无法创建 cookie，在 asp.net 核心（剃须刀页面）中获取“标题是只读的，响应已经开始”

Question

I'm trying to set a cookie, but I get "Headers are read-only, response has already started."

The method that write the cookie is in a property returning an User object in Utility.cs and Gallery.cshtml invokes the property ( img.UserVoteExists(_util.User) ) through the Utility object thats defined in ViewImports

Utility.cs :

public User User
{
    get
    {
        User result = new User();

        if (this.Context.Request.Cookies.ContainsKey(UserCookieName))
        {
            result = JsonConvert.DeserializeObject<User>(this.Context.Request.Cookies[UserCookieName]);
        }
        else
        {
            result = JsonConvert.DeserializeObject<User>(InvokePost("CreateUser", new CreateUserContextModel() { User = result }).Result);
        }

        this.Context.Response.Cookies.Append(UserCookieName, JsonConvert.SerializeObject(result), new CookieOptions() { Expires = DateTime.Now.AddDays(120) });

        return result;
    }
}

_ViewImports.cshtml :

@inject IUtility _util

Gallery.cshtml :

<div id="images" class="images-wrap list-small">
    @foreach (Image img in Model.CurrentItem.Images)
    {
        <div class="item-wrap" data-index="@img.Index" data-filename="@img.Filename">
            <img src="/IMAGES/Gallery/Items/@Html.Raw(Model.Name.ToCamelCase() + "/Thumb/" + img.Filename)" class="img-fluid item" />
            <div class="info-wrap">
                <div class="header-wrap">
                    <div class="vote-wrap">
                        <div class="circle clickable @Html.Raw(img.UserVoteExists(_util.User).AppendIfTrue("d-none"))">
                            <lc:fa name="thumbs-up" prefix="far" class="icon" />
                        </div>
                        <div class="circle @Html.Raw(img.UserVoteExists(_util.User).AppendIfFalse("d-none"))">
                            <lc:fa name="thumbs-up" prefix="fas" class="icon" />
                        </div>
                    </div>
                    <div class="caption-wrap">
                        <h2 class="caption">@img.Name</h2>
                    </div>
                </div>

                <p class="description-wrap">
                    <span class="caption">@Html.Raw(img.Description)</span>
                </p>
            </div>
        </div>
    }
</div>

Answer 1

By the time you have started rendering the view, it is probably too late, as the HTML for the view has to be rendered into the body, and the body comes after the headers in the HTTP stream. So if you're going to set something, probably best to do it in the controller before returning the view. One way to do it is to perform the check and set the cookie in a callback function, then provide the callback function to the HttpResponse's AddOnSendingHeaders method.

If you can't restructure your code, you can work around the issue by turning on response buffering , which allows random access to the HTTP response, hence allowing the headers to be written and rewritten as much as needed. However, this isn't recommended; you really should be deciding whether to buffer your output based on other factors, such as performance.

On a side note, retrieving a property's value should not have side effects-- certainly should not be setting a cookie (see Microsoft's guidance on the topic).

Answer 2

My problem here was that I was caching the HttpContext itself:

private readonly HttpContext mContext;

public MyClass(IHttpContextAccessor contextAccessor)
{
    mContext = contextAccessor.HttpContext;
}

Since my DI container cached this class as a singleton, I was using the same HttpContext for every request. Oopsie.

I changed my code to cache the accessor instead of the context itself, and added a wrapper property:

private readonly IHttpContextAccessor mContextAccessor;

public MyClass(IHttpContextAccessor contextAccessor)
{
    mContextAccessor = contextAccessor;
}

private HttpContext Context => mContextAccessor.HttpContext;

Probably obvious to most people, but I hope it helps someone!

Answer 3

I added a method for reading and writing the cookie that gets called in the constructor of the Utility object.

Utility.cs :

private void SetUser()
{
    User user = new User();

    if (this.Context.Request.Cookies.ContainsKey(UserCookieName))
    {
        user = JsonConvert.DeserializeObject<User>(this.Context.Request.Cookies[UserCookieName]);
    }
    else
    {
        user = JsonConvert.DeserializeObject<User>(InvokePost("CreateUser", new CreateUserContextModel() { User = user }).Result);
    }

    this.Context.Response.Cookies.Append(UserCookieName, JsonConvert.SerializeObject(user), new CookieOptions() { Expires = DateTime.Now.AddDays(120) });
    this.User = user;
}

public User User { get; set; }