Fix http-server-csharp crash on interfaces with template operations

.chronus/changes/fix-http-server-csharp-template-parameter-2026-03-04.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Fix crash when emitting interfaces that contain template operations. Template operations (e.g. `getItem<T>(): T`) within interfaces will simply be skipped when emitting the interface.

packages/http-server-csharp/src/lib/service.ts

@@ -784,6 +784,7 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
         usings: [],
       };
       for (const [name, operation] of iface.operations) {
+        if (isTemplateDeclaration(operation)) continue;
         const doc = getDoc(this.emitter.getProgram(), operation);
         const returnTypes: Type[] = [];
         const [httpOp, _] = getHttpOperation(this.emitter.getProgram(), operation);
@@ -1004,7 +1005,12 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
           .checker.cloneType(responseType, { name: modelName });
         responseType = returnedType;
       }
-      this.emitter.emitType(responseType);
+      // TemplateParameter types cannot be emitted (they are unresolved template placeholders).
+      // Template operations are filtered in interfaceDeclarationOperations, but this guard
+      // prevents crashes if a TemplateParameter response type is encountered via other paths.
+      if (responseType.kind !== "TemplateParameter") {
+        this.emitter.emitType(responseType);
+      }
 
       const context = this.emitter.getContext();
       const result = getCSharpType(this.emitter.getProgram(), responseType, context.namespace);

packages/http-server-csharp/test/generation.test.ts

@@ -1245,6 +1245,23 @@ it("Handles user-defined model templates", async () => {
   );
 });
 
+it("Handles template operations in interfaces without crashing", async () => {
+  // Regression test: interfaces with template operations should not crash with
+  // "Encountered type TemplateParameter which we don't know how to emit."
+  // Template operations (e.g. getItem<T>(): T) should be skipped during emission.
+  await compileAndValidateMultiple(
+    tester,
+    `
+       interface MyOps {
+         @get list(): string;
+         @get getItem<T>(): T;
+       }
+    `,
+    [["IMyOps.cs", ["interface IMyOps", "Task<string> ListAsync( );"]]],
+    [["IMyOps.cs", ["GetItemAsync"]]],
+  );
+});
+
 it("Handles void type in operations", async () => {
   await compileAndValidateMultiple(
     tester,