Send DbType.Decimal and DbType.DateTime2 as their types

DuckDB.NET.Data/PreparedStatement/ClrToDuckDBConverter.cs

@@ -6,6 +6,8 @@ internal static class ClrToDuckDBConverter
     {
         { DbType.Guid, value => NativeMethods.Value.DuckDBCreateUuid(((Guid)value).ToHugeInt(false)) },
         { DbType.Currency, value => DecimalToDuckDBValue((decimal)value) },
+        // Decimal and DateTime2 had no converter: any other value keeps the fallback below, as before.
+        { DbType.Decimal, value => value is decimal number ? DecimalToDuckDBValue(number) : NativeMethods.Value.DuckDBCreateVarchar(value.ToString()) },
         { DbType.Boolean, value => NativeMethods.Value.DuckDBCreateBool((bool)value) },
         { DbType.SByte, value => NativeMethods.Value.DuckDBCreateInt8((sbyte)value) },
         { DbType.Int16, value => NativeMethods.Value.DuckDBCreateInt16((short)value) },
@@ -43,12 +45,8 @@ internal static class ClrToDuckDBConverter
                 return NativeMethods.Value.DuckDBCreateTime(time);
             }
         },
-        { DbType.DateTime, value =>
-            {
-                var dateTime = value is DateTime dt ? dt.ToTimestampStruct(DuckDBType.Timestamp) : ((DuckDBTimestamp)value).ToDuckDBTimestampStruct();
-                return NativeMethods.Value.DuckDBCreateTimestamp(dateTime);
-            }
-        },
+        { DbType.DateTime, TimestampToDuckDBValue },
+        { DbType.DateTime2, value => value is DateTime or DuckDBTimestamp ? TimestampToDuckDBValue(value) : NativeMethods.Value.DuckDBCreateVarchar(value.ToString()) },
         { DbType.DateTimeOffset, value => NativeMethods.Value.DuckDBCreateTimestampTz(((DateTimeOffset)value).ToTimestampStruct()) },
     };
 
@@ -227,6 +225,12 @@ private static unsafe DuckDBValue BigIntegerToDuckDBValue(BigInteger value)
         }
     }
 
+    private static DuckDBValue TimestampToDuckDBValue(object value)
+    {
+        var dateTime = value is DateTime dt ? dt.ToTimestampStruct(DuckDBType.Timestamp) : ((DuckDBTimestamp)value).ToDuckDBTimestampStruct();
+        return NativeMethods.Value.DuckDBCreateTimestamp(dateTime);
+    }
+
     private static DuckDBValue DecimalToDuckDBValue(decimal value)
     {
         // The value goes to DuckDB as it is in the decimal: the integer, with the scale of the decimal. The writer of

DuckDB.NET.Data/PreparedStatement/DuckDBTypeMap.cs

@@ -13,7 +13,7 @@ internal static class DuckDBTypeMap
         {typeof(double), DbType.Double},
         {typeof(string), DbType.String},
         {typeof(Guid), DbType.Guid},
-        {typeof(decimal), DbType.Currency},
+        {typeof(decimal), DbType.Decimal},
         {typeof(byte), DbType.Byte},
         {typeof(ushort), DbType.UInt16},
         {typeof(uint), DbType.UInt32},

