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This page talks about route constraints, but does not highlight the difference in behavior [for non-compliant values] when using a route constraint vs. not using a route constraint. Simple example:

[HttpGet("resources/{id}")]
public Task<ActionResult> GetResourceById([FromRoute] Guid id)
{
  return Ok();
}

vs.

[HttpGet("resources/{id:guid}")] // ← Notice route constraint
// …same code as above

When using a non-GUID value (ex: "foo") in the URL these two actions have varying responses.

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Document Details

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• ID: 8ee6ad2f-1667-8d5c-1f36-9d0511e3720f
• Version Independent ID: 7838ccec-c8e8-9baa-2b3e-220b2af5fc58
• Content: Routing to controller actions in ASP.NET Core
• Content Source: aspnetcore/mvc/controllers/routing.md
• Product: aspnet-core
• Technology: aspnetcore-mvc
• GitHub Login: @Rick-Anderson
• Microsoft Alias: riande

Hello! Thanks for contacting us. This sounds like a general question about using ASP.NET Core. While we try to look at and respond to all issues, for questions like this we recommend posting to a community support group like Stack Overflow with the asp.net-core tag.

• Stack Overflow
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Potential bugs in the ASP.NET Core framework and product feedback can be reported at dotnet/aspnetcore issues (include full repro steps and cross-link to a repro project in GH when appropriate). For Visual Studio, use the Report a Problem gesture or Suggest a Feature gesture within VS, which open internal issues for the VS team. For Visual Studio Code, you can open an issue at microsoft/vscode issues.

@Rick-Anderson can you please re-open this? This isn't a question at all, it's a request to update the documentation to highlight a difference in behavior of 2 options that are seemingly the same.

Thank you @Rick-Anderson , I appreciate it!

See https://learn.microsoft.com/en-us/aspnet/core/fundamentals/routing?view=aspnetcore-6.0#route-constraints