Analyzer to detect greater than 1 [FromBody] usage

[mitchdenny]

Background and Motivation

When wiring up an endpoint using minimal API, the following code compiles, but results in a runtime error:

app.MapGet("/", ([FromBody]MyType t1, [FromBody] MyType t2) => {});

Currently the runtime error is:

InvalidOperationException: Failure to infer one or more parameters.
Below is the list of parameters that we found:

Parameter | Source
---------------------------------------------------------------------------------
t1 | Body (Attribute)
t2 | UNKNOWN


Did you mean to register the "UNKNOWN" parameters as a Service?

It would be better to analyze the parameters of the handler delegate during compilation and raise and error in this particular case.

Proposed Analyzer

Analyzer Behavior and Message

When the analyzer encounters a MapXXX endpoint builder method with a handler delegate with more than one [FromBody] attribute it raises an error.

Category

• Design
• Documentation
• Globalization
• Interoperability
• Maintainability
• Naming
• Performance
• Reliability
• Security
• Style
• Usage

Severity Level

• Error
• Warning
• Info
• Hidden

Usage Scenarios

In the example below both t1 and t2 parameters would be highlighted.

app.MapGet("/", ([FromBody]MyType t1, [FromBody] MyType t2) => {});

Risks

I don't think there will be any risks with this as currently doing this results in a runtime error.

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• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
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• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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[halter73]

API Review Notes:

• Usage category and error level makes sense.
• We think the message should be "At most one parameter can have a [FromBody] attribute."

API Approved!

Category: Usage
Level: Error
Message: At most one parameter can have a [FromBody] attribute.