[REQ] [csharp] Handling of nullable and required properties

[Bouillie]

Is your feature request related to a problem? Please describe.

With the current csharp generators (except generichost), there is a confusion between required and nullable fields. Nullable fields and non-required fields are both using the csharp nullable operator "?". It leads to several problems.

• When a field is defined as not required and nullable, it's impossible to know if its value is missing or null.
• Model validation is sometimes incorrect ([BUG] [CSHARP] Models from schema with a property attribute "nullable": true are generating properties not nullable #21090)

Describe the solution you'd like

Make a distinction between required and nullable fields. The solution used by generichost, using an Option struct for required fields seems good.

Describe alternatives you've considered

An alternative is moving to generichost but it's not possible for my project.

[jornhd]

I have planned to modify templates to use the "required" keyword for required properties for .NET 7 and later, and remove the constructor that take all the properties as arguments. This constructor has lead to many errors in our projects.
I can make a pull-request for this later, and refer to this issue if you like.

[Bouillie]

@jornhd I don't think removing the constructor is a good thing, we still need some validation in the constructor. Still, it needs to be fixed

[johnthagen]

Related to

• [REQ] [csharp] Partial update of only certain fields #17159
• [BUG][csharp-netcore] Nullable but required properties not respected #15735

[johnthagen]

Just wanted to add that even with the generichost client, required and nullable fields in a response cause deserialization errors as of version 7.10.0

• [BUG][csharp-netcore] Nullable but required properties not respected #15735 (comment)

This forced us to have to roll back changes to make our API more precise when responses would always return a value (i.e., required) but it could be null (i.e., nullable).