C# Generic Method, cannot implicit convert

You cannot convert to T because of the fact that T isn't known at compile time. If you want to get your code to work you can change the return type to ICar and remove the generic T return type.

You also can cast to T. This would work too. If you only using the default constructor you can also constain on new() and use new T() to get your code to work.

Samples

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;
}

Cast:

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;
}

New-constraint:

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

Your code is illegal because while you might be testing and know that your given T is BigCar or some other such type, the compiler cannot know that in advance and therefore the code is illegal. Based upon your given usage, you could have

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

The new() constraint allows you to invoke the default (parameterless) constructor on a type.

You can simplify your code

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