SERVERPROPERTY, SESSIONPROPERTY, CONNECTIONPROPERTY, COLLATIONPROPERTY, LOGINPROPERTY, SESSION_CONTEXT, SQL_VARIANT_PROPERTY and sys.configurations' value columns project true sql_variant instead of inner-type substitution, carrying per-property inner base types (COLLATIONPROPERTY Version is tinyint, sys.configurations values are int) so mixed-inner UNION/ISNULL/COALESCE/CASE stay variant, SELECT INTO creates a sql_variant column, SESSION_CONTEXT preserves the stored value's type, comparisons unwrap, the wire reports 0x62 COLMETADATA, and arithmetic rejects with Msg 257 (variant with a convertible non-variant) split enforced identically at parse and run time.

Author: RyanLamansky

SqlServerSimulator.Tests.SqlClient/SqlVariantWireTests.cs

@@ -78,6 +78,46 @@ LEFT JOIN [sys].[database_scoped_configurations] AS [dbscl] WITH (NOLOCK)
         IsFalse((bool)reader["LegacyCardinalityEstimation"]);
     }
 
+    // SERVERPROPERTY projects sql_variant over the wire (0x62 COLMETADATA) like
+    // real; each cell carries its probed inner base type — EngineEdition an int,
+    // Edition an nvarchar string.
+    [TestMethod]
+    public async Task ServerProperty_ReadOverWire_ReportsVariantWithInnerTypes()
+    {
+        var simulation = new Simulation();
+        await using var listener = await simulation.ListenAsync(0, TestContext.CancellationToken);
+        await using var connection = await Wire.OpenAsync(listener, TestContext.CancellationToken);
+        await using var command = new SqlCommand(
+            "SELECT SERVERPROPERTY('EngineEdition') AS engine, SERVERPROPERTY('Edition') AS edition", connection);
+        await using var reader = await command.ExecuteReaderAsync(TestContext.CancellationToken);
+
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        AreEqual("sql_variant", reader.GetDataTypeName(1));
+        IsTrue(await reader.ReadAsync(TestContext.CancellationToken));
+        _ = IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(3, reader.GetValue(0));
+        AreEqual("Developer Edition (64-bit)", reader.GetValue(1));
+    }
+
+    // SESSION_CONTEXT round-trips the stored value's base type through the
+    // sql_variant wire form: an int stored reads back as a boxed int.
+    [TestMethod]
+    public async Task SessionContext_ReadOverWire_PreservesIntInner()
+    {
+        var simulation = new Simulation();
+        await using var listener = await simulation.ListenAsync(0, TestContext.CancellationToken);
+        await using var connection = await Wire.OpenAsync(listener, TestContext.CancellationToken);
+        await using (var set = new SqlCommand("exec sp_set_session_context N'tenant', 42", connection))
+            _ = await set.ExecuteNonQueryAsync(TestContext.CancellationToken);
+        await using var command = new SqlCommand("SELECT SESSION_CONTEXT(N'tenant')", connection);
+        await using var reader = await command.ExecuteReaderAsync(TestContext.CancellationToken);
+
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        IsTrue(await reader.ReadAsync(TestContext.CancellationToken));
+        _ = IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(42, reader.GetValue(0));
+    }
+
     [TestMethod]
     public async Task CastWrappedInnerTypes_ReadOverWire()
     {

SqlServerSimulator.Tests/CatalogViewTests.cs

@@ -360,16 +360,18 @@ public void DmOsHostInfo_HostSkuNullOffWindows()
     }
 
     [TestMethod]
-    public void SysConfigurations_AgentXps_ValueInUseIsBigintZero()
+    public void SysConfigurations_AgentXps_ValueInUseIsVariantIntZero()
     {
         // The exact query SSMS's SMO issues during its Object-Explorer
         // database-node preamble. Msg 208 here aborts the request before the
-        // database enumeration, so the row must resolve and surface bigint.
+        // database enumeration, so the row must resolve. value_in_use is
+        // sql_variant carrying an int inner (like real).
         using var reader = new Simulation().ExecuteReader(
             "select value_in_use from sys.configurations where configuration_id = 16384");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
         IsTrue(reader.Read());
-        AreEqual(typeof(long), reader.GetFieldType(0));
-        AreEqual(0L, reader.GetInt64(0));
+        _ = Assert.IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(0, reader.GetValue(0));
         IsFalse(reader.Read());
     }
 
