Convert.ChangeType.cs

private static T getValue<T>(object source, IFormatProvider provider)
{
    Type type = Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T);
    if (source == null)
    {
        if (typeof(T).IsValueType && typeof(T) == type)
        {
            throw new InvalidCastException();
        }
        return default(T);
    }
    if (source is IConvertible)
    {
        source = System.Convert.ChangeType(source, type, provider);
    }
    return (T)source;
}