JSON ContractResolver created in every request issue - json

Question

I have an .NET Core Web Api 2.1 app in which I only serialize the properties requested by the client.
Example: GET orders/1?select=Id,TotalAmount
Example: GET orders/1?select=Id,CustomerName,DeliveryAddress,Location,ZipCode
For that, the app creates an object in every request (Lifetime Scoped) to get the requested properties.
Then, I created Custom ContractResolver class that is instanciated in every request by a global IResultFilter:
public class SerializationFilter : IResultFilter
{
private readonly IApiQueryParameters _apiQueryParameters;
public SerializationFilter(IApiQueryParameters apiQueryParameters)
{
this._apiQueryParameters = apiQueryParameters;
}
public void OnResultExecuting(ResultExecutingContext context)
{
var objectResult = context.Result as ObjectResult;
if (objectResult != null)
{
var contractResolver = new SelectiveResourceContractResolver(this._apiQueryParameters);
var serializerSettings = new JsonSerializerSettings
{
ContractResolver = contractResolver
};
var jsonFormatter = new JsonOutputFormatter(
serializerSettings,
ArrayPool<char>.Shared);
objectResult.Formatters.Add(jsonFormatter);
}
}
}
Now, this works for the first request made after the app is online.
The second request creates the ContractResolver correctly, but the response is not correct. The returned serialized properties are not the ones requested by the client.
Debugging the code, I noticed that when the SelectiveResourceContractResolver is created, it access the constructor method right,
public SelectiveResourceContractResolver(IApiQueryParameters apiQueryParameters)
{
this._apiQueryParameters = apiQueryParameters;
}
Also, it runs the CreateProperty overriden method:
protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
But in the second request, the CreatePropertyMethod is not run.
protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
{
var property = base.CreateProperty(member, memberSerialization);
property.ShouldSerialize =
instance =>
{
...
};
return property;
}
The second time on, it justs executes the property.ShouldSerialize = instance => part. I think some sort of caching is done by the app, but the wierd thing is that the ContractResolver is created in every request (it´s not set globally in Startup.cs) when the filter is executed. Also, when I debug it in the second request, the property IApiQueryParameters in the ContractResolver has all the values of the first request and not from the second execution of the constructor.
What I´m missing here?

Answer 1

I'm assuming your SelectiveResourceContractResolver inherits from DefaultContractResolver.
DefaultContractResolver.ResolveContract, which is called internally during serialization, caches contracts in a static, thread-safe dictionary, so CreateContract (which in turn calls CreateProperty) will only be called once per type for as long as your WebAPI process runs.
This is because calling CreateContract is expensive, and not caching contracts will seriously impact serialisation performance.
If you only need to cache contracts per response, you can create your own instance cache in SelectiveResourceContractResolver and override ResolveContract to use your cache instead:
public class InstanceCachingContractResolver : DefaultContractResolver
{
private readonly Dictionary<Type, JsonContract> contractCache = new Dictionary<Type, JsonContract>();
public override JsonContract ResolveContract(Type type)
{
if (!contractCache.TryGetValue(type, out JsonContract contract))
{
contract = CreateContract(type);
contractCache.Add(type, contract);
}
return contract;
}
}