Add support for C# 11 'required' modifier.

[joel1st]

Now that C# has inbuilt language support for required properties, it might be worth discussing how we can take advantage of this with SwashBuckle to provide more accurate, streamlined API documentation.

I think the below comment covers my thoughts on this well:
Since nullablility and required/not are different things where one does not necessarily imply the other, I think SwashBuckle should use the C# property nullability for swagger property nullable, and C# property required/not for the swagger property required.

[flostellbrink]

This did the trick for me:

public class SwaggerRequiredMemberFilter : ISchemaFilter
{
	public void Apply(OpenApiSchema schema, SchemaFilterContext context)
	{
		var requiredProperties = context.Type.GetProperties().Where(p => p.HasAttribute<RequiredMemberAttribute>());
		var schemaPropertyNames = schema.Properties.Keys.Where(
			sp => requiredProperties.Any(rp => string.Equals(sp, rp.Name, StringComparison.InvariantCultureIgnoreCase))
		);

		foreach (var property in schemaPropertyNames)
		{
			schema.Required.Add(property);
		}
	}
}

From a quick glance it looks like a proper implementation might get away with adding RequiredMemberAttribute here:

Swashbuckle.AspNetCore/src/Swashbuckle.AspNetCore.SwaggerGen/SwaggerGenerator/ApiParameterDescriptionExtensions.cs

Line 15 in 73610b9

private static readonly Type[] RequiredAttributeTypes = new[]

Edit: After a second look that is only for parameters, no idea how it works for properties.

Edit 2: Original filter did not handle nullable required classes type properties well.
Looks like openapi does not really have a concept of nullable ref types, so those just sort of end up being non-nullable.

This version handles that by not marking nullable ref types as required:

public class SwaggerRequiredMemberFilter : ISchemaFilter
{
	public void Apply(OpenApiSchema schema, SchemaFilterContext context)
	{
		var nullabilityContext = new NullabilityInfoContext();
		var properties = context.Type.GetProperties();
		foreach (var key in schema.Properties.Keys)
		{
			// Keys that do not correspond to a property are ignored
			var property = properties.SingleOrDefault(
				p => string.Equals(p.Name, key, StringComparison.OrdinalIgnoreCase)
			);
			if (property == null)
				continue;

			// Do not mark if the property is not required
			var required = property.HasAttribute<RequiredMemberAttribute>();
			if (!required)
				continue;

			// Check nullability of non primitive types
			var primitive = schema.Properties[key].Type != null;
			if (!primitive)
			{
				// Do not mark nullable ref types.
				// Ref types cannot be marked as nullable, so this would lead to them being non nullable.
				var nullabilityInfo = nullabilityContext.Create(property);
				if (nullabilityInfo.ReadState == NullabilityState.Nullable)
					continue;
			}

			schema.Required.Add(key);
		}
	}
}

[30drie]

I used the solution from @flostellbrink, but found one flaw. I found that not all properties where marked required and when I looked at the code I saw that this only works as long as the name of the property is the same as the json name, but if you use JsonPropertyName to use snakecase for the API while your C# property has PascalCase naming convention, this code does not work.

So I changed the code slightly, basically do the foreach on the Properties of the class instead of Properties of the schema.
This code should work in all cases.

using System.Linq;
using System.Reflection;
using System.Runtime.CompilerServices;
using System.Text.Json.Serialization;
using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;

public class RequiredMemberFilter : ISchemaFilter
{
	public void Apply(OpenApiSchema schema, SchemaFilterContext context)
	{
		var nullabilityContext = new NullabilityInfoContext();
		var properties = context.Type.GetProperties();

		foreach(var property in properties)
		{
			if (property.HasAttribute<JsonIgnoreAttribute>() || !property.HasAttribute<RequiredMemberAttribute>()) continue;

			var jsonName = property.Name;
			if (property.HasAttribute<JsonPropertyNameAttribute>())
			{
				jsonName = property.GetCustomAttribute<JsonPropertyNameAttribute>()!.Name;
			}

			var jsonKey = schema.Properties.Keys.SingleOrDefault(key =>
				string.Equals(key, jsonName, StringComparison.OrdinalIgnoreCase));

			if (string.IsNullOrWhiteSpace(jsonKey)) continue;

			// Check nullability of non primitive types
			var primitive = schema.Properties[jsonKey].Type != null;
			if (!primitive)
			{
				// Do not mark nullableref types.
				// Ref types cannot be marked as nullable, so this would lead to them being non nullable.
				var nullabilityInfo = nullabilityContext.Create(property);
				if (nullabilityInfo.ReadState == NullabilityState.Nullable) continue;
			}

			schema.Required.Add(jsonKey);
		}
	}

}

[onionhammer]

This isnt a C# 11 issue, it's a discrepancy between the ASP.NET validator treatment of non-nullable properties and swashbuckle's.

https://learn.microsoft.com/en-us/aspnet/core/mvc/models/validation?view=aspnetcore-7.0#non-nullable-reference-types-and-required-attribute

[onionhammer]

Any movement on this issue? AFAICT this discrepancy still exists

[rthomas-rdm]

Would a PR to implement support for the C# 11 required modifier be welcome and likely to create traction on this issue?

[Havunen]

FYI this should work in DotSwashbuckle

[mattiasnordqvist]

Is this related?
#2810

I think the required keyword has been incorrectly interpreted as not nullable

[martincostello]

That has already been fixed for the next release: #2879