fix(http-server-csharp): fix CS1763 - use correct type for numeric literal properties in error model constructors

Agent-Logs-Url: https://github.com/microsoft/typespec/sessions/172760bb-1724-46d4-966b-e8e1ed0aaecc

Co-authored-by: markcowl <1054056+markcowl@users.noreply.github.com>

Author: Copilot

.chronus/changes/fix-cs1763-1779128907.md

@@ -0,0 +1,9 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Fix invalid constructor generated for `@error` models with numeric literal properties
+
+When an `@error` model has a property with a numeric or floating-point literal type (e.g., `status: 404` or `retryAfter: 1.5`), the emitter now generates a correctly-typed constructor parameter (`int`/`double`) instead of `object`, which previously caused a CS1763 compiler error.

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

@@ -95,6 +95,7 @@ import {
   coalesceUnionTypes,
   ensureCSharpIdentifier,
   ensureCleanDirectory,
+  findNumericType,
   formatComment,
   getBusinessLogicCallParameters,
   getBusinessLogicDeclParameters,
@@ -492,8 +493,12 @@ export async function $onEmit(context: EmitContext<CSharpServiceEmitterOptions>)
         }
 
         const type = getCSharpType(program, prop.type);
+        const typeName =
+          prop.type.kind === "Number"
+            ? findNumericType(prop.type)[0]
+            : (type?.type.name ?? "object");
         properties.push(
-          `${type?.type.name} ${prop.name}${defaultValue ? ` = ${defaultValue}` : `${prop.optional ? " = default" : ""}`}`,
+          `${typeName} ${prop.name}${defaultValue ? ` = ${defaultValue}` : `${prop.optional ? " = default" : ""}`}`,
         );
         body.push(`\t\t${propertyName} = ${prop.name};`);
 

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

@@ -3206,6 +3206,43 @@ describe("emit correct code for `@error` models", () => {
       ],
     );
   });
+  it("emits correct constructor parameter type for numeric literal property", async () => {
+    await compileAndValidateSingleModel(
+      tester,
+      `
+        @error
+        model NotFoundProblem {
+          title: string;
+          status: 404;
+        }
+      `,
+      "NotFoundProblem.cs",
+      [
+        "public partial class NotFoundProblem : HttpServiceException {",
+        `public NotFoundProblem(string title, int status = 404) : base(400,`,
+        `value: new{title = title,status = status})`,
+        `public int Status { get; } = 404;`,
+      ],
+    );
+  });
+  it("emits correct constructor parameter type for float literal property", async () => {
+    await compileAndValidateSingleModel(
+      tester,
+      `
+        @error
+        model RateLimitError {
+          message: string;
+          retryAfter: 1.5;
+        }
+      `,
+      "RateLimitError.cs",
+      [
+        "public partial class RateLimitError : HttpServiceException {",
+        `public RateLimitError(string message, double retryAfter = 1.5) : base(400,`,
+        `public double RetryAfter { get; } = 1.5;`,
+      ],
+    );
+  });
   it("emit error constructor properties and defined in body", async () => {
     await compileAndValidateSingleModel(
       tester,