How to handle an external OpenApi spec which contains a property with a '#' in the name

[andyfurniss4]

We are consuming an external OpenApi spec which we cannot modify. For whatever reason, this spec defines several properties that are prefixed with a hash symbol. For example: #attrs.

Kiota seems unable to handle this and created the parent object with a property type of UntypedNode. We need to be able to access and use this property.

Obviously C# doesn't support hashes at the start of property names but is there any customisation available in Kiota that lets you handle weirdness like this that exists in the OpenApi spec you're consuming? Ideally I'd like to map the wonky property name to something more sensible like Attributes.

[baywet]

Hi @andyfurniss4
Thank you for using kiota and for reaching out.

Can you please share the generated code for the property declaration, the serialization and the deserialization methods?

[andyfurniss4]

Hi @baywet . Thanks for getting back to me quickly.

Below you can find an example payload segment, along with the generated class.

{
    "Bkg_Ent": {
        "Accom": {
            "Id": "1",
            "St_Dt": "2025-09-05",
            "End_Dt": "2025-09-12",
            "HtlPrd": {
                "Prom": {
                    "#attrs": {
                        "Code": "DGCR"
                    }
                },
                "Acc_Cd": "XXXX0028/034"
            },
            "Rm_Cd": {
                "Rm_No": "1",
                "Code": "ZI03",
                "BB_Cd": "AI",
                "SubServPaxs": {
                    "SubServPax": [
                        {
                            "Pax_Id": "1"
                        },
                        {
                            "Pax_Id": "2"
                        }
                    ]
                }
            }
        },
        "Item": {
            "Id": "1",
            "St_Dt": "2025-09-05",
            "Set_Type": "EXTRA",
            "Item_Type": {
                "#attrs": {
                    "Code": "WC"
                }
            },
            "Prom": {
                "#attrs": {
                    "Code": "AUCI"
                }
            },
            "Bkg_Qty": "2",
            "Ser_Sts": "FIX",
            "SubServPaxs": {
                "SubServPax": [
                    {
                        "Pax_Id": "1"
                    },
                    {
                        "Pax_Id": "2"
                    }
                ]
            },
            "Ref_Prd_Id": "1",
            "Item_Method": "PP",
            "#attrs": {
                "Code": "WCF1/A",
                "Name": "Worldcare Fund"
            }
        }
    }
}

// <auto-generated/>
#pragma warning disable CS0618
using Microsoft.Kiota.Abstractions.Extensions;
using Microsoft.Kiota.Abstractions.Serialization;
using System.Collections.Generic;
using System.IO;
using System;
namespace GeneratedStuff.Net.Booking.Models
{
    [global::System.CodeDom.Compiler.GeneratedCode("Kiota", "1.0.0")]
    #pragma warning disable CS1591
    public partial class Bkg_Ent : IParsable
    #pragma warning restore CS1591
    {
        /// <summary>The Accom property</summary>
#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER
#nullable enable
        public global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestAccommodation? Accom { get; set; }
#nullable restore
#else
        public global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestAccommodation Accom { get; set; }
#endif
        /// <summary>List of additional items in the booking</summary>
#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER
#nullable enable
        public UntypedNode? Item { get; set; }
#nullable restore
#else
        public UntypedNode Item { get; set; }
#endif
        /// <summary>The Package property</summary>
#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER
#nullable enable
        public global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestPackage? Package { get; set; }
#nullable restore
#else
        public global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestPackage Package { get; set; }
#endif
        /// <summary>
        /// Creates a new instance of the appropriate class based on discriminator value
        /// </summary>
        /// <returns>A <see cref="global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestBkg_Ent"/></returns>
        /// <param name="parseNode">The parse node to use to read the discriminator value and create the object</param>
        public static global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestBkg_Ent CreateFromDiscriminatorValue(IParseNode parseNode)
        {
            _ = parseNode ?? throw new ArgumentNullException(nameof(parseNode));
            return new global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestBkg_Ent();
        }
        /// <summary>
        /// The deserialization information for the current model
        /// </summary>
        /// <returns>A IDictionary&lt;string, Action&lt;IParseNode&gt;&gt;</returns>
        public virtual IDictionary<string, Action<IParseNode>> GetFieldDeserializers()
        {
            return new Dictionary<string, Action<IParseNode>>
            {
                { "Accom", n => { Accom = n.GetObjectValue<global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestAccommodation>(global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestAccommodation.CreateFromDiscriminatorValue); } },
                { "Item", n => { Item = n.GetObjectValue<UntypedNode>(UntypedNode.CreateFromDiscriminatorValue); } },
                { "Package", n => { Package = n.GetObjectValue<global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestPackage>(global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestPackage.CreateFromDiscriminatorValue); } },
            };
        }
        /// <summary>
        /// Serializes information the current object
        /// </summary>
        /// <param name="writer">Serialization writer to use to serialize this model</param>
        public virtual void Serialize(ISerializationWriter writer)
        {
            _ = writer ?? throw new ArgumentNullException(nameof(writer));
            writer.WriteObjectValue<global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestAccommodation>("Accom", Accom);
            writer.WriteObjectValue<UntypedNode>("Item", Item);
            writer.WriteObjectValue<global::GeneratedStuff.Net.Booking.Models.ValHolidayRequestPackage>("Package", Package);
        }
    }
}
#pragma warning restore CS0618

