System.Reflection.GetCustomAttribute(Type) returns null on .Net5

[michael-gabbay]

Description

I have a code that loads an assembly and using custom attributes to categorize data.

I'm trying to get the actual CustomAttribute object for some interfaces which have the requested Attribute.
on net framework48, the code returns the Attribute value as expected.
during migration to .Net5 - Running the code on .Net5 returns null.
I tried few overloads with no luck.

Reproduction Steps

A custom attribute:

[AttributeUsage(AttributeTargets.Interface)]
public class ApiInterfaceDescriptorAttribute : Attribute
{
    public string ApiUsageName { get; }

    public ApiInterfaceDescriptorAttribute(string apiUsageName)
    {
        ApiUsageName = apiUsageName;
    }
}

Sample interface:

[ApiInterfaceDescriptor("powercontrol")]
public interface IMyInterface 
{
   [ApiMethodDescriptor(ApiExposureLevel.Basic, "a-method...")]
    void SomeMethod();
}

usage:

public void GetApi()
{
   var assembly = Assembly.LoadFile("MyFile");
   var myInterface = assembly.DefinedTypes.FirstOrDefault(...) // get the interface
   var att = myInterface.GetCustomAttribute(typeof(ApiInterfaceDescriptorAttribute));// returns null on Net5 and value on net48
   var att = myInterface.GetCustomAttribute<ApiInterfaceDescriptorAttribute>();// returns null on Net5
}

Expected behavior

return the requested attribute

Actual behavior

method returns null

Regression?

No response

Known Workarounds

Read the CustomAtrributes object[] and itterate and query the requested attribute

Configuration

Running on Win10 x64, .Net5

Other information

No response

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Issue Details

---

Description

I have a code that loads an assembly and using custom attributes to categorize data.

I'm trying to get the actual CustomAttribute object for some interfaces which have the requested Attribute.
on net framework48, the code returns the Attribute value as expected.
during migration to .Net5 - Running the code on .Net5 returns null.
I tried few overloads with no luck.

Reproduction Steps

A custom attribute:

[AttributeUsage(AttributeTargets.Interface)]
public class ApiInterfaceDescriptorAttribute : Attribute
{
public string ApiUsageName { get; }

    public ApiInterfaceDescriptorAttribute(string apiUsageName)
    {
        ApiUsageName = apiUsageName;
    }
}

Sample interface:

[ApiInterfaceDescriptor("powercontrol")]
public interface IMyInterface
{
[ApiMethodDescriptor(ApiExposureLevel.Basic, "a-method...")]
void SomeMethod();
}

usage:

public void GetApi()
{
var assembly = Assembly.LoadFile(Path.GetFullPath("MuDiInterfaces.dll"));
var myInterface = assembly.DefinedTypes.FirstOrDefault(...) // get the interface
var att = myInterface.GetCustomAttribute(typeof(ApiInterfaceDescriptorAttribute));// returns null on Net5 and value on net48
var att = myInterface.GetCustomAttribute();// returns null on Net5
}

Expected behavior

return the requested attribute

Actual behavior

method returns null

Regression?

No response

Known Workarounds

Read the CustomAtrributes object[] and itterate and query the requested attribute

Configuration

Running on Win10 x64, .Net5

Other information

No response

Author:	mgabbay
Assignees:	-
Labels:	area-System.Reflection, untriaged
Milestone:	-

[Tornhoof]

You're manually loading your dll, where is the attribute defined?

[michael-gabbay]

Hi @Tornhoof

I Tried alos using
Assembly.GetAssembly(typeof(IMyInterface ));

The attribute is in the same project as the interface but with other namespace
the interface namespace is at 'MyProject'
the attribute namespace is at MyProject.Attributes

[vitek-karas]

I think this is because your MyAssembly.dll is loaded twice. You can check this by calling:

var defaultALC = AssemblyLoadContext.GetLoadContext(typeof(IMyInterface).Assembly);
var fileALC = AssemblyLoadContext.GetLoadContext(myInterface.Assembly);

I would expect the two variables to be different (even simple ToString should show a difference).

The root cause is the call to Assembly.LoadFile. This method always loads the specified assembly into its own AssemblyLoadContext. (More about assembly load context).

This means that you have the assembly loaded into the LoadFile load context (one copy) and then I assume you also have a direct reference to it from your main code, which will load it into the default load context (second copy). While the assembly file is exactly the same, for the runtime these two copies are distinct assemblies and all types from the two will be treated as different types.

So the call myInterface.GetCustomAttribute<ApiInterfaceDescriptorAttribute>() is looking for the attribute type from the default load context, but it's doing so on the interface from the LoadFile load context, which has the attribute type from LoadFile load context. And so it returns null - nothing found. If you were to look at all the attribute on the type, you would see the attribute you're looking for by name.

It's hard to say what you're trying to implement, so I don't know what solution to recommend. If it's anything like a plugin scenario, you might want to look at this guide/sample: https://docs.microsoft.com/en-us/dotnet/core/tutorials/creating-app-with-plugin-support

[michael-gabbay]

Hi @vitek-karas
Thanks for the explanation, I understand now what makes the strange behavior.
in my code I changed the usage of accessing the assembly and I see that the API is working as expected.

Many thanks!
issue resolved