using System;

using System.Reflection.Emit;

using System.Linq;

public class Program

{

    public static void Main()

    {

        float a = 1.2345f;

        a = (int)a;

        System.Console.WriteLine(a);

        CanAddAttribute();

    }

    public class SomeAttribute : Attribute

    {

        public SomeAttribute(string value)

        {

            this.Value = value;

        }

​

        public string Value { get; set; }

    }

​

    public class SomeClass

    {

        public string Value = "Test";

    }

​

    static public void CanAddAttribute()

    {

        var type = typeof(SomeClass);

​

        var aName = new System.Reflection.AssemblyName("SomeNamespace");

        var ab = AppDomain.CurrentDomain.DefineDynamicAssembly(aName, AssemblyBuilderAccess.Run);

        var mb = ab.DefineDynamicModule(aName.Name);

        var tb = mb.DefineType(type.Name + "Proxy", System.Reflection.TypeAttributes.Public, type);

​

        var attrCtorParams = new Type[] { typeof(string) };

        var attrCtorInfo = typeof(SomeAttribute).GetConstructor(attrCtorParams);

        var attrBuilder = new CustomAttributeBuilder(attrCtorInfo, new object[] { "Some Value" });

        tb.SetCustomAttribute(attrBuilder);

​

        var newType = tb.CreateType();

        var instance = (SomeClass)Activator.CreateInstance(newType);

​

        var attr = (SomeAttribute)instance.GetType()

            .GetCustomAttributes(typeof(SomeAttribute), false)

            .SingleOrDefault();

        System.Console.WriteLine( attr.Value );

    }

}