Pass null values to custom type handlers

Dapper/SqlMapper.cs

@@ -2817,7 +2817,14 @@ private static bool IsValueTuple(Type? type) => (type?.IsValueType == true
                     il.Emit(OpCodes.Brtrue_S, notNull);
                     // relative stack [boxed value = null]
                     il.Emit(OpCodes.Pop); // relative stack empty
-                    il.Emit(OpCodes.Ldsfld, typeof(DBNull).GetField(nameof(DBNull.Value))!); // relative stack [DBNull]
+                    if (handler is null)
+                    {
+                        il.Emit(OpCodes.Ldsfld, typeof(DBNull).GetField(nameof(DBNull.Value))!); // relative stack [DBNull]
+                    }
+                    else
+                    {
+                        il.Emit(OpCodes.Ldnull); // relative stack [null for the type handler]
+                    }
                     if (dbType == DbType.String || dbType == DbType.AnsiString)
                     {
                         EmitInt32(il, 0);

Directory.Packages.props

@@ -38,6 +38,7 @@
     <PackageVersion Include="RepoDb.SqlServer" Version="1.13.1" />
     <PackageVersion Include="ServiceStack.OrmLite.SqlServer" Version="10.0.6" />
     <PackageVersion Include="Snowflake.Data" Version="5.5.0" />
+    <PackageVersion Include="pengdows.crud.fakeDb" Version="2.0.5" />
     <PackageVersion Include="SqlMarshal" Version="0.5.0" />
     <PackageVersion Include="SubSonic" Version="3.0.0.4" />
     <PackageVersion Include="Susanoo.SqlServer" Version="1.2.4.2" />

tests/Dapper.Tests/Dapper.Tests.csproj

@@ -32,6 +32,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' != 'net481'">
+    <PackageReference Include="pengdows.crud.fakeDb" />
     <PackageReference Include="Npgsql" />
     <PackageReference Include="Snowflake.Data" />
   </ItemGroup>

tests/Dapper.Tests/FakeDbTests.Issue1234.cs

@@ -0,0 +1,56 @@
+#if !NET481
+using System;
+using System.Data;
+using pengdows.crud.fakeDb;
+using Xunit;
+
+namespace Dapper.Tests;
+
+public sealed class FakeDbIssue1234Tests
+{
+    [Fact]
+    public void NullValueIsPassedToCustomTypeHandler()
+    {
+        SqlMapper.ResetTypeHandlers();
+        var handler = new NullAwareHandler();
+        SqlMapper.AddTypeHandler(handler);
+        try
+        {
+            using var connection = new fakeDbConnection(new FakeDataStore());
+            connection.EnqueueNonQueryResult(1);
+            connection.Open();
+
+            connection.Execute("select @value", new { value = (NullAwareValue?)null });
+
+            Assert.True(handler.Called);
+            Assert.Null(handler.Value);
+            Assert.Equal(DbType.Binary, handler.LastParameter!.DbType);
+            Assert.Equal(DBNull.Value, handler.LastParameter.Value);
+        }
+        finally
+        {
+            SqlMapper.ResetTypeHandlers();
+        }
+    }
+
+    private sealed class NullAwareValue;
+
+    private sealed class NullAwareHandler : SqlMapper.TypeHandler<NullAwareValue>
+    {
+        public bool Called { get; private set; }
+        public NullAwareValue? Value { get; private set; }
+        public IDbDataParameter? LastParameter { get; private set; }
+
+        public override NullAwareValue? Parse(object value) => null;
+
+        public override void SetValue(IDbDataParameter parameter, NullAwareValue? value)
+        {
+            Called = true;
+            Value = value;
+            LastParameter = parameter;
+            parameter.DbType = DbType.Binary;
+            parameter.Value = DBNull.Value;
+        }
+    }
+}
+#endif