Change custom attribute's parameter at runtime

Question

I need change attribute's parameter during runtime. I simplified my problem to simple example.

Attribute class:

    [AttributeUsage(AttributeTargets.Property)]
    public class MyAttribute : Attribute
    {
        public string Name { get; set; }
    }

Simple entity which has decorated properties with attributes:

    public class MyEntity
    {
        [MyAttribute(Name="OldValue1")]
        public string Data1{ get; set; }

        [MyAttribute(Name = "OldValue2")]
        public string Data2 { get; set; }
    }

I created instance of class MyEntity. I can change value of object's properties, but I can't change value of attribute’s property Name on object entity. Is it possible?

Value of property on object entity I can change with this part of code:

                entityProp.SetValue(entity,"NewData",null);

but I don't how change value of attribute's property Name on object entity

This does not work:

attProp.SetValue(attribute,"NewData",null);

Value of property Name is still original.

Here is all test code.

    [TestMethod]
    public  void Test()
    {
        var entity = new MyEntity
                         {
                             Data1 = "OldData",
                             Data2 = "OldData"
                         };

        PropertyInfo[] entityProps = entity.GetType().GetProperties();

        foreach (var entityProp in entityProps)
        {
            var attribute = Attribute.GetCustomAttribute(entityProp, typeof (MyAttribute)) as MyAttribute;

            if (attribute != null)
            {
                //get attribute's property NAME
                PropertyInfo attProp= attribute.GetType().GetProperty("Name");

                //get entity property value
                var propertyValue = entityProp.GetValue(entity, null);

                //get attribute’s property NAME value
                var atributeNameValue = attProp.GetValue(entity, null);

                TestContext.WriteLine(string.Format("property name:{0} property value: {1} : atribute name value: {2}\n", 
                    entityProp.Name, propertyValue, atributeNameValue)); 

                //change values
                entityProp.SetValue(entity,"NewData",null);

                //how can I change value of property Name on object entity ?
                attProp.SetValue(attribute,"NewData",null);
                
            }

        }

        TestContext.WriteLine(string.Format("After change\n"));

        foreach (var entityProp in entityProps)
        {
            var attribute = Attribute.GetCustomAttribute(entityProp, typeof(MyAttribute)) as MyAttribute;

            if (attribute != null)
            {

                PropertyInfo attProp = attribute.GetType().GetProperty("Name");

                var propertyValue = entityProp.GetValue(entity, null);
                var atributeNameValue = attProp.GetValue(entity, null);

                TestContext.WriteLine(string.Format("property name:{0} property value: {1} : atribute name value: {2}\n",
                    entityProp.Name, propertyValue, atributeNameValue));
            }
        }
    }

EDITED: I delete original post and added very simple clear sample. Sorry

Answer 1

This is not possible with reflection, as (as already noted) the metadata is fixed. It is, however, partly possible with TypeDescriptor, which allows adding and replacing of attributes at runtime, and providing complete alternative models (TypeDescriptionProvider, etc). This approach will not be respected by any code that uses reflection, but any code using TypeDescriptor (most typically, data-binding and other UI code) will notice the changes.

Note TypeDescriptor only really works with one of each attribute-type per typ/member; multi-instance attributes are not well supported.

Answer 2

You cannot change attributes at runtime. They are embedded into the metadata of the assembly. Your method is changing the internal state of a particular instance; but when you load the attribute again, you are getting a different instance.