Add HttpSessionState.Mode (#289)

This change exposes the API and sets it to `Custom`. Everything else has a meaning and implies certain behavior. This value is the only one that makes sense for us to declare. If needed, we can make it customizable later.

Fixes #281

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

@@ -569,6 +569,7 @@ internal HttpSessionState() { }
         public bool IsReadOnly { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public bool IsSynchronized { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public object this[string name] { 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.SessionState.SessionStateMode Mode { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public string SessionID { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public object SyncRoot { get { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");} }
         public int Timeout { 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");} }
@@ -580,4 +581,12 @@ internal HttpSessionState() { }
         public void Remove(string name) { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");}
         public void RemoveAll() { throw new System.PlatformNotSupportedException("Only supported when running on ASP.NET Core or System.Web");}
     }
+    public enum SessionStateMode
+    {
+        Custom = 4,
+        InProc = 1,
+        Off = 0,
+        SQLServer = 3,
+        StateServer = 2,
+    }
 }

src/Microsoft.AspNetCore.SystemWebAdapters/Generated/TypeForwards.Framework.cs

@@ -53,3 +53,4 @@
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.Caching.CacheItemUpdateReason))]
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.Configuration.HttpCapabilitiesBase))]
 [assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.SessionState.HttpSessionState))]
+[assembly:System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Web.SessionState.SessionStateMode))]

src/Microsoft.AspNetCore.SystemWebAdapters/SessionState/HttpSessionState.cs

@@ -35,6 +35,9 @@ public int Timeout
 
     public object SyncRoot => _container.SyncRoot;
 
+    [Diagnostics.CodeAnalysis.SuppressMessage("Performance", "CA1822:Mark members as static", Justification = Constants.ApiFromAspNet)]
+    public SessionStateMode Mode => SessionStateMode.Custom;
+
     public void Abandon() => _container.IsAbandoned = true;
 
     public object? this[string name]

src/Microsoft.AspNetCore.SystemWebAdapters/SessionState/SessionStateMode.cs

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Web.SessionState;
+
+public enum SessionStateMode
+{
+    Off = 0,
+    InProc = 1,
+    StateServer = 2,
+    SQLServer = 3,
+    Custom = 4
+};

test/Microsoft.AspNetCore.SystemWebAdapters.Tests/SessionState/HttpSessionStateTests.cs

@@ -36,6 +36,21 @@ public void SessionId()
         Assert.Equal(id, result);
     }
 
+    [Fact]
+    public void Mode()
+    {
+        // Arrange
+        var session = new Mock<ISessionState>();
+
+        var state = new HttpSessionState(session.Object);
+
+        // Act
+        var result = state.Mode;
+
+        // Assert
+        Assert.Equal(SessionStateMode.Custom, result);
+    }
+
     [Fact]
     public void Count()
     {