Compelling Reasons to Use Marker Interfaces Instead of Attributes

I generally avoid "marker interfaces" because they don't allow you to unmark a derived type. But that aside, here are some of the specific cases that I have seen where marker interfaces would be preferable to built-in meta-data support:

1. Run-time performance sensitive situations.
2. Compatibility with languages that don't support annotation or attributes.
3. Any context where the interested code may not have access to the metadata.
4. Support for generic constraints and generic variance (typically of collections).

For a generic type you might want to use the same generic parameter in a marker interface. This is not achievable by an attribute:

interface MyInterface<T> {}

class MyClass<T, U> : MyInterface<U> {}

class OtherClass<T, U> : MyInterface<IDictionary<U, T>> {}

This kind of interface might be useful to associate a type with another one.

Another good use for a marker interface is when you want to create a kind of mixin:

interface MyMixin {}

static class MyMixinMethods {
  public static void Method(this MyMixin self) {}
}

class MyClass : MyMixin {
}

The acyclic visitor pattern also uses them. The term "degenerate interface" is sometimes used as well.

UPDATE:

I don't know if this one counts, but I've used them to mark classes for a post-compiler to work on.

Microsoft didn't strictly follow the guidelines when they made .NET 1.0, because the guidelines evolved together with the framework, and some of the rules they didn't learn until it was too late to change the API.

IIRC, the examples you mention belong to BCL 1.0, so that would explain it.

This is explained in Framework Design Guidelines.

That said, the book also remarks that "[A]ttribute testing is a lot more costly than type checking" (in a sidebar by Rico Mariani).

It goes on to say that sometimes you need the marker interface for compile time checking, which isn't possible with an attribute. However, I find the example given in the book (p. 88) unconvincing, so I will not repeat it here.

From performance point of view:

Marker attributes will be slower than marker interfaces because of reflection. If you don't cache reflection then calling GetCustomAttributes all the time can be a performance bottleneck. I benchmarked this before and using marker interfaces win in terms of performance even when using cached reflection.

This only applies when you use it in the code that's frequently called.

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.16299.371 (1709/FallCreatorsUpdate/Redstone3)
Intel Core i5-2400 CPU 3.10GHz (Sandy Bridge), 1 CPU, 4 logical and 4 physical cores
Frequency=3020482 Hz, Resolution=331.0730 ns, Timer=TSC
.NET Core SDK=2.1.300-rc1-008673
  [Host] : .NET Core 2.0.7 (CoreCLR 4.6.26328.01, CoreFX 4.6.26403.03), 64bit RyuJIT
  Core   : .NET Core 2.0.7 (CoreCLR 4.6.26328.01, CoreFX 4.6.26403.03), 64bit RyuJIT

Job=Core  Runtime=Core

                     Method |          Mean |      Error |     StdDev | Rank |
--------------------------- |--------------:|-----------:|-----------:|-----:|
                     CastIs |     0.0000 ns |  0.0000 ns |  0.0000 ns |    1 |
                     CastAs |     0.0039 ns |  0.0059 ns |  0.0052 ns |    2 |
            CustomAttribute | 2,466.7302 ns | 18.5357 ns | 17.3383 ns |    4 |
 CustomAttributeWithCaching |    25.2832 ns |  0.5055 ns |  0.4729 ns |    3 |

It's not significant difference though.

namespace BenchmarkStuff
{
    [AttributeUsage(AttributeTargets.All, AllowMultiple = false)]
    public class CustomAttribute : Attribute
    {

    }

    public interface ITest
    {

    }

    [Custom]
    public class Test : ITest
    {

    }

    [CoreJob]
    [RPlotExporter, RankColumn]
    public class CastVsCustomAttributes
    {
        private Test testObj;
        private Dictionary<Type, bool> hasCustomAttr;

        [GlobalSetup]
        public void Setup()
        {
            testObj = new Test();
            hasCustomAttr = new Dictionary<Type, bool>();
        }

        [Benchmark]
        public void CastIs()
        {
            if (testObj is ITest)
            {

            }
        }

        [Benchmark]
        public void CastAs()
        {
            var itest = testObj as ITest;
            if (itest != null)
            {

            }
        }

        [Benchmark]
        public void CustomAttribute()
        {
            var customAttribute = (CustomAttribute)testObj.GetType().GetCustomAttributes(typeof(CustomAttribute), false).SingleOrDefault();
            if (customAttribute != null)
            {

            }
        }

        [Benchmark]
        public void CustomAttributeWithCaching()
        {
            var type = testObj.GetType();
            bool hasAttr = false;
            if (!hasCustomAttr.TryGetValue(type, out hasAttr))
            {
                hasCustomAttr[type] = type.CustomAttributes.SingleOrDefault(attr => attr.AttributeType == typeof(CustomAttribute)) != null;
            }
            if (hasAttr)
            {

            }
        }
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run<CastVsCustomAttributes>();
        }
    }
}