C＃可空字符串错误 [英] C# nullable string error

问题描述

private string? typeOfContract
{
  get { return (string?)ViewState["typeOfContract"]; }
  set { ViewState["typeOfContract"] = value; }
}

后来在code我用这样的：

Later in the code I use it like this:

typeOfContract = Request.QueryString["type"];

我收到以下错误在typeOfContract行的声明，说明

I am getting the following error at the declaration of typeOfContract line stating

类型字符串必须是一个非空的值类型才能使用
  它作为参数的泛型类型或方法T
  System.Nullable

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

任何想法？基本上，我希望确保类型，在查询字符串中存在执行行动之前。

Any ideas? Basically, I want to make sure that "type" exists in the queryString before performing an action.

推荐答案

System.String是引用类型和已经可为空。

System.String is a reference type and already "nullable".

＆可空LT; T>和？后缀是值类型，如的Int32，双，日期时间等。

Nullable<T> and the ? suffix are for value types such as Int32, Double, DateTime, etc.

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