Note that there are multiple #attrs properties in various different places in the example payload. It seems that most of them do generate properly, but those in the Item object do not, and whole Item object is lost and becomes an UntypedNode type. For example, the Bkg_Ent.Accom.HtlPrd.Prom.#attrs generates successfully:

// <auto-generated/>
#pragma warning disable CS0618
using Microsoft.Kiota.Abstractions.Extensions;
using Microsoft.Kiota.Abstractions.Serialization;
using System.Collections.Generic;
using System.IO;
using System;
namespace GeneratedStuff.Net.Booking.Models
{
    /// <summary>
    /// Promotion object with attributes
    /// </summary>
    [global::System.CodeDom.Compiler.GeneratedCode("Kiota", "1.0.0")]
    public partial class Prom : IParsable
    {
        /// <summary>Container for attribute-style properties</summary>
#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER
#nullable enable
        public global::GeneratedStuff.Net.Booking.Models.Prom_Attrs? Attrs { get; set; }
#nullable restore
#else
        public global::GeneratedStuff.Net.Booking.Models.Prom_Attrs Attrs { get; set; }
#endif
        /// <summary>
        /// Creates a new instance of the appropriate class based on discriminator value
        /// </summary>
        /// <returns>A <see cref="global::GeneratedStuff.Net.Booking.Models.Prom"/></returns>
        /// <param name="parseNode">The parse node to use to read the discriminator value and create the object</param>
        public static global::GeneratedStuff.Net.Booking.Models.Prom CreateFromDiscriminatorValue(IParseNode parseNode)
        {
            _ = parseNode ?? throw new ArgumentNullException(nameof(parseNode));
            return new global::GeneratedStuff.Net.Booking.Models.Prom();
        }
        /// <summary>
        /// The deserialization information for the current model
        /// </summary>
        /// <returns>A IDictionary&lt;string, Action&lt;IParseNode&gt;&gt;</returns>
        public virtual IDictionary<string, Action<IParseNode>> GetFieldDeserializers()
        {
            return new Dictionary<string, Action<IParseNode>>
            {
                { "#attrs", n => { Attrs = n.GetObjectValue<global::GeneratedStuff.Net.Booking.Models.Prom_Attrs>(global::GeneratedStuff.Net.Booking.Models.Prom_Attrs.CreateFromDiscriminatorValue); } },
            };
        }
        /// <summary>
        /// Serializes information the current object
        /// </summary>
        /// <param name="writer">Serialization writer to use to serialize this model</param>
        public virtual void Serialize(ISerializationWriter writer)
        {
            _ = writer ?? throw new ArgumentNullException(nameof(writer));
            writer.WriteObjectValue<global::GeneratedStuff.Net.Booking.Models.Prom_Attrs>("#attrs", Attrs);
        }
    }
}
#pragma warning restore CS0618

I'm speculating that if the #attrs is the only property in the object, then it works, but if there are other properties, it doesn't. That may not necessarily be accurate, but it seems that way for this example.

[andyfurniss4]

I've just discovered that the issue is actually being caused by something being defined as an array of arrays in the OpenApi spec. It seems Kiota is unable to handle this. Is that even valid in OpenApi?

[baywet]

Thank you for the additional information.

That should lead to an array of UntypedNodes in the generated code. It'd be helpful to share the relevant sections of your description.