show friendlier error when players have two copies of a mod

4 files changed, 56 insertions, 8 deletions

diff --git a/src/StardewModdingAPI/Framework/Exceptions/SAssemblyLoadFailedException.cs b/src/StardewModdingAPI/Framework/Exceptions/SAssemblyLoadFailedException.cs
new file mode 100644
index 00000000..ec9279f1
--- /dev/null
+++ b/src/StardewModdingAPI/Framework/Exceptions/SAssemblyLoadFailedException.cs
@@ -0,0 +1,16 @@
+using System;
+
+namespace StardewModdingAPI.Framework.Exceptions
+{
+    /// <summary>An exception thrown when an assembly can't be loaded by SMAPI, with all the relevant details in the message.</summary>
+    internal class SAssemblyLoadFailedException : Exception
+    {
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="message">The error message.</param>
+        public SAssemblyLoadFailedException(string message)
+            : base(message) { }
+    }
+}
diff --git a/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoadStatus.cs b/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoadStatus.cs
new file mode 100644
index 00000000..11be19fc
--- /dev/null
+++ b/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoadStatus.cs
@@ -0,0 +1,15 @@
+namespace StardewModdingAPI.Framework.ModLoading
+{
+    /// <summary>Indicates the result of an assembly load.</summary>
+    internal enum AssemblyLoadStatus
+    {
+        /// <summary>The assembly was loaded successfully.</summary>
+        Okay = 1,
+
+        /// <summary>The assembly could not be loaded.</summary>
+        Failed = 2,
+
+        /// <summary>The assembly is already loaded.</summary>
+        AlreadyLoaded = 3
+    }
+}
diff --git a/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoader.cs b/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoader.cs
index 406d49e1..b14ae56f 100644
--- a/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoader.cs
+++ b/src/StardewModdingAPI/Framework/ModLoading/AssemblyLoader.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -6,6 +6,7 @@ using System.Reflection;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
 using StardewModdingAPI.AssemblyRewriters;
+using StardewModdingAPI.Framework.Exceptions;
 
 namespace StardewModdingAPI.Framework.ModLoading
 {
@@ -65,16 +66,27 @@ namespace StardewModdingAPI.Framework.ModLoading
                 AssemblyDefinitionResolver resolver = new AssemblyDefinitionResolver();
                 HashSet<string> visitedAssemblyNames = new HashSet<string>(AppDomain.CurrentDomain.GetAssemblies().Select(p => p.GetName().Name)); // don't try loading assemblies that are already loaded
                 assemblies = this.GetReferencedLocalAssemblies(new FileInfo(assemblyPath), visitedAssemblyNames, resolver).ToArray();
-                if (!assemblies.Any())
-                    throw new InvalidOperationException($"Could not load '{assemblyPath}' because it doesn't exist.");
-                resolver.Add(assemblies.Select(p => p.Definition).ToArray());
             }
 
+            // validate load
+            if (!assemblies.Any() || assemblies[0].Status == AssemblyLoadStatus.Failed)
+            {
+                throw new SAssemblyLoadFailedException(!File.Exists(assemblyPath)
+                    ? $"Could not load '{assemblyPath}' because it doesn't exist."
+                    : $"Could not load '{assemblyPath}'."
+                );
+            }
+            if (assemblies[0].Status == AssemblyLoadStatus.AlreadyLoaded)
+                throw new SAssemblyLoadFailedException($"Could not load '{assemblyPath}' because it was already loaded. Do you have two copies of this mod?");
+
             // rewrite & load assemblies in leaf-to-root order
             bool oneAssembly = assemblies.Length == 1;
             Assembly lastAssembly = null;
             foreach (AssemblyParseResult assembly in assemblies)
             {
+                if (assembly.Status == AssemblyLoadStatus.AlreadyLoaded)
+                    continue;
+
                 bool changed = this.RewriteAssembly(assembly.Definition, assumeCompatible, logPrefix: "   ");
                 if (changed)
                 {
@@ -143,7 +155,7 @@ namespace StardewModdingAPI.Framework.ModLoading
 
             // skip if already visited
             if (visitedAssemblyNames.Contains(assembly.Name.Name))
-                yield break;
+                yield return new AssemblyParseResult(file, null, AssemblyLoadStatus.AlreadyLoaded);
             visitedAssemblyNames.Add(assembly.Name.Name);
 
             // yield referenced assemblies
@@ -155,7 +167,7 @@ namespace StardewModdingAPI.Framework.ModLoading
             }
 
             // yield assembly
-            yield return new AssemblyParseResult(file, assembly);
+            yield return new AssemblyParseResult(file, assembly, AssemblyLoadStatus.Okay);
         }
 
         /****
diff --git a/src/StardewModdingAPI/Framework/ModLoading/AssemblyParseResult.cs b/src/StardewModdingAPI/Framework/ModLoading/AssemblyParseResult.cs
index 69c99afe..b56a776c 100644
--- a/src/StardewModdingAPI/Framework/ModLoading/AssemblyParseResult.cs
+++ b/src/StardewModdingAPI/Framework/ModLoading/AssemblyParseResult.cs
@@ -15,6 +15,9 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>The assembly definition.</summary>
         public readonly AssemblyDefinition Definition;
 
+        /// <summary>The result of the assembly load.</summary>
+        public AssemblyLoadStatus Status;
+
 
         /*********
         ** Public methods
@@ -22,10 +25,12 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>Construct an instance.</summary>
         /// <param name="file">The original assembly file.</param>
         /// <param name="assembly">The assembly definition.</param>
-        public AssemblyParseResult(FileInfo file, AssemblyDefinition assembly)
+        /// <param name="status">The result of the assembly load.</param>
+        public AssemblyParseResult(FileInfo file, AssemblyDefinition assembly, AssemblyLoadStatus status)
         {
             this.File = file;
             this.Definition = assembly;
+            this.Status = status;
         }
     }
-}
\ No newline at end of file
+}