类型转化方法（处理System.Nullable类型）

 public static object ChangeType(object value, Type conversionType)
        {
            if (conversionType.IsGenericType && conversionType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
            {
                if (value != null)
                {
                    NullableConverter nullableConverter = new NullableConverter(conversionType);
                    conversionType = nullableConverter.UnderlyingType;
                }
            }
            return Convert.ChangeType(value, conversionType);
        }