TypeLoadException when attempting serialization of value type defined in referenced .NET Framework class library

[zspitz]

I have the following project dependency structure:

• Entry (.NET Core 2.2)
  • Fx (.NET Framework 4.7.2 class library)

Fx contains two types:

• struct EndNodeData
• class TestClass

Serializing an instance of TestClass succeeds:

var stream = File.Create(Path.GetTempFileName());
var formatter = new BinaryFormatter();

var testClass = new TestClass() {
    Foo = "a",
    Bar = "b"
};
formatter.Serialize(stream, testClass);

but serializing a value of EndNodeType fails:

var endnodeData = new EndNodeData {
    Closure = null,
    Name = null,
    Type = "int",
    Value = "5"
};
formatter.Serialize(stream, endnodeData);

with the following exception:

System.TypeLoadException: 'Could not load type 'System.Runtime.CompilerServices.IsReadOnlyAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.'

Commenting out the string properties doesn't help.

Sample code

Related StackOverflow question

---

Running Windows 10 Home 1903 64-bit.
dotnet --version prints 2.2.300

Using these functions I get the following output:

.NET Framework Version: 4.8 or later

v2.0.50727  2.0.50727.4927  SP2
v3.0  3.0.30729.4926  SP2
v3.5  3.5.30729.4926  SP1
v4.0  
  Client  4.0.0.0

[0xd4d]

System.Runtime.CompilerServices.IsReadOnlyAttribute doesn't exist in .NET Core 2.2's mscorlib.dll (as a type forwarder), but it does exist in .NET Framework's mscorlib (net48, don't know about earlier versions)

workarounds:

• Don't use auto implemented properties: add your own backing fields or don't use properties, just public fields. The compiler adds [IsReadOnly] to the getters.
• Define your own System.Runtime.CompilerServices.IsReadOnlyAttribute in the same assembly or some common assembly

	namespace System.Runtime.CompilerServices {
		[AttributeUsage(AttributeTargets.All, Inherited = false)]
		sealed class IsReadOnlyAttribute : Attribute {
			public IsReadOnlyAttribute() { }
		}
	}

• Upgrade your project from netcoreapp2.2 to netcoreapp3.0

[zspitz]

@0xd4d

Closing, as the workarounds are sufficient.

Why does this happen only on struct and not on a class with auto-implemented properties?

[0xd4d]

I think it's the new 'readonly member' feature in C#8, and only structs can have readonly members. Seems like you're compiling your code with the latest Roslyn compiler.

[zspitz]

@0xd4d Even when I explicitly set the compiler version to 7.3, and to 7.0, in both the .NET Framework and .NET Core projects, I still get the same error, and not while serializing the class, only when serializing the struct.

[0xd4d]

Yes, you could report it to roslyn, I'm not sure if it's a feature or a bug to add [IsReadOnly] to the getter. https://github.com/dotnet/roslyn/issues