@@ -389,10 +391,12 @@ public void SysConfigurations_ColumnShape()
         IsTrue(reader.Read());
         AreEqual(typeof(int), reader.GetFieldType(0));
         AreEqual(typeof(string), reader.GetFieldType(1));
-        AreEqual(typeof(long), reader.GetFieldType(2));
-        AreEqual(typeof(long), reader.GetFieldType(3));
-        AreEqual(typeof(long), reader.GetFieldType(4));
-        AreEqual(typeof(long), reader.GetFieldType(5));
+        // value / minimum / maximum / value_in_use are sql_variant (object
+        // field type), matching real — each wraps an int inner.
+        AreEqual(typeof(object), reader.GetFieldType(2));
+        AreEqual(typeof(object), reader.GetFieldType(3));
+        AreEqual(typeof(object), reader.GetFieldType(4));
+        AreEqual(typeof(object), reader.GetFieldType(5));
         AreEqual(typeof(string), reader.GetFieldType(6));
         AreEqual(typeof(bool), reader.GetFieldType(7));
         AreEqual(typeof(bool), reader.GetFieldType(8));
@@ -409,10 +413,11 @@ public void SysConfigurations_AgentXpsRow_MatchesReference()
         IsTrue(reader.Read());
         AreEqual(16384, reader.GetInt32(0));
         AreEqual("Agent XPs", reader.GetString(1));
-        AreEqual(0L, reader.GetInt64(2));
-        AreEqual(0L, reader.GetInt64(3));
-        AreEqual(1L, reader.GetInt64(4));
-        AreEqual(0L, reader.GetInt64(5));
+        // The four sql_variant columns wrap an int inner (probe-confirmed).
+        AreEqual(0, reader.GetValue(2));
+        AreEqual(0, reader.GetValue(3));
+        AreEqual(1, reader.GetValue(4));
+        AreEqual(0, reader.GetValue(5));
         AreEqual("Enable or disable Agent XPs", reader.GetString(6));
         IsTrue(reader.GetBoolean(7));
         IsTrue(reader.GetBoolean(8));

SqlServerSimulator.Tests/CollationMetadataTests.cs

@@ -306,9 +306,9 @@ public void DatabasePropertyEx_MissingCloseParen_RaisesSyntaxError()
             "SELECT DATABASEPROPERTYEX('simulated', 'Status' extra"));
 
     // === COLLATIONPROPERTY(collation_name, property) ===
-    // Probe-confirmed against SQL Server 2025 (2026-07-14). Real SQL Server
-    // projects sql_variant; the simulator surfaces the bare base type
-    // (int for CodePage/LCID/ComparisonStyle/Version, nvarchar for Name).
+    // Probe-confirmed against SQL Server 2025 (2026-07-14). Like real, the
+    // result is sql_variant carrying a per-property inner base type: int for
+    // CodePage/LCID/ComparisonStyle, tinyint for Version, nvarchar for Name.
 
     [TestMethod]
     public void CollationProperty_CodePage_ReturnsAnsiCodePage()
@@ -327,19 +327,32 @@ public void CollationProperty_ComparisonStyle_ReturnsBitmask()
 
     [TestMethod]
     public void CollationProperty_Version_SqlCollation_IsZero()
-        => AreEqual(0, new Simulation().ExecuteScalar(
+        => AreEqual((byte)0, new Simulation().ExecuteScalar(
             "SELECT COLLATIONPROPERTY('SQL_Latin1_General_CP1_CI_AS', 'Version')"));
 
