Different model binding behaviour for nullable query params with default value in asp net core

[mnerubashchenko]

Is there an existing issue for this?

• [x] I have searched the existing issues

Describe the bug

Hello! I noticed different behaviour in Model Binding for nullable query parameters with default value after upgade from .net 6 to 7 version. I will show this by examples.

I have 2 applications, one of them using .net v6, another one - .net v7. I will send the same request to both of applications. This is the request I will send - WeatherForecast?a=.

Endpoint in first application with .net 6:

Endpoint in second application with .net 7:

As you can see there are different behaviours after upgrade version of .net. I was trying to find any information about this change in many resource included in Microsoft articles about breaking changes. But I can't find. Maybe this is a bug?

Can you explain this and how can I fix it in my app. Now I have a couple options: rewrite GET to POST and use body instead of query params (it works). And may be use custom model binder (not tried yet).

Thanks in advance!

Expected Behavior

No response

Steps To Reproduce

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Exceptions (if any)

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.NET Version

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Anything else?

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[captainsafia]

@mnerubashchenko Thanks for filing this issue!

I believe this behavior you are seeing is a result of a change that we introduced in .NET 7 to prioritize the new TryParseModelBinder in MVC over the pre-existing TryParseConverters. We received several bug reports at the time and eventually reverted this behavior in .NET 8.

Can you try this in .NET 8 and see if you repro the same issue?

As a side note: .NET 7 is out-of-support at the moment so I'd recommend upgrading from .NET 6 to .NET 8 anyways. 😄

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