.net web API的CORS问题

我在API中面临CORS问题。得到以下回应：

> {"$id":"1","Message":"The requested resource does not support http method 'OPTIONS'."}

在web.config中完成允许“选项”方法的设置。但仍然不起作用。下面是我的web.config设置

`<system.webServer>
      <httpProtocol>
        <customHeaders>
          <add name="Access-Control-Allow-Origin" value="*" />
          <add name="Access-Control-Allow-Headers" value="Content-Type" />
          <add name="Access-Control-Allow-Methods" value="GET,POST,PUT,DELETE,OPTIONS" />
         <add name="Access-Control-Allow-Credentials" value="true" />
        </customHeaders>
      </httpProtocol>
 </system.webServer>
`

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