DuckDB.NET.Test/Parameters/DecimalParameterTest.cs

@@ -456,4 +456,58 @@ public void ParameterWithMoreDigitsThanItsColumnIsRefused()
         Command.CommandText = "SELECT list(value ORDER BY rowid)::VARCHAR FROM DecimalParameterRefused";
         Command.ExecuteScalar().Should().Be("[999, -999, 999]");
     }
+
+    // ADO.NET infers DbType.Decimal for a decimal value, and GetSchema("DataTypes") maps DECIMAL to System.Decimal.
+    [Fact]
+    public void DecimalValueInfersDbTypeDecimal()
+    {
+        new DuckDBParameter(1.5m).DbType.Should().Be(DbType.Decimal);
+    }
+
+    // DbType.Decimal had no converter, so a parameter that set it was sent as decimal.ToString() in the current
+    // culture: "1234,5" under fr-FR, stored as such in a VARCHAR column and refused by a DOUBLE one.
+    [Theory]
+    [InlineData(DbType.Decimal)]
+    [InlineData(DbType.Currency)]
+    public void ExplicitDecimalDbTypeIsSentAsADecimalInAnyCulture(DbType dbType)
+    {
+        var defaultCulture = System.Threading.Thread.CurrentThread.CurrentCulture;
+        try
+        {
+            System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("fr-FR");
+
+            Command.Parameters.Clear();
+            Command.CommandText = "SELECT ?";
+            Command.Parameters.Add(new DuckDBParameter(1234.5m) { DbType = dbType });
+            Command.ExecuteScalar().Should().BeOfType<decimal>().Subject.Should().Be(1234.5m);
+
+            Command.CommandText = "CREATE OR REPLACE TABLE DecimalDbTypeTests (text VARCHAR, number DOUBLE)";
+            Command.Parameters.Clear();
+            Command.ExecuteNonQuery();
+
+            Command.CommandText = "INSERT INTO DecimalDbTypeTests VALUES (?, ?)";
+            Command.Parameters.Add(new DuckDBParameter(1234.5m) { DbType = dbType });
+            Command.Parameters.Add(new DuckDBParameter(1234.5m) { DbType = dbType });
+            Command.ExecuteNonQuery();
+
+            Command.Parameters.Clear();
+            Command.CommandText = "SELECT text || ' ' || number::VARCHAR FROM DecimalDbTypeTests";
+            Command.ExecuteScalar().Should().Be("1234.5 1234.5");
+        }
+        finally
+        {
+            System.Threading.Thread.CurrentThread.CurrentCulture = defaultCulture;
+        }
+    }
+
+    // Other values with DbType.Decimal are still sent as before.
+    [Fact]
+    public void ExplicitDecimalDbTypeWithAnIntegerValueIsStillSent()
+    {
+        Command.CommandText = "SELECT ?::DECIMAL(4,1)";
+        Command.Parameters.Clear();
+        Command.Parameters.Add(new DuckDBParameter(5) { DbType = DbType.Decimal });
+
+        Command.ExecuteScalar().Should().Be(5.0m);
+    }
 }

DuckDB.NET.Test/Parameters/TimestampTests.cs

@@ -216,6 +216,38 @@ public void BindParameterWithoutTable_DateTimeOffset()
         result.Should().Be(value);
     }
 
+    // DbType.DateTime2 had no converter, so the value was sent as DateTime.ToString() in the current culture:
+    // text a statement could misread or refuse, and a VARCHAR column stored as such.
+    [Fact]
+    public void ExplicitDateTime2DbTypeIsSentAsATimestampInAnyCulture()
+    {
+        var value = new DateTime(2026, 10, 18, 13, 45, 30, 123);
+        var defaultCulture = System.Threading.Thread.CurrentThread.CurrentCulture;
+        try
+        {
+            System.Threading.Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.GetCultureInfo("de-DE");
+
+            Command.CommandText = "SELECT ?;";
+            Command.Parameters.Add(new DuckDBParameter(value) { DbType = DbType.DateTime2 });
+
+            Command.ExecuteScalar().Should().BeOfType<DateTime>().Subject.Should().Be(value);
+        }
+        finally
+        {
+            System.Threading.Thread.CurrentThread.CurrentCulture = defaultCulture;
+        }
+    }
+
+    // Other values with DbType.DateTime2 are still sent as before.
+    [Fact]
+    public void ExplicitDateTime2DbTypeWithATextValueIsStillSent()
+    {
+        Command.CommandText = "SELECT ?::TIMESTAMP;";
+        Command.Parameters.Add(new DuckDBParameter("2026-10-18 13:45:30") { DbType = DbType.DateTime2 });
+
+        Command.ExecuteScalar().Should().Be(new DateTime(2026, 10, 18, 13, 45, 30));
+    }
+
     public static TimeOnly Trim(TimeOnly date, long ticks) => new(date.Ticks - date.Ticks % ticks);
 
     public static DateTime Trim(DateTime date, long ticks) => new(date.Ticks - date.Ticks % ticks, date.Kind);