CommandFactory does not use registered TypeHandler for parameter binding

[alagonterie]

Repository: DapperLib/DapperAOT
Version: 1.0.48

Description

When a custom TypeHandler<T> is registered via [module: TypeHandler<T, THandler>], the generated CommandFactory does not use THandler.SetValue() for parameter binding. Instead, it generates p.Value = AsValue(typed.Property), which sets the raw custom type directly as the parameter value — causing SqlClient to throw:

System.ArgumentException: No mapping exists from object type MyCustomType to a known managed provider native type.

The TypeHandler<T> works correctly for result parsing (RowFactory uses Parse()), but is not used for parameter binding (CommandFactory uses AsValue() instead of SetValue()).

Reproduction

// Custom value type for typed SQL string parameters
public readonly record struct DbString(string? Value, bool IsAnsi, int Length, bool IsFixedLength);

// AOT TypeHandler
public sealed class DbStringHandler : Dapper.TypeHandler<DbString>
{
    public override void SetValue(DbParameter parameter, DbString value)
    {
        parameter.Value = value.Value ?? (object)DBNull.Value;
        parameter.DbType = value.IsAnsi ? DbType.AnsiString : DbType.String;
        if (value.Length > 0) parameter.Size = value.Length;
    }
    public override DbString Parse(DbParameter parameter) => new(parameter.Value as string);
}

// Registration
[module: DapperAot]
[module: SqlSyntax(SqlSyntax.SqlServer)]
[module: TypeHandler<DbString, DbStringHandler>]

// Usage — flat parameter overload (intercepted)
await connection.ExecuteAsync(
    "UPDATE Table SET Name = @Name WHERE Id = @Id",
    new { Name = new DbString("test", IsAnsi: true, Length: 25, IsFixedLength: false), Id = 1 }
);

Expected behavior

The generated CommandFactory.AddParameters() should call DbStringHandler.SetValue(parameter, value) for DbString properties, producing:

p.ParameterName = "Name";
TypeHandlerInstance.SetValue(p, typed.Name);  // sets DbType, Size, Value
ps.Add(p);

Actual behavior

The generated CommandFactory.AddParameters() produces:

p.ParameterName = "Name";
p.Direction = ParameterDirection.Input;
p.Value = AsValue(typed.Name);  // sets raw DbString struct as Value — SqlClient throws
ps.Add(p);

No DbType or Size is set. The TypeHandler.SetValue() is never called.

Generated code excerpt

private sealed class CommandFactory2 : global::Dapper.CommandFactory<object?>
// <anonymous type: int Id, DbString Name>
{
    public override void AddParameters(in global::Dapper.UnifiedCommand cmd, object? args)
    {
        var typed = Cast(args, static () => new { Id = default(int), Name = default(DbString) });
        var ps = cmd.Parameters;
        DbParameter p;
        p = cmd.CreateParameter();
        p.ParameterName = "Id";
        p.DbType = DbType.Int32;
        p.Direction = ParameterDirection.Input;
        p.Value = AsValue(typed.Id);
        ps.Add(p);

        p = cmd.CreateParameter();
        p.ParameterName = "Name";
        p.Direction = ParameterDirection.Input;
        p.Value = AsValue(typed.Name);  // <-- should use TypeHandler.SetValue() here
        ps.Add(p);
    }
}

Workaround

Use CommandDefinition overloads for calls with custom TypeHandler parameters. CommandDefinition falls back to vanilla Dapper which respects SqlMapper.AddTypeHandler():

// Works — vanilla Dapper path uses TypeHandler
await connection.ExecuteAsync(new CommandDefinition(sql, new { Name = new DbString(...) }));

// Fails — AOT intercepted path ignores TypeHandler
await connection.ExecuteAsync(sql, new { Name = new DbString(...) });

Selectively disable AOT on affected methods:

[DapperAot(false)]
public async Task Handle(MyEvent @event) { ... }

Environment

• Dapper.AOT 1.0.48
• Dapper 2.1.72
• .NET 10
• Microsoft.Data.SqlClient 7.0.0

[alagonterie]

This also affects ICustomQueryParameter (table-valued parameters). When [DapperAot] is active, the generated CommandFactory calls p.Value = AsValue(...) on TVP parameters too, instead of delegating to ICustomQueryParameter.AddParameter().

Same workaround: [DapperAot(false)] on the method.

// Fails — AOT intercepted path sets DataTable directly as p.Value
await connection.QueryAsync<T>(sql, new { Ids = table.AsTableValuedParameter("tt_UniqueidentifierList") });

// Works with [DapperAot(false)] — vanilla Dapper calls ICustomQueryParameter.AddParameter()