     [TestMethod]
     public void CollationProperty_Version_HundredSeriesCollation_IsTwo()
-        => AreEqual(2, new Simulation().ExecuteScalar(
+        => AreEqual((byte)2, new Simulation().ExecuteScalar(
             "SELECT COLLATIONPROPERTY('Latin1_General_100_CI_AS', 'Version')"));
 
     [TestMethod]
     public void CollationProperty_Name_ReturnsCollationName()
         => AreEqual("SQL_Latin1_General_CP1_CI_AS", new Simulation().ExecuteScalar(
             "SELECT COLLATIONPROPERTY('SQL_Latin1_General_CP1_CI_AS', 'Name')"));
 
+    // The result projects as sql_variant; CodePage carries an int inner, Version
+    // a tinyint inner (probe-confirmed against SQL Server 2025).
+    [TestMethod]
+    public void CollationProperty_ReportsSqlVariantMetadataWithInnerType()
+    {
+        using var reader = new Simulation().ExecuteReader(
+            "SELECT COLLATIONPROPERTY('SQL_Latin1_General_CP1_CI_AS', 'CodePage')");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        IsTrue(reader.Read());
+        _ = Assert.IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(1252, reader.GetValue(0));
+    }
+
     // ComparisonStyle tracks the suffix flags — accent-insensitive adds bit 2,
     // case-sensitive clears bit 1, KS/WS clear the ignore-kana/width bits, and
     // binary collations report 0. Probe-confirmed values.

SqlServerSimulator.Tests/FormatMessageAndLoginScalarTests.cs

@@ -280,22 +280,25 @@ public void PwdCompare_VerifiesRealServerGeneratedHash()
 
     // ---- LOGINPROPERTY ----
 
+    // Each property surfaces its probed inner base type through the
+    // sql_variant result: datetime for the time properties, int for the
+    // counters / flags, nvarchar for the name properties.
     [TestMethod]
     public void LoginProperty_KnownLogin_TimeAndCountProperties()
     {
-        AreEqual("2020-01-01 00:00:00.000", Scalar("select loginproperty('dbo', 'PasswordLastSetTime')"));
-        AreEqual("0", Scalar("select loginproperty('dbo', 'BadPasswordCount')"));
-        AreEqual("1900-01-01 00:00:00.000", Scalar("select loginproperty('dbo', 'BadPasswordTime')"));
-        AreEqual("1900-01-01 00:00:00.000", Scalar("select loginproperty('dbo', 'LockoutTime')"));
-        AreEqual("0", Scalar("select loginproperty('dbo', 'HistoryLength')"));
+        AreEqual(new DateTime(2020, 1, 1), Scalar("select loginproperty('dbo', 'PasswordLastSetTime')"));
+        AreEqual(0, Scalar("select loginproperty('dbo', 'BadPasswordCount')"));
+        AreEqual(new DateTime(1900, 1, 1), Scalar("select loginproperty('dbo', 'BadPasswordTime')"));
+        AreEqual(new DateTime(1900, 1, 1), Scalar("select loginproperty('dbo', 'LockoutTime')"));
+        AreEqual(0, Scalar("select loginproperty('dbo', 'HistoryLength')"));
     }
 
     [TestMethod]
     public void LoginProperty_KnownLogin_BooleanFlags()
     {
-        AreEqual("0", Scalar("select loginproperty('dbo', 'IsExpired')"));
-        AreEqual("0", Scalar("select loginproperty('dbo', 'IsLocked')"));
-        AreEqual("0", Scalar("select loginproperty('dbo', 'IsMustChange')"));
+        AreEqual(0, Scalar("select loginproperty('dbo', 'IsExpired')"));
+        AreEqual(0, Scalar("select loginproperty('dbo', 'IsLocked')"));
+        AreEqual(0, Scalar("select loginproperty('dbo', 'IsMustChange')"));
     }
 
     [TestMethod]
@@ -313,9 +316,22 @@ public void LoginProperty_KnownLogin_NameProperties()
         AreEqual("us_english", Scalar("select loginproperty('dbo', 'DefaultLanguage')"));
     }
 
