Question

It seems as if, once you set the cacheability on Response.Cache to NoCache, there’s no way to change it again. Here is a simple but complete illustration of the issue:

protected void Page_Load(object sender, EventArgs e)
{
    FieldInfo fi = typeof(HttpCachePolicy).GetField(
        "_cacheability",
        BindingFlags.NonPublic | BindingFlags.Instance);

    // Default value = 6
    HttpCacheability first = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can change it to Public
    Response.Cache.SetCacheability(HttpCacheability.Public);
    HttpCacheability second = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can change it to Private
    Response.Cache.SetCacheability(HttpCacheability.Private);
    HttpCacheability third = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can change it to NoCache
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    HttpCacheability fourth = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can't go back to Private!  Stuck on NoCache
    Response.Cache.SetCacheability(HttpCacheability.Private);
    HttpCacheability fifth = (HttpCacheability)fi.GetValue(Response.Cache);
}

Am I missing something? Is there a way to do this?

EDIT: Of course, it works if I set it with Reflection, but I’m worried that there’s something else happening when you set to HttpCacheability.NoCache that I would miss if I went behind the scenes.. and would prefer to do it in an officially-supported way anyway.

EDIT2: Same thing seems to happen with Private; can you only go more restrictive?

protected void Page_Load(object sender, EventArgs e)
{
    FieldInfo fi = typeof(HttpCachePolicy).GetField(
        "_cacheability",
        BindingFlags.NonPublic | BindingFlags.Instance);

    // Default value = 6
    HttpCacheability first = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can change it to Private
    Response.Cache.SetCacheability(HttpCacheability.Private);
    HttpCacheability second = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can't change to Public!  Stuck on Private
    Response.Cache.SetCacheability(HttpCacheability.Public);
    HttpCacheability third = (HttpCacheability)fi.GetValue(Response.Cache);

    // Can change to NoCache - Can only go more restrictive?
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    HttpCacheability fourth = (HttpCacheability)fi.GetValue(Response.Cache);
}

Answer 1

Cracked open Reflector and took a look inside HttpCachePolicy:

public void SetCacheability(HttpCacheability cacheability)
{
    if ((cacheability < HttpCacheability.NoCache) || (HttpCacheability.ServerAndPrivate < cacheability))
    {
        throw new ArgumentOutOfRangeException("cacheability");
    }
    if (s_cacheabilityValues[(int) cacheability] < s_cacheabilityValues[(int) this._cacheability])
    {
        this.Dirtied();
        this._cacheability = cacheability;
    }
}

s_cacheabilityValues is set during the static constructor:

s_cacheabilityValues = new int[] { -1, 0, 2, 1, 4, 3, 100 };

Dirtied() is called, but it just seems to set some flags:

private void Dirtied()
{
    this._isModified = true;
    this._useCachedHeaders = false;
}

It does look ilke there are rules for changing the values, but there doesn’t look like they have much effect. As such, probably safe to just change using reflection.

fi.SetValue(Response.Cache, HttpCacheability.Private);