[C#] Add support for IEnumerable<T> for Views' return types

crates/bindings-csharp/Codegen.Tests/Tests.cs

@@ -300,16 +300,15 @@ public static async Task ViewInvalidReturnHighlightsReturnType()
                         Method = method,
                     })
             )
-            .Single(entry => entry.Method.Identifier.Text == "ViewDefIEnumerableReturnFromIter");
+            .Single(entry => entry.Method.Identifier.Text == "ViewDefWrongReturn");
 
         var returnTypeSpan = method.Method.ReturnType.Span;
         var diagnostics = runResult
             .Results.SelectMany(result => result.Diagnostics)
             .Where(d => d.Id == "STDB0024")
             .ToList();
         var diagnostic = diagnostics.FirstOrDefault(d =>
-            d.GetMessage().Contains("ViewDefIEnumerableReturnFromIter")
-            && d.Location.SourceTree == method.Tree
+            d.GetMessage().Contains("ViewDefWrongReturn") && d.Location.SourceTree == method.Tree
         );
 
         Assert.NotNull(diagnostic);
@@ -319,6 +318,6 @@ public static async Task ViewInvalidReturnHighlightsReturnType()
         var returnTypeText = method
             .Root.ToFullString()
             .Substring(returnTypeSpan.Start, returnTypeSpan.Length);
-        Assert.Contains("IEnumerable", returnTypeText);
+        Assert.Contains("Player", returnTypeText);
     }
 }

crates/bindings-csharp/Codegen.Tests/fixtures/diag/Lib.cs

@@ -569,14 +569,14 @@ public static Player ViewDefWrongReturn(ViewContext ctx)
         return new Player { Identity = new() };
     }
 
-    // Invalid: IEnumerable<T> return type (from Iter()) is not List<T> or T?
+    // Valid: IEnumerable<T> return type (from Iter()) is supported
     [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_iter", Public = true)]
     public static IEnumerable<Player> ViewDefIEnumerableReturnFromIter(ViewContext ctx)
     {
         return ctx.Db.Player.Iter();
     }
 
-    // Invalid: IEnumerable<T> return type (from Filter()) is not List<T> or T?
+    // Valid: IEnumerable<T> return type (from Filter()) is supported
     [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_filter", Public = true)]
     public static IEnumerable<TestScheduleIssues> ViewDefIEnumerableReturnFromFilter(
         ViewContext ctx

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/ExtraCompilationErrors.verified.txt

@@ -256,7 +256,7 @@ public partial struct TestDefaultFieldValues
 
             var returnValue = Module.ViewDefWrongContext((SpacetimeDB.ViewContext)ctx);
                                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-                SpacetimeDB.BSATN.List<Player, Player.BSATN> returnRW = new();
+                var listSerializer = new SpacetimeDB.BSATN.List<Player, Player.BSATN>();
 */
     Message: Argument 1: cannot convert from 'SpacetimeDB.ViewContext' to 'SpacetimeDB.ReducerContext',
     Severity: Error,
@@ -325,7 +325,7 @@ SpacetimeDB.Internal.Module.RegisterClientVisibilityFilter(global::Module.MY_THI
 
             var returnValue = Module.ViewDefNoContext((SpacetimeDB.ViewContext)ctx);
                                      ^^^^^^^^^^^^^^^^
-                SpacetimeDB.BSATN.List<Player, Player.BSATN> returnRW = new();
+                var listSerializer = new SpacetimeDB.BSATN.List<Player, Player.BSATN>();
 */
     Message: No overload for method 'ViewDefNoContext' takes 1 arguments,
     Severity: Error,

