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I'm running into a problem where I can't deserialize a string into a Task using System.Text.Json (.Net 5).

JsonSerializer.Deserialize<Task<TItem>>(serializedItem)

Background

I have a localized cache and store in it items retrieved from the DB.

I can't store the actual item in the cache, because any subsequent manipulation of the object results in the cached item being manipulation, affecting all further uses. I therefore store a serialized copy of the object.

Regarding performance...

I'm using the async/await pattern to call the DB (the whole app is async).

I read an article (may have been a video) in which Stephen Toub described the performance advantage of caching the Task. This SO article When to cache Tasks? goes into the details. Anyhow, I thought I'd try to take advantage of this (it works perfectly without serialization) using the following in my local cache "layer":

  1. If Task is in my cache, await it and return the result.
  2. Otherwise, call the DB method without awaiting it and add the resultant task to the cache

When I add serialization, then the deserialization of the task:

Task<TItem>? cachedItem = JsonSerializer.Deserialize<Task<TItem>>(serializedItem);

results in

Deserialization of types without a parameterless constructor, a singular parameterized constructor, or a parameterized constructor annotated with 'JsonConstructorAttribute' is not supported.

DrGriff
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  • How is this related to `async-await`? Does it work if it's synchronous? – Paulo Morgado Sep 25 '20 at 09:23
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    You can't serialize or deserialize a `Task`, it is an operation that returns its result asynchronously, not a data object from which a meaningful contract can be derived. It wraps an `Action` or other delegate, and you can't serialize a delegate. See: [how come BinaryFormatter can serialize an Action<> but Json.net cannot](https://stackoverflow.com/a/49139733/3744182). Of course that answer is for [tag:json.net], but [tag:system.text.json] is even more limited and doesn't even attempt to support `[Serializable]` serialization, as it doesn't serialize members, only properties. – dbc Sep 25 '20 at 14:28
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    And while `Action` is technically marked as `[Serializable]`, `Task` [is not](https://referencesource.microsoft.com/#mscorlib/system/threading/tasks/Future.cs,5ca7b77f3df89fc6), so there's no reason to think it could be serialized. – dbc Sep 25 '20 at 14:31
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    Is that sufficient for an answer? – dbc Sep 26 '20 at 00:57

1 Answers1

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The answer here is relatively straightforward - serialize the data inside the Task result, not the task itself.

If you aren't using something like memcached but rather using something like a simple Dictionary in-process object cache then serializing and deserializing seems like a rather heavy approach to cloning objects though.

Mike Marynowski
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