[Dev tip] When does attribute code run?

class Program
static void Main(string[] args)
Console.WriteLine("Creating MyClass instance");
MyClass mc = new MyClass();
Console.WriteLine("Setting value in MyClass instance");
mc.Value = 1;
Console.WriteLine("Getting attributes for MyClass type");
object[] attributes = typeof(MyClass).GetCustomAttributes(true);


public class MyAttribute : Attribute
public MyAttribute()
Console.WriteLine("Running constructor");

class MyClass
public int Value { get; set; }

And what is the output?

Creating MyClass instance
Setting value in MyClass instance
Getting attributes for MyClass type
Running constructor
So, the attribute constructor is run when we start to examine the attribute. Note that the attribute is fetched from the type, not the instance of the type.

Call validation attribute run manually

var vc = new ValidationContext(myObject, null, null);
return Validator.TryValidateObject(myObject, vc, null, true);

You can also validate attributes on a single named property. You can also optionally pass in a list in order to access the error messages:

var results = new List<ValidationResult>();
var vc = new ValidationContext(myObject, null, null) { MemberName = "UserName"};
var isValid = Validator.TryValidateProperty(value, vc, results);

// get all the errors
var errors = Array.ConvertAll(results.ToArray(), o => o.ErrorMessage);

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