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

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/Module.verified.txt

@@ -335,12 +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>,
+        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
@@ -369,12 +369,12 @@ public partial struct TestScheduleIssues
                   ^^^^^^^^^^^^^^^^^^^
     {
 */
-      Message: View 'ViewDefIEnumerableReturnFromIter' must return T?, List<T>, or IQuery<T>,
+      Message: View 'ViewDefIEnumerableReturnFromIter' 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>,
+        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
@@ -403,12 +403,12 @@ public partial struct TestScheduleIssues
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         ViewContext ctx
 */
-      Message: View 'ViewDefIEnumerableReturnFromFilter' must return T?, List<T>, or IQuery<T>,
+      Message: View 'ViewDefIEnumerableReturnFromFilter' 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>,
+        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/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
@@ -1192,6 +1194,24 @@ 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);
@@ -1210,11 +1230,12 @@ method.ReturnType is INamedTypeSymbol
             diag.Report(ErrorDescriptor.ViewContextParam, methodSyntax);
         }
 
-        // Validate return type: must be List<T> or T?
+        // Validate return type: must be List<T>, T?, or IEnumerable<T>
         if (
             !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.ValueOption")
             && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.RefOption")
             && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.List")
+            && !ReturnsEnumerable
         )
         {
             diag.Report(ErrorDescriptor.ViewInvalidReturn, methodSyntax);
@@ -1292,6 +1313,18 @@ 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.

docs/docs/00200-core-concepts/00200-functions/00500-views.md

@@ -135,9 +135,9 @@ public static partial class Module
         return ctx.Db.Player.Identity.Find(ctx.Sender) as Player?;
     }
 
-    // Multiple rows: return a list
+    // Multiple rows: return a list or enumerable
     [SpacetimeDB.View(Accessor = "PlayersForLevel", Public = true)]
-    public static List<PlayerAndLevel> PlayersForLevel(AnonymousViewContext ctx)
+    public static IEnumerable<PlayerAndLevel> PlayersForLevel(AnonymousViewContext ctx)
     {
         var rows = new List<PlayerAndLevel>();
         foreach (var player in ctx.Db.PlayerLevel.Level.Filter(1))
@@ -159,7 +159,7 @@ public static partial class Module
 }
 ```
 
-Views must be static methods and can return either a single row (`T?`) or a list of rows (`List<T>` or `T[]`) where `T` can be a table type or any product type.
+Views must be static methods and can return either a single row (`T?`) or multiple rows (`IEnumerable<T>`, `List<T>`, or `T[]`) where `T` can be a table type or any product type.
 
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