BinaryFormatter serialization APIs produce compiler errors

As part of the BinaryFormatter long-term deprecation plan, we continue to cull BinaryFormatter functionality from our libraries and to wean developers off of the type. Starting in .NET 7, calls to the following APIs produce compile-time errors across all C# and Visual Basic project types:

Previous behavior

Since .NET 5, using the affected Serialize and Deserialize methods produced a compiler warning with ID SYSLIB0011. For more information, see BinaryFormatter serialization methods are obsolete and prohibited in ASP.NET apps (.NET 5).

Using the Exception.SerializeObjectState event did not produce an error.

New behavior

Starting in .NET 7, using any of the affected APIs in code produces a compiler error with the same ID, SYSLIB0011. Your project will be affected if it meets all of the following criteria:

  • It's a C# or Visual Basic project.
  • It targets net7.0 or higher.
  • It directly calls one of the affected APIs.
  • It's not already suppressing the SYSLIB0011 warning code.

Version introduced

.NET 7 RC 1

Type of breaking change

This change can affect source compatibility.

Reason for change

As part of the BinaryFormatter long-term deprecation plan, we continue to cull BinaryFormatter functionality from our libraries and to wean developers off of the type.

The best course of action is to migrate away from BinaryFormatter due to its security and reliability flaws. BinaryFormatter may be removed from .NET in a future release. The .NET libraries team has already taken a stance that recent types such as System.Half and System.DateOnly won't be compatible with BinaryFormatter.

If you must suppress the errors, you can do so by following the guidelines in the original obsoletion article. You can also disable the error project-wide by setting a project property that converts the error back to a warning (to match the .NET 5/6 behavior).

Warning

Setting this property might change host behavior. See <EnableUnsafeBinaryFormatterSerialization> property.

<PropertyGroup>
    ...
    <EnableUnsafeBinaryFormatterSerialization>true</EnableUnsafeBinaryFormatterSerialization>
</PropertyGroup>

Note

If your project compiles with "warnings as errors" enabled, compilation will still fail. (This matches the behavior that shipped in the .NET 5 and .NET 6 SDKs.) If that's the case, you'll still need to suppress the SYSLIB0011 warning in source or in your project file's <NoWarn> element.

<EnableUnsafeBinaryFormatterSerialization> property

The <EnableUnsafeBinaryFormatterSerialization property was introduced in .NET 5. With .NET 7, the behavior of this switch has changed to control both compilation and host run-time behavior. The meaning of this switch differs based on the project type, as described in the following table.

Type of project Property set to true Property set to false Property omitted
Library/shared component1 The affected APIs are obsolete as warning. Compilation will succeed unless you have "warnings as errors" enabled for your application or you've suppressed the SYSLIB0011 warning code. The affected APIs are obsolete as error, and calls from your code to those APIs will fail at compile time unless the error is suppressed. (Same as for false.)
Blazor and MAUI apps2 Calls to BinaryFormatter will fail at run time. Calls to BinaryFormatter will fail at run time. Calls to BinaryFormatter will fail at run time.
ASP.NET app The affected APIs are obsolete as warning. Compilation will succeed unless you have "warnings as errors" enabled for your application or you've suppressed the SYSLIB0011 warning code. The runtime will allow calls to BinaryFormatter, regardless of whether the call originates from your code or from a dependency that you consume. The affected APIs are obsolete as error, and calls from your code to those APIs will fail at compile time unless the error is suppressed. The runtime will forbid calls to BinaryFormatter, regardless of whether the call originates from your code or from a dependency that you consume. (Same as for false.)
Desktop apps and all other app types The affected APIs are obsolete as warning. Compilation will succeed unless you have "warnings as errors" enabled for your application or you've suppressed the SYSLIB0011 warning code. The runtime will allow calls to BinaryFormatter, regardless of whether the call originates from your code or from a dependency that you consume. The affected APIs are obsolete as error, and calls from your code to those APIs will fail at compile time unless the error is suppressed. The runtime will forbid calls to BinaryFormatter, regardless of whether the call originates from your code or from a dependency that you consume. The affected APIs are obsolete as error, and calls from your code to those APIs will fail at compile time unless the error is suppressed. The runtime will allow calls to BinaryFormatter, regardless of whether the call originates from your code or from a dependency that you consume.

1Runtime policy is controlled by the app host. Calls into BinaryFormatter might still fail at run time even if <EnableUnsafeBinaryFormatterSerialization> is set to true within your library's project file. Libraries can't override the app host's runtime policy.

2The Blazor and MAUI runtimes forbid calls to BinaryFormatter. Regardless of any value you set for <EnableUnsafeBinaryFormatterSerialization>, the calls will fail at run time. Don't call these APIs from Blazor or MAUI applications or from libraries intended to be consumed by Blazor or MAUI apps.

Affected APIs

See also