I recently had a use case where I needed to conditionally map a property when mapping an object. In the past, I would have mapped the object as you typically do with AutoMapper like so var barInstance = mapper.Map<Bar>(fooInstance) and then set the property on barInstance depending on the state of fooInstance. I found out that all this can be done in your map configuration with Conditional Mappings in AutoMapper.

For example, lets say we have the following two classes:

class Foo {
  public string FirstName { get; set; }
  public string LastName { get; set; }
}

class Bar {
  public string FullName { get; set; }
}

These objects are not going to map one-to-one, but we can use a PreCondition in our AutoMapper configuration to map FirstName and LastName to FullName only if both properties (FirstName, LastName) are set. (Yes, we lose data in the case of one of them not being set, this is really a contrived example to just talk about using a PreCondition.)

To do that, we need to setup the PreCondition in our CreateMap like so:

CreateMap<Foo, Bar>()
  .ForMember(dest => dest.FullName, opt => {
    opt.PreCondition(src => !string.IsNullOrWhiteSpace(src.FirstName) && !string.IsNullOrWhiteSpace(src.LastName));
    opt.MapFrom(src => string.Format("{0} {1}", src.FirstName, src.LastName));
  });

When mapping from Foo to Bar objects now, the FullName property on Bar will only be set if FirstName and LastName are not null or whitespace on Foo. This makes mapping objects a whole lot cleaner once you realize you can do this in your map configuration instead of in your code.

There is also the ability to specify Conditional Mapping similar to PreCondition that works if properties have the same name it will only map them if the Condition is true.