From 55297ab63d2066d58f93c8d3eb348c63eb2bcbfd Mon Sep 17 00:00:00 2001
From: "Dailey, Alaric" <alaricd@pengdows.com>
Date: Sat, 24 Oct 2026 07:37:40 -0500
Subject: [PATCH] Use type handlers for list expansion parameters

---
 Dapper/SqlMapper.cs                         | 24 ++++++---
 Directory.Packages.props                    |  1 +
 tests/Dapper.Tests/Dapper.Tests.csproj      |  5 +-
 tests/Dapper.Tests/FakeDbTests.Issue1649.cs | 58 +++++++++++++++++++++
 4 files changed, 79 insertions(+), 9 deletions(-)
 create mode 100644 tests/Dapper.Tests/FakeDbTests.Issue1649.cs

diff --git a/Dapper/SqlMapper.cs b/Dapper/SqlMapper.cs
index 8f3956cab..c5f0df2ad 100644
--- a/Dapper/SqlMapper.cs
+++ b/Dapper/SqlMapper.cs
@@ -2180,9 +2180,10 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                 bool byPosition = ShouldPassByPosition(command.CommandText);
                 var list = value as IEnumerable;
                 var count = 0;
-                bool isString = value is IEnumerable<string>;
-                bool isDbString = value is IEnumerable<DbString>;
-                DbType? dbType = null;
+                bool isString = value is IEnumerable<string>;
+                bool isDbString = value is IEnumerable<DbString>;
+                DbType? dbType = null;
+                ITypeHandler? handler = null;
 
                 int splitAt = SqlMapper.Settings.InListStringSplitCount;
                 bool viaSplit = splitAt >= 0
@@ -2199,9 +2200,9 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                             {
                                 throw new NotSupportedException("The first item in a list-expansion cannot be null");
                             }
-                            if (!isDbString)
-                            {
-                                dbType = LookupDbType(item.GetType(), "", true, out var handler);
+                            if (!isDbString)
+                            {
+                                dbType = LookupDbType(item.GetType(), "", true, out handler);
                             }
                         }
                         var nextName = namePrefix + count.ToString();
@@ -2222,7 +2223,16 @@ public static void PackListParameters(IDbCommand command, string namePrefix, obj
                                 }
                             }
 
-                            var tmp = listParam.Value = SanitizeParameterValue(item);
+                            object? tmp;
+                            if (handler is null)
+                            {
+                                tmp = listParam.Value = SanitizeParameterValue(item);
+                            }
+                            else
+                            {
+                                handler.SetValue(listParam, item!);
+                                tmp = listParam.Value;
+                            }
                             if (tmp is not null && tmp is not DBNull)
                                 lastValue = tmp; // only interested in non-trivial values for padding
 
diff --git a/Directory.Packages.props b/Directory.Packages.props
index 2f91a3cca..8f2772419 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -35,6 +35,7 @@
     <PackageVersion Include="Norm.net" Version="5.4.0" />
     <PackageVersion Include="Npgsql" Version="10.0.2" />
     <PackageVersion Include="PetaPoco" Version="5.1.306" />
+    <PackageVersion Include="pengdows.crud.fakeDb" Version="2.0.5" />
     <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" />
diff --git a/tests/Dapper.Tests/Dapper.Tests.csproj b/tests/Dapper.Tests/Dapper.Tests.csproj
index 93b498618..09947a89a 100644
--- a/tests/Dapper.Tests/Dapper.Tests.csproj
+++ b/tests/Dapper.Tests/Dapper.Tests.csproj
@@ -31,8 +31,9 @@
     <PackageReference Include="log4net" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(TargetFramework)' != 'net481'">
-    <PackageReference Include="Npgsql" />
+  <ItemGroup Condition="'$(TargetFramework)' != 'net481'">
+    <PackageReference Include="pengdows.crud.fakeDb" />
+    <PackageReference Include="Npgsql" />
     <PackageReference Include="Snowflake.Data" />
   </ItemGroup>
 
diff --git a/tests/Dapper.Tests/FakeDbTests.Issue1649.cs b/tests/Dapper.Tests/FakeDbTests.Issue1649.cs
new file mode 100644
index 000000000..f9068ba28
--- /dev/null
+++ b/tests/Dapper.Tests/FakeDbTests.Issue1649.cs
@@ -0,0 +1,58 @@
+#if !NET481
+using System;
+using System.Collections.Generic;
+using System.Data;
+using pengdows.crud.fakeDb;
+using Xunit;
+
+namespace Dapper.Tests
+{
+    public sealed class FakeDbIssue1649Tests
+    {
+        [Fact]
+        public void TypeHandlerIsUsedForEachListExpansionParameter()
+        {
+            var handler = new EncodedValueHandler();
+            SqlMapper.AddTypeHandler(handler);
+            try
+            {
+                using var conn = new fakeDbConnection(new FakeDataStore());
+                conn.EnqueueNonQueryResult(2);
+                conn.Open();
+
+                var count = conn.Execute("select 1 where Id in @ids", new
+                {
+                    ids = new[] { new EncodedValue(7), new EncodedValue(8) }
+                });
+
+                Assert.Equal(2, count);
+                Assert.Equal(new[] { "encoded:7", "encoded:8" }, handler.Values);
+            }
+            finally
+            {
+                SqlMapper.ResetTypeHandlers();
+            }
+        }
+
+        private readonly struct EncodedValue
+        {
+            public EncodedValue(int value) => Value = value;
+            public int Value { get; }
+        }
+
+        private sealed class EncodedValueHandler : SqlMapper.TypeHandler<EncodedValue>
+        {
+            public List<string> Values { get; } = new();
+
+            public override EncodedValue Parse(object value) => new(Convert.ToInt32(value));
+
+            public override void SetValue(IDbDataParameter parameter, EncodedValue value)
+            {
+                var encoded = $"encoded:{value.Value}";
+                Values.Add(encoded);
+                parameter.Value = encoded;
+            }
+        }
+    }
+}
+#endif