JSON is pretty much the de facto standard for moving data between systems, which works across operating systems and programming languages.

This, however, lends it to abuse, where it is used in scenarios that stretch the imagination.

Recently, I was updating some old code and found a class that looked like this (exact definition has been camouflaged to protect the guilty).

public sealed class Movie
{
    public required string Title { get; init; }
    public required int Length { get; init; }
    public required DateOnly ReleaseDate { get; init; }
    public required Image Poster { get; init; }
}

A request has been made to serialize this as JSON for consumption by an external system.

This presents a number of problems:

  1. Image is not a cross-platform type
  2. Image is not serializable

The code in the wild worked like this:

var movie = new Movie
{
    Title = "The Old Woman With The Knife",
    Length = 183,
    ReleaseDate = new DateOnly(2025, 5, 1),
    Poster = Image.Load<Rgba32>("movie.jpeg")
};

// Do things with the movie here

Here, Image.Load is from the excellent SixLabors ImageSharp library, which is cross-platform.

You might be tempted to try something like this:

var json = JsonSerializer.Serialize(movie);

This will unceremoniously throw an exception:

Unhandled exception. System.InvalidOperationException: The type 'System.ReadOnlySpan`1[System.Byte]' of property 'Preamble' on type 'System.Text.Encoding' is invalid for serialization or deserialization because it is a pointer type, is a ref struct, or contains generic parameters that have not been replaced by specific types.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_CannotSerializeInvalidType(Type typeToConvert, Type declaringType, MemberInfo memberInfo)
   at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.CreatePropertyInfo(JsonTypeInfo typeInfo, Type typeToConvert, MemberInfo memberInfo, NullabilityInfoContext nullabilityCtx, JsonSerializerOptions options, Boolean shouldCheckForRequiredKeyword, Boolean hasJsonIncludeAttribute)
   at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.AddMembersDeclaredBySuperType(JsonTypeInfo typeInfo, Type currentType, NullabilityInfoContext nullabilityCtx, Boolean constructorHasSetsRequiredMembersAttribute, PropertyHierarchyResolutionState& state)
   at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.PopulateProperties(JsonTypeInfo typeInfo, NullabilityInfoContext nullabilityCtx)
   at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.CreateTypeInfoCore(Type type, JsonConverter converter, JsonSerializerOptions options)
   at System.Text.Json.Serialization.Metadata.DefaultJsonTypeInfoResolver.GetTypeInfo(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoNoCaching(Type type)
   at System.Text.Json.JsonSerializerOptions.CachingContext.CreateCacheEntry(Type type, CachingContext context)
--- End of stack trace from previous location ---
   at System.Text.Json.JsonSerializerOptions.CachingContext.CacheEntry.GetResult()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.ConfigureProperties()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.Configure()
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.<EnsureConfigured>g__ConfigureSynchronized|174_0()
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoInternal(Type type, Boolean ensureConfigured, Nullable`1 ensureNotNull, Boolean resolveIfMutable, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.JsonSerializerOptions.GetTypeInfoForRootType(Type type, Boolean fallBackToNearestAncestorType)
   at System.Text.Json.JsonSerializer.GetTypeInfo[T](JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)
   at Program.<Main>$(String[] args) in /Users/rad/Projects/BlogCode/ImageSerialization/Program.cs:line 14

The simplest way is to create a data transfer object DTO specifically for this purpose.

namespace ImageSerialization;

public sealed record MovieDTO
{
    public required string Title { get; init; }
    public required int Length { get; init; }
    public required DateOnly ReleaseDate { get; init; }
    public required byte[] PosterData { get; init; } = [];
}

Note here the new property PosterData that is a byte array. This is safely portable.

Then we load the DTO directly.

// Create our DTO
var movieDTO = new MovieDTO
{
    Title = "The Old Woman With The Knife",
    Length = 183,
    ReleaseDate = new DateOnly(2025, 5, 1),
    PosterData = await File.ReadAllBytesAsync("movie.jpeg")
};

This can now be safely serialized.

// Serialize for transfer
var json = JsonSerializer.Serialize(movieDTO);

We will simulate receiving this JSON in another system.

// Deserialize for processing (assume this is a different system)
var receivedDTO = JsonSerializer.Deserialize<MovieDTO>(json)!;

// Map to a movie
var movie = new Movie
{
    Title = receivedDTO.Title,
    Length = receivedDTO.Length,
    ReleaseDate = receivedDTO.ReleaseDate,
    Poster = Image.Load<Rgba32>(receivedDTO.PosterData)
};

// Use our movie here
Log.Information("The poster for {MovieTitle} measures {BoundsWidth} x  {BoundsHeight}",
    movie.Title, movie.Poster.Bounds.Width, movie.Poster.Bounds.Height);

If we run this code, it will print the following:

[21:40:19 INF] The poster for The Old Woman With The Knife measures 459 x  680

In this way, we can avoid platform and serialization complications.

The code is in my GitHub.

Happy hacking!