+    // The result projects as sql_variant carrying the property's inner base
+    // type — an int for a counter, a datetime for a time property.
+    [TestMethod]
+    public void LoginProperty_ReportsSqlVariantMetadataWithInnerType()
+    {
+        using var reader = new Simulation().ExecuteReader(
+            "select loginproperty('dbo', 'IsExpired') as flag, loginproperty('dbo', 'PasswordLastSetTime') as pwset");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        IsTrue(reader.Read());
+        _ = Assert.IsInstanceOfType<int>(reader.GetValue(0));
+        _ = Assert.IsInstanceOfType<DateTime>(reader.GetValue(1));
+    }
+
     [TestMethod]
     public void LoginProperty_CaseInsensitiveLoginAndProperty()
-        => AreEqual("0", Scalar("select loginproperty('DBO', 'isexpired')"));
+        => AreEqual(0, Scalar("select loginproperty('DBO', 'isexpired')"));
 
     [TestMethod]
     public void LoginProperty_UnknownLogin_ReturnsNull()

SqlServerSimulator.Tests/LoginDdlTests.cs

@@ -127,7 +127,7 @@ public void LoginProperty_IsLocked_CreatedLogin_ReturnsZero()
     {
         var sim = new Simulation();
         _ = sim.ExecuteNonQuery("create login t with password = 'Xy!12345'");
-        AreEqual("0", sim.ExecuteScalar("select loginproperty('t', 'IsLocked')"));
+        AreEqual(0, sim.ExecuteScalar("select loginproperty('t', 'IsLocked')"));
     }
 
     [TestMethod]

SqlServerSimulator.Tests/ServerPropertyTests.cs

@@ -22,12 +22,14 @@ public void ProductLevel_ReturnsRTM()
     public void EngineEdition_Returns3AsInt()
         => AreEqual(3, new Simulation().ExecuteScalar("select serverproperty('EngineEdition')"));
 
+    // The projection is sql_variant (like real); each cell surfaces its inner
+    // CLR type — EngineEdition an int, SqlCharSet / SqlSortOrder a tinyint byte.
     [TestMethod]
-    public void EngineEdition_SurfacesAsIntType()
+    public void EngineEdition_SurfacesAsVariantWithIntInner()
     {
         using var reader = new Simulation().ExecuteReader("select serverproperty('EngineEdition')");
         IsTrue(reader.Read());
-        AreEqual("int", reader.GetDataTypeName(0));
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
         _ = Assert.IsInstanceOfType<int>(reader.GetValue(0));
         AreEqual(3, reader.GetValue(0));
     }
@@ -37,7 +39,7 @@ public void SqlCharSet_ReturnsTinyIntOne()
     {
         using var reader = new Simulation().ExecuteReader("select serverproperty('SqlCharSet')");
         IsTrue(reader.Read());
-        AreEqual("tinyint", reader.GetDataTypeName(0));
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
         AreEqual((byte)1, reader.GetValue(0));
     }
 
@@ -46,7 +48,7 @@ public void SqlSortOrder_ReturnsTinyInt52OnDefaultCollation()
     {
         using var reader = new Simulation().ExecuteReader("select serverproperty('SqlSortOrder')");
         IsTrue(reader.Read());
-        AreEqual("tinyint", reader.GetDataTypeName(0));
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
         AreEqual((byte)52, reader.GetValue(0));
     }
 
@@ -74,20 +76,30 @@ public void ProductUpdateReference_ReturnsKb()
     public void ResourceVersion_ReturnsReferenceBuild()
         => AreEqual("17.00.4065", new Simulation().ExecuteScalar("select serverproperty('ResourceVersion')"));
 
