don't warn for NetCollection conversion to implemented interface (#471)

src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/Netcode/NetCollection.cs

@@ -0,0 +1,10 @@
+// ReSharper disable CheckNamespace -- matches Stardew Valley's code
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+
+namespace Netcode
+{
+    /// <summary>A simplified version of Stardew Valley's <c>Netcode.NetCollection</c> for unit testing.</summary>
+    public class NetCollection<T> : Collection<T>, IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable { }
+}

src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Farmer.cs

@@ -1,7 +1,6 @@
 // ReSharper disable CheckNamespace, InconsistentNaming -- matches Stardew Valley's code
 #pragma warning disable 649 // (never assigned) -- only used to test type conversions
 using System.Collections.Generic;
-using Netcode;
 
 namespace StardewValley
 {
@@ -10,11 +9,5 @@ namespace StardewValley
     {
         /// <summary>A sample field which should be replaced with a different property.</summary>
         public readonly IDictionary<string, int[]> friendships;
-
-        /// <summary>A sample net list.</summary>
-        public readonly NetList<int> eventsSeen;
-
-        /// <summary>A sample net object list.</summary>
-        public readonly NetObjectList<int> netObjectList;
     }
 }

src/SMAPI.ModBuildConfig.Analyzer.Tests/Mock/StardewValley/Item.cs

@@ -20,5 +20,14 @@ namespace StardewValley
 
         /// <summary>A generic net ref property with no equivalent non-net property.</summary>
         public NetRef<object> netRefProperty { get; } = new NetRef<object>();
+
+        /// <summary>A sample net list.</summary>
+        public readonly NetList<int> netList = new NetList<int>();
+
+        /// <summary>A sample net object list.</summary>
+        public readonly NetObjectList<int> netObjectList = new NetObjectList<int>();
+
+        /// <summary>A sample net collection.</summary>
+        public readonly NetCollection<int> netCollection = new NetCollection<int>();
     }
 }

src/SMAPI.ModBuildConfig.Analyzer.Tests/NetFieldAnalyzerTests.cs

@@ -86,7 +86,8 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests
         [TestCase("SObject obj = null; if (obj.netRefField != null);", 24, "obj.netRefField", "NetRef", "object")]
         [TestCase("SObject obj = null; if (obj.netRefProperty == null);", 24, "obj.netRefProperty", "NetRef", "object")]
         [TestCase("SObject obj = null; if (obj.netRefProperty != null);", 24, "obj.netRefProperty", "NetRef", "object")]
-        [TestCase("SFarmer farmer = new SFarmer(); object list = farmer.eventsSeen;", 46, "farmer.eventsSeen", "NetList", "object")] // ↓ NetList field converted to a non-interface type
+        [TestCase("Item item = new Item(); object list = item.netList;", 38, "item.netList", "NetList", "object")] // ↓ NetList field converted to a non-interface type
+        [TestCase("Item item = new Item(); object list = item.netCollection;", 38, "item.netCollection", "NetCollection", "object")]
         public void AvoidImplicitNetFieldComparisons_RaisesDiagnostic(string codeText, int column, string expression, string fromType, string toType)
         {
             // arrange
@@ -105,10 +106,11 @@ namespace SMAPI.ModBuildConfig.Analyzer.Tests
 
         /// <summary>Test that the net field analyzer doesn't raise any warnings for safe member access.</summary>
         /// <param name="codeText">The code line to test.</param>
-        [TestCase("SFarmer farmer = new SFarmer(); System.Collections.IEnumerable list = farmer.eventsSeen;")]
-        [TestCase("SFarmer farmer = new SFarmer(); System.Collections.Generic.IEnumerable<int> list = farmer.eventsSeen;")]
-        [TestCase("SFarmer farmer = new SFarmer(); System.Collections.Generic.IList<int> list = farmer.eventsSeen;")]
-        [TestCase("SFarmer farmer = new SFarmer(); System.Collections.Generic.IList<int> list = farmer.netObjectList;")] // subclass of NetList
+        [TestCase("Item item = new Item(); System.Collections.IEnumerable list = farmer.eventsSeen;")]
+        [TestCase("Item item = new Item(); System.Collections.Generic.IEnumerable<int> list = farmer.netList;")]
+        [TestCase("Item item = new Item(); System.Collections.Generic.IList<int> list = farmer.netList;")]
+        [TestCase("Item item = new Item(); System.Collections.Generic.ICollection<int> list = farmer.netCollection;")]
+        [TestCase("Item item = new Item(); System.Collections.Generic.IList<int> list = farmer.netObjectList;")] // subclass of NetList
         public void AvoidImplicitNetFieldComparisons_AllowsSafeAccess(string codeText)
         {
             // arrange

src/SMAPI.ModBuildConfig.Analyzer/NetFieldAnalyzer.cs

@@ -22,6 +22,9 @@ namespace StardewModdingAPI.ModBuildConfig.Analyzer
         /// <summary>The full name for Stardew Valley's <c>Netcode.NetList</c> type.</summary>
         private readonly string NetListTypeFullName = "Netcode.NetList";
 
+        /// <summary>The full name for Stardew Valley's <c>Netcode.NetCollection</c> type.</summary>
+        private readonly string NetCollectionTypeFullName = "Netcode.NetCollection";
+
         /// <summary>Maps net fields to their equivalent non-net properties where available.</summary>
         private readonly IDictionary<string, string> NetFieldWrapperProperties = new Dictionary<string, string>
         {
@@ -232,6 +235,14 @@ namespace StardewModdingAPI.ModBuildConfig.Analyzer
                     return false;
             }
 
+            // collection conversion to an implemented interface is OK
+            if (AnalyzerUtilities.GetConcreteTypes(typeInfo.Type).Any(p => p.ToString().StartsWith(this.NetCollectionTypeFullName))) // StartsWith to ignore generics
+            {
+                string toType = typeInfo.ConvertedType.ToString();
+                if (toType.StartsWith(typeof(IEnumerable<>).Namespace) || toType == typeof(IEnumerable).FullName)
+                    return false;
+            }
+
             // avoid any other conversions
             return true;
         }