Required Schema filter to remove nullable properties from required list in Swagger generation in aspnetcore

SwaggerRequiredSchemaFilter.cs

namespace App.Swagger.Filters;

using System;
using System.Linq;
using System.Reflection;
using Microsoft.OpenApi.Models;
using Swashbuckle.AspNetCore.SwaggerGen;

/*
services.AddSwaggerGen(c =>
{
      c.SupportNonNullableReferenceTypes();
      // this filter is needed by some typescript generators that use the required list only (not nullable prop)
      c.SchemaFilter<SwaggerRequiredSchemaFilter>();
});
*/


/// <summary>
/// Adds required list of properties in schema objects for openapi generation
/// </summary>
public class SwaggerRequiredSchemaFilter : ISchemaFilter
{
    public void Apply(OpenApiSchema schema, SchemaFilterContext context)
    {
        if (schema.Properties == null)
        {
            return;
        }

        foreach (var schemProp in schema.Properties)
        {
            var propertyInfo = context.Type.GetProperty(schemProp.Key);
            
            if (propertyInfo == null)
            {
                continue;
            }

            // Check if the property is explicitly marked as nullable
            bool isNullable = IsPropertyNullable(context, propertyInfo);

            // Only mark as required if it's NOT nullable
            if (!isNullable && !schema.Required.Contains(schemProp.Key))
            {
                schema.Required.Add(schemProp.Key);
            }
            else
            {
                if (schema.Required.Contains(schemProp.Key))
                {
                    schema.Required.Remove(schemProp.Key);
                }
            }
        }
    }

    private static readonly NullabilityInfoContext nullabilityContext = new NullabilityInfoContext();

    private static bool IsPropertyNullable(SchemaFilterContext context, PropertyInfo propertyInfo)
    {
        // nullable value type (e.g., int?, DateTime?)
        if (Nullable.GetUnderlyingType(propertyInfo.PropertyType) != null)
        {
            return true;
        }

        // nullable reference type (Type?) by inspecting metadata
        if (!propertyInfo.PropertyType.IsValueType)
        {
            var nullabilityInfo = nullabilityContext.Create(propertyInfo);
            if (nullabilityInfo.WriteState is NullabilityState.Nullable)
            {
                return true;
            }
        }

        // Otherwise, assume it's required (non-nullable)
        return false;
    }
}