fix: NPC discovery scans mods built against any version of S1API

S1API.Tests/Internal/Utils/ReflectionUtilsTests.cs

@@ -121,6 +121,57 @@ public void DerivedTypeScanDoesNotFollowSameNameAssembliesWithDifferentIdentitie
             AppDomain.CurrentDomain.GetAssemblies()));
     }
 
+    [Theory]
+    [InlineData("S1API, Version=3.0.1.0, Culture=neutral, PublicKeyToken=null")]
+    [InlineData("S1API, Version=2.9.2.0, Culture=neutral, PublicKeyToken=null")]
+    [InlineData("S1API, Version=3.2.1.0, Culture=neutral, PublicKeyToken=null")]
+    [InlineData("s1api, Version=4.0.0.0, Culture=neutral, PublicKeyToken=null")]
+    public void AReferenceToAnyVersionOfTheBaseAssemblyBindsToIt(string reference)
+    {
+        var loaded = new AssemblyName("S1API, Version=3.2.1.0, Culture=neutral, PublicKeyToken=null");
+
+        Assert.True(ReflectionUtils.ReferenceBindsToDefinition(new AssemblyName(reference), loaded));
+    }
+
+    [Fact]
+    public void AReferenceToADifferentAssemblyNameDoesNotBind()
+    {
+        var loaded = new AssemblyName("S1API, Version=3.2.1.0, Culture=neutral, PublicKeyToken=null");
+
+        Assert.False(ReflectionUtils.ReferenceBindsToDefinition(
+            new AssemblyName("S1APILoader, Version=3.2.1.0, Culture=neutral, PublicKeyToken=null"),
+            loaded));
+    }
+
+    [Fact]
+    public void AReferenceWithADifferentPublicKeyTokenDoesNotBind()
+    {
+        var signed = new AssemblyName("Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089");
+        var unsigned = new AssemblyName("Example, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null");
+
+        Assert.False(ReflectionUtils.ReferenceBindsToDefinition(signed, unsigned));
+        Assert.False(ReflectionUtils.ReferenceBindsToDefinition(unsigned, signed));
+        Assert.True(ReflectionUtils.ReferenceBindsToDefinition(signed, signed));
+    }
+
+    [Fact]
+    public void ALoadedAssemblyOfADifferentVersionIsFollowedOnlyWhenItIsTheOnlyOneOfThatName()
+    {
+        var reference = new AssemblyName("Library, Version=1.5.0.0, Culture=neutral, PublicKeyToken=null");
+        var newer = new AssemblyName("Library, Version=1.6.0.0, Culture=neutral, PublicKeyToken=null");
+
+        Assert.True(ReflectionUtils.ShouldFollowLoadedReference(newer, reference, loadedAssembliesWithThatName: 1));
+        Assert.False(ReflectionUtils.ShouldFollowLoadedReference(newer, reference, loadedAssembliesWithThatName: 2));
+    }
+
+    [Fact]
+    public void ALoadedAssemblyWithTheExactIdentityIsAlwaysFollowed()
+    {
+        var reference = new AssemblyName("Library, Version=1.5.0.0, Culture=neutral, PublicKeyToken=null");
+
+        Assert.True(ReflectionUtils.ShouldFollowLoadedReference(reference, reference, loadedAssembliesWithThatName: 2));
+    }
+
     private static AssemblyBuilder CreateDynamicAssembly(string name, Version version)
     {
         var assemblyName = new AssemblyName(name)

S1API/Internal/Utils/ReflectionUtils.cs

@@ -137,7 +137,7 @@ private static bool ReferencesAssemblyTransitively(
 
             foreach (AssemblyName referencedAssembly in referencedAssemblies)
             {
-                if (AssemblyIdentityMatches(referencedAssembly, baseAssemblyName))
+                if (ReferenceBindsToDefinition(referencedAssembly, baseAssemblyName))
                     return true;
 
                 string referencedName = referencedAssembly.Name ?? string.Empty;
@@ -147,7 +147,10 @@ private static bool ReferencesAssemblyTransitively(
 
                 foreach (Assembly loadedReference in loadedReferences)
                 {
-                    if (!AssemblyIdentityMatches(loadedReference.GetName(), referencedAssembly))
+                    if (!ShouldFollowLoadedReference(
+                            loadedReference.GetName(),
+                            referencedAssembly,
+                            loadedReferences.Length))
                         continue;
 
                     if (ReferencesAssemblyTransitively(
@@ -164,6 +167,45 @@ private static bool ReferencesAssemblyTransitively(
             return false;
         }
 
+        /// <summary>
+        /// INTERNAL: Does this reference bind to that assembly? The simple name and public key token must match; the
+        /// version need not, because the runtime binds a reference to whichever assembly of that name is loaded. A mod
+        /// built against an older S1API still runs against the loaded one, so it must still be scanned for the types
+        /// that derive from it.
+        /// </summary>
+        internal static bool ReferenceBindsToDefinition(
+            AssemblyName referenceAssemblyName,
+            AssemblyName definitionAssemblyName)
+        {
+            if (!string.Equals(
+                    referenceAssemblyName.Name,
+                    definitionAssemblyName.Name,
+                    StringComparison.OrdinalIgnoreCase))
+                return false;
+
+            byte[] referenceToken = referenceAssemblyName.GetPublicKeyToken() ?? Array.Empty<byte>();
+            byte[] definitionToken = definitionAssemblyName.GetPublicKeyToken() ?? Array.Empty<byte>();
+            return referenceToken.SequenceEqual(definitionToken);
+        }
+
+        /// <summary>
+        /// INTERNAL: Should the scan follow a reference into this loaded assembly? An exact identity always; a different
+        /// version only when it is the one assembly of that name loaded, since then the reference can only bind to it.
+        /// With several same-named assemblies loaded it is unknown which one the reference means, so only an exact
+        /// match is followed.
+        /// </summary>
+        internal static bool ShouldFollowLoadedReference(
+            AssemblyName loadedAssemblyName,
+            AssemblyName referenceAssemblyName,
+            int loadedAssembliesWithThatName)
+        {
+            if (AssemblyIdentityMatches(loadedAssemblyName, referenceAssemblyName))
+                return true;
+
+            return loadedAssembliesWithThatName == 1
+                   && ReferenceBindsToDefinition(referenceAssemblyName, loadedAssemblyName);
+        }
+
         private static bool AssemblyIdentityMatches(
             AssemblyName referenceAssemblyName,
             AssemblyName definitionAssemblyName) =>