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/Module.verified.txt

@@ -335,80 +335,12 @@ public partial struct TestScheduleIssues
                   ^^^^^^
     {
 */
-      Message: View 'ViewDefWrongReturn' must return T?, List<T>, or IQuery<T>,
+      Message: View 'ViewDefWrongReturn' must return T?, List<T>, IQuery<T>, or IEnumerable<T>.,
       Severity: Error,
       Descriptor: {
         Id: STDB0024,
-        Title: Views must return T?, List<T>, or IQuery<T>,
-        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
-        Category: SpacetimeDB,
-        DefaultSeverity: Error,
-        IsEnabledByDefault: true
-      }
-    },
-    {/*
-    [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_iter", Public = true)]
-    public static IEnumerable<Player> ViewDefIEnumerableReturnFromIter(ViewContext ctx)
-                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    {
-*/
-      Message: BSATN implementation for System.Collections.Generic.IEnumerable<Player> is not found: Unsupported system type System.Collections.Generic.IEnumerable<T>,
-      Severity: Error,
-      Descriptor: {
-        Id: BSATN0001,
-        Title: Unsupported type,
-        MessageFormat: BSATN implementation for {0} is not found: {1},
-        Category: SpacetimeDB.BSATN,
-        DefaultSeverity: Error,
-        IsEnabledByDefault: true
-      }
-    },
-    {/*
-    [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_iter", Public = true)]
-    public static IEnumerable<Player> ViewDefIEnumerableReturnFromIter(ViewContext ctx)
-                  ^^^^^^^^^^^^^^^^^^^
-    {
-*/
-      Message: View 'ViewDefIEnumerableReturnFromIter' must return T?, List<T>, or IQuery<T>,
-      Severity: Error,
-      Descriptor: {
-        Id: STDB0024,
-        Title: Views must return T?, List<T>, or IQuery<T>,
-        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
-        Category: SpacetimeDB,
-        DefaultSeverity: Error,
-        IsEnabledByDefault: true
-      }
-    },
-    {/*
-    [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_filter", Public = true)]
-    public static IEnumerable<TestScheduleIssues> ViewDefIEnumerableReturnFromFilter(
-                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-        ViewContext ctx
-*/
-      Message: BSATN implementation for System.Collections.Generic.IEnumerable<TestScheduleIssues> is not found: Unsupported system type System.Collections.Generic.IEnumerable<T>,
-      Severity: Error,
-      Descriptor: {
-        Id: BSATN0001,
-        Title: Unsupported type,
-        MessageFormat: BSATN implementation for {0} is not found: {1},
-        Category: SpacetimeDB.BSATN,
-        DefaultSeverity: Error,
-        IsEnabledByDefault: true
-      }
-    },
-    {/*
-    [SpacetimeDB.View(Accessor = "view_def_ienumerable_return_from_filter", Public = true)]
-    public static IEnumerable<TestScheduleIssues> ViewDefIEnumerableReturnFromFilter(
-                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-        ViewContext ctx
-*/
-      Message: View 'ViewDefIEnumerableReturnFromFilter' must return T?, List<T>, or IQuery<T>,
-      Severity: Error,
-      Descriptor: {
-        Id: STDB0024,
-        Title: Views must return T?, List<T>, or IQuery<T>,
-        MessageFormat: View '{0}' must return T?, List<T>, or IQuery<T>,
+        Title: Views must return T?, List<T>, IQuery<T>, or IEnumerable<T>,
+        MessageFormat: View '{0}' must return T?, List<T>, IQuery<T>, or IEnumerable<T>.,
         Category: SpacetimeDB,
         DefaultSeverity: Error,
         IsEnabledByDefault: true

crates/bindings-csharp/Codegen.Tests/fixtures/explicitnames/snapshots/Module#FFI.verified.cs

@@ -366,13 +366,14 @@ public byte[] Invoke(
         try
         {
             var returnValue = Reducers.DemoView((SpacetimeDB.ViewContext)ctx);
-            SpacetimeDB.BSATN.List<DemoTable, DemoTable.BSATN> returnRW = new();
+            var listSerializer = new SpacetimeDB.BSATN.List<DemoTable, DemoTable.BSATN>();
+            var listValue = global::System.Linq.Enumerable.ToList(returnValue);
             var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
             var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
             using var output = new System.IO.MemoryStream();
             using var writer = new System.IO.BinaryWriter(output);
             headerRW.Write(writer, header);
-            returnRW.Write(writer, returnValue);
+            listSerializer.Write(writer, listValue);
             return output.ToArray();
         }
         catch (System.Exception e)

crates/bindings-csharp/Codegen/Diag.cs

@@ -215,8 +215,9 @@ string typeName
     public static readonly ErrorDescriptor<MethodDeclarationSyntax> ViewInvalidReturn =
         new(
             group,
-            "Views must return T?, List<T>, or IQuery<T>",
-            method => $"View '{method.Identifier}' must return T?, List<T>, or IQuery<T>",
+            "Views must return T?, List<T>, IQuery<T>, or IEnumerable<T>",
+            method =>
+                $"View '{method.Identifier}' must return T?, List<T>, IQuery<T>, or IEnumerable<T>.",
             method => method.ReturnType
         );
 

crates/bindings-csharp/Codegen/Module.cs

@@ -1120,6 +1120,7 @@ record ViewDeclaration
     public readonly bool IsAnonymous;
     public readonly bool IsPublic;
     public readonly bool ReturnsQuery;
+    public readonly bool ReturnsEnumerable;
     public readonly TypeUse ReturnType;
     public readonly EquatableArray<MemberDeclaration> Parameters;
     public readonly Scope Scope;
@@ -1155,6 +1156,7 @@ public ViewDeclaration(GeneratorAttributeSyntaxContext context, DiagReporter dia
         IsAnonymous = isAnonymousContext;
 
