What causes "Error building constructor parameter mapping strategy." in LuckyPennySoftware/AutoMapper?

Inspect the InnerException of the AutoMapperMappingException for the root cause.

LuckyPennySoftware/AutoMapper · error · AutoMapperMappingException

Error building constructor parameter mapping strategy.

Error message

Error building constructor parameter mapping strategy.

What it means

Thrown during execution-plan compilation when building the expression for a constructor parameter mapping fails with a non-AutoMapperConfigurationException error. The ConstructorMapping method catches the inner exception and rethrows it as AutoMapperMappingException wrapping the ConstructorParameterMap context. This happens at configuration-build time when resolving how each constructor parameter gets its value.

Source

Thrown at src/AutoMapper/Execution/TypeMapPlanBuilder.cs:395

            interfaceType),
        _ => ObjectFactory.GenerateConstructorExpression(DestinationType, _configuration)
    };

    private Expression ConstructorMapping(ConstructorMap constructorMap)
    {
        List<ParameterExpression> variables = [];
        List<Expression> body = [];
        foreach (var parameter in constructorMap.CtorParams)
        {
            try
            {
                var variable = Variable(parameter.DestinationType, parameter.DestinationName);
                variables.Add(variable);
                body.Add(Assign(variable, CreateConstructorParameterExpression(parameter)));
            }
            catch (Exception e) when (e is not AutoMapperConfigurationException)
            {
                throw new AutoMapperMappingException("Error building constructor parameter mapping strategy.", e, parameter);
            }
        }

        body.Add(CheckReferencesCache(New(constructorMap.Ctor, variables)));
        return Block(variables, body);
    }

    private Expression CreateConstructorParameterExpression(ConstructorParameterMap ctorParamMap)
    {
        var defaultValue = ctorParamMap.DefaultValue(_configuration);
        var customSource = GetCustomSource(ctorParamMap);
        var resolvedExpression = BuildValueResolverFunc(ctorParamMap, customSource, defaultValue);
        var resolvedValue = Variable(resolvedExpression.Type, "resolvedValue");
        var mapMember = MapMember(ctorParamMap, resolvedValue, defaultValue);
        _variables.Clear();
        _variables.Add(resolvedValue);
        _expressions.Clear();
        _expressions.Add(Assign(resolvedValue, resolvedExpression));

View on GitHub (pinned to dfa6dd587c)

Solutions

Inspect the InnerException of the AutoMapperMappingException for the root cause.
Verify that each constructor parameter's resolved type matches its declared type, or add a converter.
Use ForCtorParam with an explicit MapFrom lambda that returns the correct type.
If the parameter type is complex, register a CreateMap or ConvertUsing for it.

Example fix

// before — ctor param type mismatch
public Dest(int id) { }
CreateMap<Source, Dest>().ForCtorParam("id", opt => opt.MapFrom(s => s.IdString));
// s.IdString is string but ctor param is int

// after — convert in MapFrom
CreateMap<Source, Dest>().ForCtorParam("id", opt => opt.MapFrom(s => int.Parse(s.IdString)));

Defensive patterns

Strategy: try-catch

Try / catch

try
{
    var config = new MapperConfiguration(cfg => cfg.CreateMap<Source, Dest>()
        .ForCtorParam("id", opt => opt.MapFrom(s => s.Id)));
    config.CompileMaps();
}
catch (AutoMapperMappingException ex) when (ex.Message.Contains("constructor parameter mapping strategy"))
{
    // ex.InnerException has the root cause
    logger.LogError(ex.InnerException, "Ctor param plan build failed.");
}

Prevention

Ensure each constructor parameter's resolved type matches its declared type or add a converter.
Use AssertConfigurationIsValid in tests to catch plan-building errors at test time.
Inspect the InnerException for the specific type mismatch.
Prefer lambda-based ForCtorParam MapFrom so the compiler checks types.

When it happens

Trigger: A ForCtorParam configuration or an auto-resolved constructor parameter whose source-to-destination type resolution produces an invalid expression tree — e.g., the ctor param type doesn't match the source member type and no converter is registered, or a MapFrom on the ctor param returns an incompatible type.

Common situations: Constructor parameter type mismatch (e.g., ctor param is int but source member is string); ForCtorParam MapFrom returning wrong type; missing type map for a complex ctor parameter.

Related errors

AI-assisted analysis of LuckyPennySoftware/AutoMapper@dfa6dd587c (2026-08-13). Data as JSON: /api/errors/1f08d1d418c7733d. Report an issue: GitHub.