A (probably) quite common problem with numerics

1. functional programming style for template programming
2. define default behavior in a templated item, add specialization for the types in question (e.g. for the several numeric types)
3. C++ allows to have values as template parameters (e.g. not only types but also compile-time constants)

[AttributeUsage(AttributeTargets.Class)]
public class MyAttrAttribute : Attribute
{
    public dynamic Arg { get; private set; } // dynamic attribute
    public MyAttrAttribute(object arg)
    {
        Arg = arg;
    }
    public static MyAttrAttribute GetAttribute(Type owner)
    {
        return (from a in owner.GetCustomAttributes(false)
                where a.GetType() == typeof(MyAttrAttribute)
                select a
                ).FirstOrDefault() as MyAttrAttribute;
    }
}

[MyAttr(123)]
public class MyClassWithIntAttr
{
    public MyClassWithIntAttr()
    {
        var attr = MyAttrAttribute.GetAttribute(
                     MethodBase.GetCurrentMethod().DeclaringType);
        Console.WriteLine("{0} = {1}", attr, attr.Arg + 42); // may throw exception
    }
}
[MyAttr(3.14159265358)]
public class MyClassWithDoubleAttr
{
    public MyClassWithDoubleAttr()
    {
        var attr = MyAttrAttribute.GetAttribute(
                     MethodBase.GetCurrentMethod().DeclaringType);
        Console.WriteLine("{0} = {1}", attr, attr.Arg + 42); // may throw exception
    }
}

class Program
{
    public static void Main()
    {
        MyClassWithIntAttr c1 = new MyClassWithIntAttr();
        MyClassWithDoubleAttr c2 = new MyClassWithDoubleAttr();
    }

Sample.MyAttrAttribute = 165
Sample.MyAttrAttribute = 45.14159265358

[AttributeUsage(AttributeTargets.Class)]
public class MyAttrAttribute : Attribute
{
    private object _arg;
    public MyAttrAttribute(object arg)
    {
        _arg = arg;
    }
    // cast to a needed type
    public static T As<T>(Type owner)
    {
        var attr = (from a in owner.GetCustomAttributes(false)
                where a.GetType() == typeof(MyAttrAttribute)
                select a
                ).FirstOrDefault() as MyAttrAttribute;
        return attr != null ? (T)attr._arg : default(T);
    }

}

[MyAttr(123)]
public class MyClassWithIntAttr
{
    public MyClassWithIntAttr()
    {
        int value = MyAttrAttribute.As<int>(this.GetType());
        Console.WriteLine("attr + 42 = {0}", value + 42);
    }
}
[MyAttr(3.14159265358)]
public class MyClassWithDoubleAttr
{
    public MyClassWithDoubleAttr()
    {
        double value = MyAttrAttribute.As<double>(this.GetType());
        Console.WriteLine("attr + 42 = {0}", value + 42);
    }
}
class Program
{
    public static void Main()
    {
        MyClassWithIntAttr c1 = new MyClassWithIntAttr();
        MyClassWithDoubleAttr c2 = new MyClassWithDoubleAttr();
    }
}

attr + 42 = 165
attr + 42 = 45.14159265358