Managed C++/CLI

// EXAMPLE 1: C#
using System;

namespace ManagedCode {
    public static class DateTimeExtension {
        public static double Foo(this DateTime dt) {
            return 83.5;
        }
    }
}

#using "ManagedCode.dll"
#using <mscorlib.dll>
#using <system.dll>

using namespace ManagedCode;
using ManagedCode::DateTimeExtension;
using System::DateTime;

public int main(void) {
    DateTime foo;
    double temp = foo.Foo();
    return 0;
}

// Example 2: C++
#using <mscorlib.dll>
#using <system.dll>

class DateTime : public System::DateTime {
public:
    DateTime();
    double Foo();
};

// Example 3: C++
#using <mscorlib.dll>
#using <system.dll>

ref class DateTime : public System::DateTime {
protected:
    ~DateTime();
public:
    DateTime();
    !DateTime();
    double Foo();
};