Starting with .NET 9, using the in-box BinaryFormatter implementation throws PlatformNotSupportedException. The public APIs remain, but the old compatibility switch no longer re-enables the in-box implementation. Microsoft recommends migrating to another serializer; if you must process existing BinaryFormatter data, use APIs that read its NRBF format without creating arbitrary types encoded in the payload.
What changed in .NET 9?
Microsoft removed the in-box implementation, not the public BinaryFormatter API surface. The APIs are still present, but the in-box implementation throws at runtime in all cases, including when earlier settings had enabled BinaryFormatter. Microsoft introduced the change in .NET 9 Preview 6, and its breaking-change notice says it applies to all project types.
The setting System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization alone is therefore not a fix for .NET 9. A call that previously serialized or deserialized data may now fail when it reaches the formatter.
Why did Microsoft remove BinaryFormatter?
The risk is deserialization that lets input influence which objects are constructed. Microsoft associates this with CWE-502, “Deserialization of Untrusted Data,” and says BinaryFormatter cannot be made secure. Its removal is the final step in BinaryFormatter’s obsoletion plan. BinaryFormatter dates to the initial .NET Framework release in 2002, but that history does not make it suitable for handling untrusted data today.
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Changing from a binary format to a text format does not, by itself, make deserialization safe. Assess the replacement serializer and how the application handles input, especially data from outside a trusted boundary. Microsoft’s migration guide strongly recommends against continued BinaryFormatter use: BinaryFormatter security guide.
Which serializer should you choose?
There is no drop-in replacement. Choose based on the required wire format, whether you control both the producer and consumer, the shape and visibility of the types being serialized, and requirements such as ahead-of-time (AOT) compilation. If both ends are under your control, changing them together is generally simpler than preserving the old format. Microsoft documents these options in its serializer selection guide.
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| Option | Format and fit | Trade-offs to consider |
|---|---|---|
System.Text.Json |
JSON; the official .NET library. | Human-readable and broadly interoperable. Non-public and read-only members need additional handling, and the serializer does not support the [Serializable] attribute. |
DataContractSerializer |
XML; included in .NET. | Supports the BinaryFormatter programming model, including [Serializable] and ISerializable, which may reduce migration effort. Known types generally must be specified. Microsoft describes it as less modern or performant than other choices. Do not confuse it with the unsafe NetDataContractSerializer. |
| MessagePack for C# | Compact binary. | Can be configured for AOT and non-public or read-only members. Attributes and contracts affect integration; the guide notes built-in LZ4 compression. |
| protobuf-net | Protocol Buffers binary. | Contract-based and feature-rich, with support for non-public members and fields; many cases require attributes. |
These formats have different compatibility and integration costs. Microsoft’s comparison does not establish a universal performance winner, so test against your workload rather than choosing on an assumed ranking.
How can you handle existing BinaryFormatter data?
Replacing a serializer and reading old payloads are separate problems. If persisted data cannot all be converted at once, or producers and consumers must migrate on different schedules, Microsoft points to APIs that read the NRBF (Binary Format) payload without instantiating the types encoded in it. You can inspect or transform legacy data as part of a controlled transition, rather than invoking general-purpose BinaryFormatter deserialization. See Microsoft’s migration guide for the NRBF reading approach and its limits.
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When is the compatibility package an option?
For an application that cannot migrate immediately, Microsoft documents the unsupported System.Runtime.Serialization.Formatters NuGet package. Adding the package to the application project restores a functioning BinaryFormatter implementation, with the same vulnerabilities and risks. Microsoft recommends migrating away rather than depending on it. Treat it only as a temporary exception with a defined migration plan—not as a security fix or a normal .NET 9 remedy.
The former System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization switch by itself does not restore the in-box implementation. Package setup and the switch are described in Microsoft’s compatibility-package guidance.
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What about WPF, Windows Forms, and ResX resources?
WPF and Windows Forms
.NET 9’s WPF and Windows Forms frameworks use a limited internal subset for certain clipboard, drag-and-drop, and journal scenarios. Common primitives, strings, dates, and arrays or lists of supported types continue to work without migration. For other types, a fallback can reach the throwing BinaryFormatter implementation. If your application passes custom types through these workflows, follow the framework-specific migration steps in Microsoft’s WPF guidance and consult the corresponding Windows Forms instructions.
ResX managed resources
Common resource types such as strings and icons work without BinaryFormatter. Custom types may need the compatibility package and switch to load at runtime; this does not mean every ResX resource is affected. Microsoft covers this case in its migration guide.
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