为什么我在“HttpException”的 Web API 项目中收到缺少命名空间错误

问：

在 ASP.NET Web Api 2 Building a REST Service from Start to Finish 一书之后，我遇到了一个命名空间错误，我无法针对 HttpException 参考专门解决该错误。

var httpException = exception as HttpException;

我正在使用 C#，我只看到 VB.NET 中使用的样式，因此预期的语法样式可能存在一些问题。更正建议一直尝试将 h 小写以使 httpException，但 HttpException 是预期的

以下是完整代码：

    using System.Net;
    using System.Net.Http;
    using System.Web; // bad practice: not usually supposed to be used on non 
    // web application projects
    using System.Web.Http.ExceptionHandling;
    using WebApi2Book.Data.Exceptions;

    namespace WebApi2Book.Web.Common.ErrorHandling
    {
    public class GlobalExceptionHandler : ExceptionHandler
    {
        public override void Handle(ExceptionHandlerContext context)
        {
            var exception = context.Exception;

            var httpException = exception as HttpException; <- red squiggle
            if (httpException != null)
            {
                context.Result = new SimpleErrorResult(context.Request, (HttpStatusCode)httpException.GetHttpCode(), httpException.Message);
                return;
            }
            if (exception is RootObjectNotFoundException)
            {
                context.Result = new SimpleErrorResult(context.Request, HttpStatusCode.NotFound, exception.Message);
                return;
            }
            if (exception is ChildObjectNotFoundException)
            {
                context.Result = new SimpleErrorResult(context.Request, HttpStatusCode.Conflict, exception.Message);
                return;
            }
            context.Result = new SimpleErrorResult(context.Request, HttpStatusCode.InternalServerError, exception.Message);
        }
    }
}