Avoid passing DBNull to scalar type handlers

Dapper/SqlMapper.cs

@@ -1983,10 +1983,10 @@ private static Func<DbDataReader, object> GetDeserializer(Type type, DbDataReade
             return GetSimpleValueDeserializer(type, underlyingType ?? type, startBound, useGetFieldValue);
         }
 
-        private static Func<DbDataReader, object> GetHandlerDeserializer(ITypeHandler handler, Type type, int startBound)
-        {
-            return reader => handler.Parse(type, reader.GetValue(startBound))!;
-        }
+        private static Func<DbDataReader, object> GetHandlerDeserializer(ITypeHandler handler, Type type, int startBound)
+        {
+            return reader => reader.IsDBNull(startBound) ? null! : handler.Parse(type, reader.GetValue(startBound))!;
+        }
 
         private static Exception MultiMapException(IDataRecord reader, string? splitOn = null)
         {

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" />

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>
 

tests/Dapper.Tests/FakeDbTests.Issue224.cs

@@ -0,0 +1,55 @@
+#if !NET481
+using System;
+using System.Collections.Generic;
+using System.Data;
+using pengdows.crud.fakeDb;
+using Xunit;
+
+namespace Dapper.Tests
+{
+    public sealed class FakeDbIssue224Tests
+    {
+        [Fact]
+        public void NullableScalarTypeHandlerDoesNotReceiveDbNull()
+        {
+            var handler = new MoneyHandler();
+            SqlMapper.AddTypeHandler(handler);
+            try
+            {
+                using var conn = new fakeDbConnection(new FakeDataStore());
+                conn.EnqueueReaderResult(new[] { new Dictionary<string, object?> { ["Value"] = DBNull.Value } });
+                conn.Open();
+
+                var value = conn.QuerySingle<Money?>("select Value from Values");
+
+                Assert.Null(value);
+                Assert.False(handler.ParseCalled);
+            }
+            finally
+            {
+                SqlMapper.ResetTypeHandlers();
+            }
+        }
+
+        private readonly struct Money
+        {
+            public Money(int value) => Value = value;
+            public int Value { get; }
+        }
+
+        private sealed class MoneyHandler : SqlMapper.TypeHandler<Money>
+        {
+            public bool ParseCalled { get; private set; }
+
+            public override Money Parse(object value)
+            {
+                ParseCalled = true;
+                return new Money(Convert.ToInt32(value));
+            }
+
+            public override void SetValue(IDbDataParameter parameter, Money value)
+                => parameter.Value = value.Value;
+        }
+    }
+}
+#endif