为什么此代码返回“不支持指定的方法”。

Question

为什么此代码返回“不支持指定的方法”。

using System;
using System.IO;
using System.Security.Cryptography;
using System.Web.Mvc;

public class ETagAttribute : ActionFilterAttribute
{
    private string GetToken(Stream stream) {
        MD5 md5 = MD5.Create();
        byte [] checksum = md5.ComputeHash(stream);
        return Convert.ToBase64String(checksum, 0, checksum.Length);
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        filterContext.HttpContext.Response.AppendHeader("ETag", GetToken(filterContext.HttpContext.Response.OutputStream));
        base.OnResultExecuted(filterContext);
    }
}

这应该有效，但不是。

显然微软覆盖了 System.Web.HttpResponseStream.Read(Byte[] buffer, Int32 offset, Int32 count) 以便它返回“不支持指定的方法。”，不知道为什么他们会这样做，因为它继承了系统。 IO.Stream 基类...

堆栈跟踪：

[NotSupportedException: Specified method is not supported.]
   System.Web.HttpResponseStream.Read(Byte[] buffer, Int32 offset, Int32 count) +29
   ETagAttribute.ReadFully(Stream input) in Filters\ETagAttribute.cs:11
   ETagAttribute.GetToken(Stream stream) in Filters\ETagAttribute.cs:22
   ETagAttribute.OnResultExecuted(ResultExecutedContext filterContext) in Filters\ETagAttribute.cs:29
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +282
   System.Web.Mvc.<>c__DisplayClass1e.<InvokeActionResultWithFilters>b__1b() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +260
   System.Web.Mvc.<>c__DisplayClass1e.<InvokeActionResultWithFilters>b__1b() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +177
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +343
   System.Web.Mvc.Controller.ExecuteCore() +116
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +97
   System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +10
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +37
   System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +21
   System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +50
   System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7
   System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +22
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +60
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8920029
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

Answer 1

HttpResponse.OutputStream用于写入，而不是从. 它基本上是一个只写流 - 就像Output属性返回一个TextWriter.