NullReferenceException with V4.X

[KolyaNET]

Hello. I used FastExpressionCompiler version 3.3.4 and everything worked. After updating to version 4.1.0 I started getting errors. I also use Mapster 7.4.0.

I was able to find the problem using a NUnit test

using FastExpressionCompiler;
using Mapster;
using NUnit.Framework;

namespace Garda.Unit.Tests.Mapster;

[TestFixture]
[Parallelizable(ParallelScope.Default)]
public sealed class NullReferenceExceptionFastExpressionCompilerTests
{
    public sealed record AgentStatusDto
    {
        public int ErrorCode { get; init; }

        public int CompleteTaskType { get; set; }
    }

    public sealed record ResulAgent(ResponseAgent Common);

    public sealed record ResponseAgent(int ErrorType, int TaskType, int Code);

    internal class AgentStatusDtoAdaptRule : TypeAdapterRule
    {
        internal AgentStatusDtoAdaptRule()
        {
            TypeAdapterConfig<ResulAgent, AgentStatusDto>
                .NewConfig()
                .Map(dst => dst.ErrorCode, src => src.Common.Code)
                .AfterMapping(ToDtoMap);
        }

        private static void ToDtoMap(ResulAgent res, AgentStatusDto dto)
        {
            dto.CompleteTaskType = res.Common.TaskType switch
            {
                2 => 3,
                _ => default
            };
        }
    }

    [TestCase(CompilerFlags.Default)]
    [TestCase(CompilerFlags.NoInvocationLambdaInlining)]
    public void NullReferenceException(CompilerFlags compilerFlags)
    {
        // Arrange
        var result = new ResulAgent(new ResponseAgent(1, 2, 3));

        TypeAdapterConfig.RulesTemplate.Add(new AgentStatusDtoAdaptRule());

        TypeAdapterConfig.GlobalSettings.Compiler = exp => exp.CompileFast(flags: compilerFlags);
        TypeAdapterConfig.GlobalSettings.Compile();

        // Act
        if (compilerFlags == CompilerFlags.Default)
        {
            Assert.Throws<NullReferenceException>(() => result.Adapt<AgentStatusDto>());
        }
        else
        {
            var resultStatus = result.Adapt<AgentStatusDto>();

            // Assert
            Assert.NotNull(resultStatus, nameof(resultStatus));
        }
    }
}

If I revert to version 3.3.4 or use the NoInvocationLambdaInlining flag the NullReferenceException will no longer be thrown.

Similar problems:

• NullReferenceException with V4.X when using long?[] #405
• NullReferenceException with V4.X when trying to map #381
• NullReferenceException with FastExpressionCompiler MapsterMapper/Mapster#676