string Name { get; set; }
[MustHaveMoreThanOneItem]
public IList<Employee> { get; set; }
}
You already know that it's not going to work because it was strongly typed only for List<Client>. So I decided to use Generic there to solve this problem.
But to my disappointment, the _Attribute interface doesn't support Generic. There's no additional implementation like _Attribute<T> : Attribute and therefore, no ValidationAttribute<T> alas!! I just cannot use Generic here !!
public class Department
{
public string Name { get; set; }
// No way to use this syntax.
[MustHaveMoreThanOneItem<Employee>]
public IList<Employee> { get; set; }
}
So I made a conclusion that Attribute must have been designed for a fixed set of validations like email format, card format, null check, and etc IMAO.
But I still want to use an attribute and give a lot of flexibilities in it to prevent the duplicated, verbose validation codes like this.
if(model.Clients.Count > 0) ...
if(model.Name != null) ...
if(model.Clients.GroupBy(x => x.Country == Country.USA).Count >= 1) ...
if(model.Clients.Where(x => x.CompanyName == Company.Google).ToList().Count > 1 ) ...
.
.
.
I want to pose two questions here.
If Attirbute supports Generic, this problem will be solved?
Is there any way to implement Generic Attribute? in order to use
[MustHaveMoreThanOneItem<Employee>] annotation on a class member?

You can generically check any object that implements IEnumerable like this:
public class MustHaveMoreThanOneItemAttribute : ValidationAttribute
{
public override bool IsValid(object value)
{
// omitted null checking
var enumerable = value as IEnumerable;
var enumerator = enumerable.GetEnumerator();
if (!enumerator.MoveNext())
{
return false;
}
if (!enumerator.MoveNext())
{
return false;
}
return true;
}
}

C# by definition does not support generic type attributes, although this has been requested actively for a long time:
https://github.com/dotnet/roslyn/issues/953
https://github.com/dotnet/csharplang/issues/124
However, you can still inject a type into a validation attribute via constructor. You then can use reflection or whatever you need to define your custom validation criteria.
public class MustHaveMoreThanOneItemAttribute : ValidationAttribute
{
public Type EnumerableType { get; }
public MustHaveMoreThanOneItemAttribute(Type t)
=> this.EnumerableType = typeof(ICollection<>).MakeGenericType(t);
public override bool IsValid(object value)
{
var count = this.EnumerableType.GetProperty("Count").GetValue(value) as int?;
return (count ?? 0) > 1;
}
}
Now this allows you to use something similar to your goal:
public class Department
{
public string Name { get; set; }
[MustHaveMoreThanOneItem(typeof(Employee))]
public IList<Employee> { get; set; }
}

C# access custom attributes of a partial class on MVC with EF

My situation is the following:
I'm coding a MVC website on Visual Studio 2013 using database-first approach with Entity Framework.
EF automatically generates the models. But I need to add custom attributes (~NOT~ necessarily for data validation but also for internal processes) and, via reflection, access those custom attributes.
Let's say I have
public partial class Application {
public int AppID {get; set;}
public string Name {get; set;}
//etc...
}
I've tried the following approaches:
• On a different file I continue the partial class:
public partial class Application {
[MyAttributeOne]
public int AppID { get; set; }
[DataType(DataType.Text)]
[MyAttributeTwo]
public string Name { get; set; }
}
• Use the MetaData class
public class ApplicationMetadata {
[MyAttributeOne]
public int SolutionID { get; set; }
[DataType(DataType.Text)]
[MyAttributeTwo]
public string Name { get; set; }
}
[MetadataType(typeof(ApplicationMetadata))]
public partial class Application { }
• Inherit the class with attributes:
public class ApplicationMetadata {
[MyAttributeOne]
public int SolutionID { get; set; }
[DataType(DataType.Text)]
[MyAttributeTwo]
public string Name { get; set; }
}
public partial class Application : ApplicationMetadata { }
• And the 'Buddy class' approach where I do basically the previous 2 approaches but instead I define the class with the attributes inside the 'Application' class.
Am I doing something wrong? Or is this simply impossible?
I need to be able to make the following code work:
foreach (PropertyInfo propertyInfo in currentObject.GetType().GetProperties())
{
foreach (CustomAttributeData attrData in propertyInfo.GetCustomAttributesData())
{
if (typeof(attrData) == typeof(MyAttributeOne))
//stuff
else if (typeof(attrData) == typeof(MyAttributeTwo))
//different stuff
else
//yet more stuff
}
}
Thank you very much for the attention!
Regards.

OK, this is a little involved but it's fairly simple. This is also really a bit of a brain dump but it does work and gives you enough to work with. Lets set up with some basics:
//A couple of custom attributes
public class MyAttributeOne : Attribute { }
public class MyAttributeTwo : Attribute { }
//A metadata class where we can use the custom attributes
public sealed class MyEntityMetadata
{
//This property has the same name as the class it is referring to
[MyAttributeOne]
public int SomeProperty { get; set; }
}
//And an entity class where we use System.ComponentModel.DataAnnotations.MetadataType
//to tell our function where the metadata is stored
[MetadataType(typeof(MyEntityMetadata))]
public class MyEntity
{
public int SomeProperty { get; set; }
}
OK, still with me? Now we need a function to process the properties in the same way you did earlier:
public void DoStuff(object currentObject)
{
//Lets see if our entity class has associated metadata
var metaDataAttribute = currentObject.GetType()
.GetCustomAttributes()
.SingleOrDefault(a => a is MetadataTypeAttribute) as MetadataTypeAttribute;
PropertyInfo[] metaProperties = null;
//Cache the metadata properties here
if (metaDataAttribute != null)
{
metaProperties = metaDataAttribute.MetadataClassType.GetProperties();
}
//As before loop through each property...
foreach (PropertyInfo propertyInfo in currentObject.GetType().GetProperties())
{
//Refactored this out as it's called again later
ProcessAttributes(propertyInfo.GetCustomAttributes());
//Now check the metadata class
if (metaProperties != null)
{
//Look for a matching property in the metadata class
var metaPropertyInfo = metaProperties
.SingleOrDefault(p => p.Name == propertyInfo.Name);
if (metaPropertyInfo != null)
{
ProcessAttributes(metaPropertyInfo.GetCustomAttributes());
}
}
}
}
And of course, here is the refactored method to process the attributes:
private void ProcessAttributes(IEnumerable<Attribute> attributes)
{
foreach (var attr in attributes)
{
if (attr is MyAttributeOne)
{
Console.WriteLine("MyAttributeOne found");
}
else if (attr is MyAttributeTwo)
{
Console.WriteLine("MyAttributeTwo found");
}
else
{
}
}
}