         ReturnsQuery = false;
+        ReturnsEnumerable = false;
         INamedTypeSymbol? iquery = null;
         if (
             method.ReturnType is INamedTypeSymbol
@@ -1192,9 +1194,40 @@ method.ReturnType is INamedTypeSymbol
             // Match Rust semantics: Query<T> is described as a nullable row (T?).
             ReturnType = new ReferenceUse(opt, opt);
         }
+        else if (
+            method.ReturnType
+                is INamedTypeSymbol
+                {
+                    OriginalDefinition: var originalDefinition,
+                    TypeArguments: [var enumerableElementType]
+                }
+            && originalDefinition.ToString() == "System.Collections.Generic.IEnumerable<T>"
+        )
+        {
+            ReturnsEnumerable = true;
+            var elementType = TypeUse.Parse(method, enumerableElementType, diag);
+            var elementTypeName = SymbolToName(enumerableElementType);
+            var listTypeName = $"System.Collections.Generic.List<{elementTypeName}>";
+            var listTypeInfo =
+                $"SpacetimeDB.BSATN.List<{elementTypeName}, {elementType.BSATNName}>";
+            ReturnType = new ListUse(listTypeName, listTypeInfo, elementType);
+        }
         else
         {
             ReturnType = TypeUse.Parse(method, method.ReturnType, diag);
+
+            if (
+                method.ReturnType
+                    is INamedTypeSymbol
+                    {
+                        OriginalDefinition: var listDefinition,
+                        TypeArguments.Length: 1,
+                    }
+                && listDefinition.ToString() == "System.Collections.Generic.List<T>"
+            )
+            {
+                ReturnsEnumerable = true;
+            }
         }
         Scope = new Scope(methodSyntax.Parent as MemberDeclarationSyntax);
 
@@ -1210,12 +1243,12 @@ method.ReturnType is INamedTypeSymbol
             diag.Report(ErrorDescriptor.ViewContextParam, methodSyntax);
         }
 
-        // Validate return type: must be List<T> or T?
-        if (
-            !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.ValueOption")
-            && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.RefOption")
-            && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.List")
-        )
+        // Validate return type: must be List<T>, T?, or IEnumerable<T>
+        var isOption =
+            ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.ValueOption")
+            || ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.RefOption");
+
+        if (!ReturnsQuery && !ReturnsEnumerable && !isOption)
         {
             diag.Report(ErrorDescriptor.ViewInvalidReturn, methodSyntax);
         }
@@ -1292,6 +1325,18 @@ public string GenerateDispatcherClass(uint index)
                         listSerializer.Write(writer, listValue);
                         return output.ToArray();
                     """
+            : ReturnsEnumerable
+                ? $$$"""
+                        var listSerializer = new {{{ReturnType.BSATNName}}}();
+                        var listValue = global::System.Linq.Enumerable.ToList(returnValue);
+                        var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
+                        var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
+                        using var output = new System.IO.MemoryStream();
+                        using var writer = new System.IO.BinaryWriter(output);
+                        headerRW.Write(writer, header);
+                        listSerializer.Write(writer, listValue);
+                        return output.ToArray();
+                    """
             : $$$"""
                     {{{ReturnType.BSATNName}}} returnRW = new();
                     var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);

docs/docs/00200-core-concepts/00100-databases/00500-cheat-sheet.md

@@ -631,9 +631,9 @@ public static Player? MyPlayer(ViewContext ctx)
 
 // Return potentially multiple rows
 [SpacetimeDB.View(Accessor = "TopPlayers", Public = true)]
-public static List<Player> TopPlayers(ViewContext ctx)
+public static IEnumerable<Player> TopPlayers(ViewContext ctx)
 {
-    return ctx.Db.Player.Score.Filter(1000).ToList();
+    return ctx.Db.Player.Score.Filter(1000);
 }
 
 // Perform a generic filter using the query builder.