ASP.NET Web API: How to handle partial update requests?

Chris B 1

Hi,

I'm wondering how to handle update requests in my API, where the request does not necessarily include all the properties that the API is able to update.
The API might be able to receive a DTO with 10 properties, but if your JSON only contains 5 of those, the remaining ones will be considered null.
This might lead to null being written into the database, even though the property wasn't actually present in the request and not supposed to be changed at all.
Generally though, actually passing a property with null as its value is absolutely fine and should be updated accordingly.

So the main question is, how to differentiate between null values and not present values?

Regards,
Chris

AgaveJoe 29,281 Reputation points

2022-12-05T15:40:55.923+00:00

Typically, the record is fetched by Id. Next, only the inputs that have values are used to update the fetched record. The null coalesce operator makes the assignment rather simple.

?? and ??= operators - the null-coalescing operators

Or, you can use a conditional statement.

I believe mapping utilities (DTO -> Entity) also handle this situation through configuration.

So the main question is, how to differentiate between null values and not present values?

Determining "not present" is possible by keeping track of what values where returned to the client by using some kind identifier. The identifier is needed to update the record and the identifier has a timeout period.

Or you can simply using standard validation and require all required fields. If the client is missing a filed they get a 400 response with an error message that describes the missing fields.

1 answer

Bruce (SqlWork.com) 69,276 Reputation points

2022-12-05T16:56:15.893+00:00

c# is not the easiest language for this. you will want to preserve the json structure:

using StreamReader reader = new StreamReader(Request.Body, Encoding.UTF8);
var jsonString = reader.ReadToEndAsync();

then parse to JObject or JsonElement depending on your json library of choice. your code can then walk json tree and build insert/update statements based on the request.

if the updates are not random collection of values, then you can use custom request / response classes and build a transaction oriented api.

note: you should never use DTO classes as action parameters.
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Chris B 1 Reputation point

2022-12-08T08:28:56.087+00:00

Hey,

I was hoping there is some standard way of handling these kind of requests, as I suspect many people to have a similar need.

While reading out the plain JSON and using it to later check whether some property was there or not works, it does feel like a workaround,
and especially tricky when using AutoMapper.

What exactly do you mean about not using DTO classes as action parameters?

Regards

Bruce (SqlWork.com) 69,276 Reputation points

2022-12-09T23:10:01.46+00:00

the issue is that .net does't support sparse objects like json (javascript). what you really want

update() { var updateData = getPostData(); var id = getId(updateData); var data = db.getById(id); data.applyUpdateData(updateData); db.Save(); }

you need updateData to a format easy to reflect. you could use an anonymous object, but json is probably the easiest.
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ASP.NET Web API: How to handle partial update requests?

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