Memory Cache .Net Core Not Persisting Values - entity-framework

Question

I have a .NET Core 2.1 application. In Startup.cs configuration method, I use:
services.AddDbContext<ApplicationDbContext>(options =
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
...
services.AddMemoryCache();
Then in my controller:
public class DropDownListController : Controller
{
private readonly ApplicationDbContext _context;
private readonly IMemoryCache _memoryCache;
private const string ProvidersCacheKey = "providers";
private const string AgenciesCacheKey = "agencies";
public DropDownListController(ApplicationDbContext context, IMemoryCache memoryCache )
{
_context = context;
_memoryCache = memoryCache;
}
}
and in the controller also, the method to get the dropdownlist:
public JsonResult GetProvider()
{
IEnumerable<DropDownListCode.NameValueStr> providerlist;
if (_memoryCache.TryGetValue(ProvidersCacheKey, out providerlist))
{
return Json(providerlist);
}
else
{
MemoryCacheEntryOptions cacheExpirationOptions = new MemoryCacheEntryOptions();
cacheExpirationOptions.AbsoluteExpiration = DateTime.Now.AddDays(30);
cacheExpirationOptions.Priority = CacheItemPriority.Normal;
DropDownListCode um = new DropDownListCode(_context);
var result = um.GetProviderList();
_memoryCache.Set(ProvidersCacheKey, result);
return Json(result);
}
}
When I set a breakpoint on the line:
return Json(providerlist);
I see the ProvidersCacheKey is in the _memoryCache, but it has no value.
What happened to the data?
When I do a Quick Watch on _memoryCache, I can see the DbContext object was destroyed. But how can that be, the code works fine but the cache object does not have the data I saved to it.
Any help would be appreciated.

Answer 1

The method to get providers is:
public IEnumerable<NameValueStr> GetProviderList()
{
var providerlist = (from a in _context.AgencyProvider
where a.Provider == a.AgencyId
select new NameValueStr
{
id = a.Provider,
name = a.Name
});
return providerlist.Distinct();
}
Adding "ToList()" in the calling method worked:
MemoryCacheEntryOptions cacheExpirationOptions = new MemoryCacheEntryOptions();
cacheExpirationOptions.AbsoluteExpiration = DateTime.Now.AddMinutes(30);
cacheExpirationOptions.Priority = CacheItemPriority.Normal;
DropDownListCode um = new DropDownListCode(_context);
var result = um.GetProviderList().ToList();
_memoryCache.Set(ProvidersCacheKey, result);
return Json(result);
All credit goes to Steve Py… Thank you sir!