Configuration Validation

Hand-rolled mapping code, though tedious, has the advantage of being testable. One of the inspirations behind AutoMapper was to eliminate not just the custom mapping code, but eliminate the need for manual testing. Because the mapping from source to destination is convention-based, you will still need to test your configuration.

AutoMapper provides configuration testing in the form of the AssertConfigurationIsValid method. Suppose we have slightly misconfigured our source and destination types:

public class Source
	public int SomeValue { get; set; }

public class Destination
	public int SomeValuefff { get; set; }

In the Destination type, we probably fat-fingered the destination property. Other typical issues are source member renames. To test our configuration, we simply create a unit test that sets up the configuration and executes the AssertConfigurationIsValid method:

var configuration = new MapperConfiguration(cfg =>
  cfg.CreateMap<Source, Destination>());


Executing this code produces an AutoMapperConfigurationException, with a descriptive message. AutoMapper checks to make sure that every single Destination type member has a corresponding type member on the source type.

Overriding configuration errors

To fix a configuration error (besides renaming the source/destination members), you have three choices for providing an alternate configuration:

With the third option, we have a member on the destination type that we will fill with alternative means, and not through the Map operation.

var configuration = new MapperConfiguration(cfg =>
  cfg.CreateMap<Source, Destination>()
	.ForMember(dest => dest.SomeValuefff, opt => opt.Ignore())

Selecting members to validate

By default, AutoMapper uses the destination type to validate members. It assumes that all destination members need to be mapped. To modify this behavior, use the CreateMap overload to specify which member list to validate against:

var configuration = new MapperConfiguration(cfg =>
  cfg.CreateMap<Source, Destination>(MemberList.Source);
  cfg.CreateMap<Source2, Destination2>(MemberList.None);

To skip validation altogether for this map, use MemberList.None. That’s the default for ReverseMap.

Custom validations

You can add custom validations through an extension point. See here.