LuckyPennySoftware/AutoMapper · error · AutoMapperMappingException

Error building member mapping strategy.

Error message

Error building member mapping strategy.

What it means

Thrown during execution-plan compilation when building the expression for a ForMember property mapping fails with a non-AutoMapperConfigurationException error. The AddPropertyMaps method catches the inner exception and rethrows it as AutoMapperMappingException wrapping the PropertyMap context. This occurs at configuration-build time when the execution plan is compiled.

Source

Thrown at src/AutoMapper/Execution/TypeMapPlanBuilder.cs:314

            if (!propertyMap.CanResolveValue)
            {
                continue;
            }

            try
            {
                var property = TryMemberMap(propertyMap,
                    CreatePropertyMapFunc(propertyMap, _destination, propertyMap.DestinationMember));
                if (_typeMap.ConstructorParameterMatches(propertyMap.DestinationName))
                {
                    property = _initialDestination.IfNullElse(_configuration.Default(property.Type), property);
                }

                actions.Add(property);
            }
            catch (Exception e) when (e is not AutoMapperConfigurationException)
            {
                throw new AutoMapperMappingException("Error building member mapping strategy.", e, propertyMap);
            }
        }
    }

    private Expression TryPathMap(PathMap pathMap)
    {
        var destination =
            ((MemberExpression)_configuration.ConvertReplaceParameters(pathMap.DestinationExpression, _destination))
            .Expression;
        var pathMapFunc = CreatePropertyMapFunc(pathMap, destination, pathMap.MemberPath.Last);
        _expressions.Clear();
        foreach (var member in destination.GetMemberExpressions())
        {
            var setter = GetSetter(member);
            var ifNull = setter == null
                ? Throw(Constant(new NullReferenceException($"{member} cannot be null because it's used by ForPath.")),
                    member.Type)
                : (Expression)Assign(setter, ObjectFactory.GenerateConstructorExpression(member.Type, _configuration));

View on GitHub (pinned to dfa6dd587c)

Solutions

1. Inspect the InnerException of the AutoMapperMappingException for the precise failure.
2. Add a CreateMap or custom type converter (ConvertUsing) for the member's source-to-destination type pair.
3. Fix the MapFrom lambda so it returns the correct destination member type.
4. If the member is complex, consider a custom IValueResolver.

Example fix

// before — incompatible member types, no converter
CreateMap<Source, Dest>()
    .ForMember(d => d.Amount, opt => opt.MapFrom(s => s.AmountString));
// Amount is decimal, AmountString is string

// after — add explicit conversion in MapFrom
CreateMap<Source, Dest>()
    .ForMember(d => d.Amount, opt => opt.MapFrom(s => decimal.Parse(s.AmountString)));

try
{
    var config = new MapperConfiguration(cfg => cfg.CreateMap<Source, Dest>()
        .ForMember(d => d.Amount, opt => opt.MapFrom(s => s.AmountValue)));
    config.CompileMaps();
}
catch (AutoMapperMappingException ex) when (ex.Message.Contains("member mapping strategy"))
{
    // ex.InnerException has the real error; ex.MemberMap identifies the failing property
    logger.LogError(ex.InnerException, "Member plan build failed for {Member}", ex.MemberMap?.DestinationMember);
}

When it happens

Trigger: A ForMember configuration whose resolver, MapFrom expression, or type conversion produces an invalid expression tree — e.g., source member type and destination member type are incompatible with no registered type map or converter, or a custom resolver returns the wrong type.

Common situations: Type mismatch between source and destination member types with no implicit conversion or registered type map; invalid custom resolver returning incompatible types; null-reference in a MapFrom lambda that fails during expression compilation.

Related errors

• Error building path mapping strategy.
• Error building constructor parameter mapping strategy.
• Unexpected member.
• Expression '{lambdaExpression}' must resolve to top-level me

AI-assisted analysis of LuckyPennySoftware/AutoMapper@dfa6dd587c (2026-08-13). Data as JSON: /api/errors/71096b7278a819c4. Report an issue: GitHub.