LuckyPennySoftware/AutoMapper · error · AutoMapperMappingException

Error building path mapping strategy.

Error message

Error building path mapping strategy.

What it means

Thrown during execution-plan compilation when building the expression for a ForPath mapping fails with a non-AutoMapperConfigurationException error. The AddPathMaps method catches the inner exception and rethrows it as an AutoMapperMappingException wrapping the PathMap context. This is a compile-time (configuration build) error, not a runtime mapping error.

Source

Thrown at src/AutoMapper/Execution/TypeMapPlanBuilder.cs:267

        if (pathMaps.Count == 0)
        {
            return;
        }

        foreach (var pathMap in pathMaps)
        {
            if (pathMap.Ignored)
            {
                continue;
            }

            try
            {
                actions.Add(TryPathMap(pathMap));
            }
            catch (Exception e) when (e is not AutoMapperConfigurationException)
            {
                throw new AutoMapperMappingException("Error building path mapping strategy.", e, pathMap);
            }
        }
    }

    private void AddBeforeMap(List<Expression> actions)
    {
        var beforeMap = _typeMap.BeforeMapActions;
        if (beforeMap.Count == 0)
        {
            return;
        }

        foreach (var beforeMapAction in beforeMap)
        {
            actions.Add(ReplaceParameters(beforeMapAction));
        }
    }

View on GitHub (pinned to dfa6dd587c)

Solutions

1. Inspect the InnerException of the AutoMapperMappingException for the root cause.
2. Verify that the ForPath destination expression and the MapFrom source expression produce compatible types at every path segment.
3. Simplify the ForPath configuration to isolate which segment fails, then re-add complexity.
4. Ensure intermediate objects in the ForPath path have settable properties or can be auto-constructed.

Example fix

// before — type mismatch in ForPath
CreateMap<Source, Dest>()
    .ForPath(d => d.Address.ZipCode, opt => opt.MapFrom(s => s.ZipCodeString));
// Address.ZipCode is int but s.ZipCodeString is string

// after — map the correct type or add a converter
CreateMap<Source, Dest>()
    .ForPath(d => d.Address.ZipCode, opt => opt.MapFrom(s => int.Parse(s.ZipCodeString)));

try
{
    var config = new MapperConfiguration(cfg => cfg.CreateMap<Source, Dest>()
        .ForPath(d => d.Address.City, opt => opt.MapFrom(s => s.City)));
    config.CompileMaps(); // triggers plan building for ForPath
}
catch (AutoMapperMappingException ex) when (ex.Message.Contains("path mapping strategy"))
{
    // ex.InnerException holds the root cause; inspect it for type mismatches
    logger.LogError(ex.InnerException, "ForPath plan build failed for {Types}", ex.Types);
}

When it happens

Trigger: A ForPath configuration whose destination path or source expression produces an invalid expression tree during plan building — e.g., a type mismatch in an intermediate path segment, a null-path that cannot be constructed, or an incompatible MapFrom expression type.

Common situations: ForPath to a nested object where intermediate property types don't align; ForPath combined with custom ConvertUsing that returns incompatible types; ForPath where the source MapFrom lambda has a type mismatch.

Related errors

• Error building member mapping strategy.
• Error building constructor parameter mapping strategy.
• Unexpected member.
• Only member accesses are allowed. {destinationMember}
• sourceExpression may not be null when mapping {DestinationMe

AI-assisted analysis of LuckyPennySoftware/AutoMapper@dfa6dd587c (2026-08-13). Data as JSON: /api/errors/497542a9a0ac262a. Report an issue: GitHub.