How to make AutoMapper truncate strings according to MaxLength attribute?

Question

I have a DTO I want to map to an entity. The entity has some properties decorated with the MaxLength attribute.

I would like AutoMapper to truncate all the strings coming from the DTO when mapping to my entity according to the MaxLength for each property, so that I don't get validation errors when saving the entity.

So, if entity is defined like this:

public class Entity 
{
    [MaxLength(10)]
    string Name { get; set; }
}

I would like that doing this:

var myDto = new MyDto() { Name = "1231321312312312312312" };
var entity = Mapper.Map<Entity>(myDto);

The resulting entity should have its Name limited to a maximum of 10 characters.

Answer 1

I'm not sure that it's a good place to put that logic, but here is an example that should work in your case (AutoMapper 4.x): Custom Mapping with AutoMapper

In this example, I'm reading a custom MapTo property on my entity, you could do the same with MaxLength.

Here a full example with the current version of AutoMapper (6.x)

class Program
{
    static void Main(string[] args)
    {
        Mapper.Initialize(configuration =>
            configuration.CreateMap<Dto, Entity>()
                .ForMember(x => x.Name, e => e.ResolveUsing((dto, entity, value, context) =>
                {
                    var result = entity.GetType().GetProperty(nameof(Entity.Name)).GetCustomAttribute<MaxLengthAttribute>();
                    return dto.MyName.Substring(0, result.Length);
                })));

        var myDto = new Dto { MyName = "asadasdfasfdaasfasdfaasfasfd12" };
        var myEntity = Mapper.Map<Dto, Entity>(myDto);
    }
}

public class Entity
{
    [MaxLength(10)]
    public string Name { get; set; }
}

public class Dto
{
    public string MyName { get; set; }
}