Add HttpResponse.HeadersWritten

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/Ref.Standard.cs

@@ -427,6 +427,7 @@ internal HttpResponse() { }
         public string ContentType { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public System.Web.HttpCookieCollection Cookies { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public System.Collections.Specialized.NameValueCollection Headers { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
+        public bool HeadersWritten { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public bool IsClientConnected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public bool IsRequestBeingRedirected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public System.IO.TextWriter Output { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
@@ -467,6 +468,7 @@ public partial class HttpResponseBase
         public virtual string ContentType { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public virtual System.Web.HttpCookieCollection Cookies { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public virtual System.Collections.Specialized.NameValueCollection Headers { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
+        public virtual bool HeadersWritten { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public virtual bool IsClientConnected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public virtual bool IsRequestBeingRedirected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public virtual System.IO.TextWriter Output { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
@@ -500,6 +502,7 @@ public partial class HttpResponseWrapper : System.Web.HttpResponseBase
         public override string ContentType { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public override System.Web.HttpCookieCollection Cookies { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public override System.Collections.Specialized.NameValueCollection Headers { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
+        public override bool HeadersWritten { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public override bool IsClientConnected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public override bool IsRequestBeingRedirected { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public override System.IO.TextWriter Output { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} set { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }

src/Microsoft.AspNetCore.SystemWebAdapters/HttpResponse.cs

@@ -168,6 +168,11 @@ public TextWriter Output
 
         public void AppendHeader(string name, string value) => _response.Headers.Append(name, value);
 
+        public bool HeadersWritten
+        {
+            get => _response.HasStarted;
+        }
+
         public string? RedirectLocation
         {
             get => _response.Headers.Location;

src/Microsoft.AspNetCore.SystemWebAdapters/HttpResponseBase.cs

@@ -35,6 +35,14 @@ public virtual NameValueCollection Headers
             get => throw new NotImplementedException();
         }
 
+        public virtual bool HeadersWritten
+        {
+            get
+            {
+                throw new NotImplementedException();
+            }
+        }
+
         public virtual bool TrySkipIisCustomErrors
         {
             get => throw new NotImplementedException();

src/Microsoft.AspNetCore.SystemWebAdapters/HttpResponseWrapper.cs

@@ -4,6 +4,7 @@
 using System.Collections.Specialized;
 using System.IO;
 using System.Text;
+using Microsoft.AspNetCore.Http;
 
 namespace System.Web
 {
@@ -47,6 +48,8 @@ public override HttpCookieCollection Cookies
 
         public override NameValueCollection Headers => _response.Headers;
 
+        public override bool HeadersWritten => _response.HeadersWritten;
+
         public override bool IsClientConnected => _response.IsClientConnected;
 
         public override TextWriter Output