+    // The result is always sql_variant, whether or not the name argument is a
+    // compile-time constant (a runtime @variable resolves the same value).
     [TestMethod]
-    public void NonConstantName_FallsBackToNVarchar()
-        => AreEqual("3", new Simulation().ExecuteScalar(
+    public void NonConstantName_StillReturnsVariantInner()
+        => AreEqual(3, new Simulation().ExecuteScalar(
             "declare @p nvarchar(30) = 'EngineEdition'; select serverproperty(@p)"));
 
-    // Typed values must survive set-op schema unification. int (EngineEdition)
-    // and tinyint (SqlCharSet) promote to int per data-type precedence. Mixing
-    // a numeric property with a string one instead raises a conversion error
-    // (int outranks nvarchar) — a deliberate divergence from real SQL Server,
-    // where every SERVERPROPERTY is sql_variant so no promotion occurs.
+    // sql_variant UNION ALL keeps each row's own inner base type — no schema
+    // unification / promotion (matching real, where SERVERPROPERTY is
+    // sql_variant). Mixing a numeric property with a string one now succeeds
+    // where the old bare-type substitution raised a conversion error.
     [TestMethod]
-    public void UnionOfNumericProperties_PromotesToInt()
-        => AreEqual(3, new Simulation().ExecuteScalar(
-            "SELECT SERVERPROPERTY('EngineEdition') UNION ALL SELECT SERVERPROPERTY('SqlCharSet')"));
+    public void UnionOfMixedInnerProperties_KeepsPerRowInnerTypes()
+    {
+        using var reader = new Simulation().ExecuteReader(
+            "SELECT SERVERPROPERTY('EngineEdition') AS v UNION ALL SELECT SERVERPROPERTY('Edition')");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        IsTrue(reader.Read());
+        _ = Assert.IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(3, reader.GetValue(0));
+        IsTrue(reader.Read());
+        _ = Assert.IsInstanceOfType<string>(reader.GetValue(0));
+        AreEqual("Developer Edition (64-bit)", reader.GetValue(0));
+    }
 
     [TestMethod]
     public void Collation_ReturnsServerCollation()

SqlServerSimulator.Tests/SessionContextTests.cs

@@ -6,9 +6,11 @@ namespace SqlServerSimulator;
 [TestClass]
 public sealed class SessionContextTests
 {
+    // SESSION_CONTEXT preserves the stored value's base type through the
+    // sql_variant result: an int round-trips as int, an nvarchar as nvarchar.
     [TestMethod]
-    public void SetAndRead_Named()
-        => AreEqual("42", ExecuteScalar(
+    public void SetAndRead_Named_PreservesIntType()
+        => AreEqual(42, ExecuteScalar(
             "exec sp_set_session_context @key = N'TenantId', @value = 42; select session_context(N'TenantId')"));
 
     [TestMethod]
@@ -20,6 +22,29 @@ public void SetAndRead_Positional()
     public void MissingKey_ReturnsNull()
         => IsInstanceOfType<DBNull>(ExecuteScalar("select session_context(N'nope')"));
 
+    // SESSION_CONTEXT projects sql_variant (like real); a stored int surfaces
+    // its int inner and compares against an int column by unwrapping.
+    [TestMethod]
+    public void ReportsSqlVariantMetadata()
+    {
+        using var reader = new Simulation().ExecuteReader(
+            "exec sp_set_session_context N'k', 7; select session_context(N'k')");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        AreEqual(typeof(object), reader.GetFieldType(0));
+        IsTrue(reader.Read());
+        _ = IsInstanceOfType<int>(reader.GetValue(0));
+        AreEqual(7, reader.GetValue(0));
+    }
+
+    [TestMethod]
+    public void ConnectionProperty_ReportsSqlVariantMetadata()
+    {
+        using var reader = new Simulation().ExecuteReader("select connectionproperty('net_transport')");
+        AreEqual("sql_variant", reader.GetDataTypeName(0));
+        IsTrue(reader.Read());
+        AreEqual("TCP", reader.GetValue(0));
+    }
+
     [TestMethod]
     public void Key_IsCaseSensitive()
         => IsInstanceOfType<DBNull>(ExecuteScalar(