ResourceContents.cs

using System.ComponentModel;
using System.Diagnostics;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ModelContextProtocol.Protocol;

/// <summary>
/// Provides a base class representing contents of a resource in the Model Context Protocol.
/// </summary>
/// <remarks>
/// <para>
/// <see cref="ResourceContents"/> serves as the base class for different types of resources that can be 
/// exchanged through the Model Context Protocol. Resources are identified by URIs and can contain
/// different types of data.
/// </para>
/// <para>
/// This class is abstract and has two concrete implementations:
/// <list type="bullet">
///   <item><description><see cref="TextResourceContents"/> - For text-based resources</description></item>
///   <item><description><see cref="BlobResourceContents"/> - For binary data resources</description></item>
/// </list>
/// </para>
/// <para>
/// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for more details.
/// </para>
/// </remarks>
[JsonConverter(typeof(Converter))]
public abstract class ResourceContents
{
    private protected ResourceContents()
    {
    }

    /// <summary>
    /// Gets or sets the URI of the resource.
    /// </summary>
    [JsonPropertyName("uri")]
    public string Uri { get; set; } = string.Empty;

    /// <summary>
    /// Gets or sets the MIME type of the resource content.
    /// </summary>
    [JsonPropertyName("mimeType")]
    public string? MimeType { get; set; }

    /// <summary>
    /// Provides a <see cref="JsonConverter"/> for <see cref="ResourceContents"/>.
    /// </summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public class Converter : JsonConverter<ResourceContents>
    {
        /// <inheritdoc/>
        public override ResourceContents? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.Null)
            {
                return null;
            }

            if (reader.TokenType != JsonTokenType.StartObject)
            {
                throw new JsonException();
            }

            string? uri = null;
            string? mimeType = null;
            string? blob = null;
            string? text = null;

            while (reader.Read() && reader.TokenType != JsonTokenType.EndObject)
            {
                if (reader.TokenType != JsonTokenType.PropertyName)
                {
                    continue;
                }

                string? propertyName = reader.GetString();
                bool success = reader.Read();
                Debug.Assert(success, "STJ must have buffered the entire object for us.");

                switch (propertyName)
                {
                    case "uri":
                        uri = reader.GetString();
                        break;

                    case "mimeType":
                        mimeType = reader.GetString();
                        break;

                    case "blob":
                        blob = reader.GetString();
                        break;

                    case "text":
                        text = reader.GetString();
                        break;

                    default:
                        break;
                }
            }

            if (blob is not null)
            {
                return new BlobResourceContents
                {
                    Uri = uri ?? string.Empty,
                    MimeType = mimeType,
                    Blob = blob
                };
            }

            if (text is not null)
            {
                return new TextResourceContents
                {
                    Uri = uri ?? string.Empty,
                    MimeType = mimeType,
                    Text = text
                };
            }

            return null;
        }

        /// <inheritdoc/>
        public override void Write(Utf8JsonWriter writer, ResourceContents value, JsonSerializerOptions options)
        {
            if (value is null)
            {
                writer.WriteNullValue();
                return;
            }

            writer.WriteStartObject();
            writer.WriteString("uri", value.Uri);
            writer.WriteString("mimeType", value.MimeType);
            
            Debug.Assert(value is BlobResourceContents or TextResourceContents);
            if (value is BlobResourceContents blobResource)
            {
                writer.WriteString("blob", blobResource.Blob);
            }
            else if (value is TextResourceContents textResource)
            {
                writer.WriteString("text", textResource.Text);
            }

            writer.WriteEndObject();
        }
    }
}