public static T GetCar<T>() where T : ICar
{
    T objCar = default(T);

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar("");
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar("");
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar("");
    }

    return objCar;
}

public ICar GetCar<T>()
    where T : ICar
{
    ICar objCar = null;

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar();
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar();
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar();
    }

    return objCar;
}

public T GetCar<T>()
    where T : ICar
{
    Object objCar = null;

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar();
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar();
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar();
    }

    return (T)objCar;
}

public T GetCar<T>()
    where T : ICar, new()
{
    return new T();
}

public static T GetCar<T>() where T : ICar, new()
{
    return new T();
}

public static T GetCar<T>()
    where T : ICar, new()
{
    return new T();
}

class Program
{
    static void Main(string[] args)
    {
        ICar smallCar = Helper.GetCar<SmallCar>("car 1");
        ICar mediumCar = Helper.GetCar<MediumCar>("car 2");

        Console.ReadLine();
    }
}

static class Helper
{
    public static T GetCar<T>(string carName) where T : ICar
    {
        ICar objCar = default(T);

        if (typeof(T) == typeof(SmallCar))
        {
            objCar = new SmallCar { CarName = carName };
        }
        else if (typeof(T) == typeof(MediumCar))
        {
            objCar = new MediumCar { CarName = carName };
        }

        return (T)objCar;
    }

}

interface ICar
{
    string CarName { get; set; }
}

class SmallCar : ICar
{
    public string CarName { get; set ; }
}

class MediumCar : ICar
{
    public string CarName { get; set; }
}