[Analyzer] Header dictionary indexer analyzer and fixer

[JamesNK]

Background and Motivation

IHeaderDictionary has DIM properties for many commonly used headers: https://github.com/dotnet/aspnetcore/blob/ff2148be7592be83f1f88e2f372ef04f02fdfdb8/src/Http/Http.Features/src/IHeaderDictionary.Keyed.cs

These properties are better because:

• Strongly typed so no chance of typos.
• Faster because they directly fetch the known header value from the underlying collection.

Proposed API

An analyzer that looks for indexer use with a name that matches a property on IHeaderDictionary. This will only apply to header names that have a property, e.g.

var accepts = context.Request.Headers["accepts"]; // <- valid to fix
var id = context.Request.Headers["contoso-id"]; // <- ignored

Usage Examples

Alternative Designs

Risks

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• 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.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[halter73]

API Review Notes:

• The fix looks nicer and is faster! Approved.
• Should it be Performance? Usage? Style? Usage wins the vote. The perf impact isn't big, but it also doesn't really avoid bugs because it only flags string literals that are known correct header names.

Category: Usage
Severity: Info