Lets imagine model entity:

[HierarchyRoot]
public class User: Entity
{
  [Field]
  public string Name {get;set;]
}

and opposite Dto class:

public class UserDto: IDataErrorInfo
{
  public string _Key {get;set;}
  public string Name {get;set;}

  public string this[string columnName]
  {
     get
     {
        if (columnName == "Name")
        {
           if (string.IsNullOrEmpty(this.Name))
           {
             return "Name cannot be empty!";
           }
        }

        return null;
     }
  }
}

Now creating mapping builder:

var builder = new Xtensive.ObjectMapping.MappingBuilder()
  .MapType<User, UserDto, string>(entity => entity.Key.Format(), objDto => objDto._Key)
  .IgnoreProperty(objDto => objDto.Error)
  .TrackChanges(dto => dto.ID, false);

And when i trying to create Mapper from this mapping description, it fails on:

The primitive property System.String Item [System.String] in UserDto is bound to the property System.Object Item [System.String] in User that isn't primitive.

I can imagine(maybe wrong) why it throws this, because yours Xtensive.Orm.Entity has also indexer with return type object, not like mine string. My question is, how can i tell to mapping description to ignore indexer ? something like IgnoreProperty but for indexer, maybe IgnoreIndexer or something?

Problem is i need such indexer in UserDto class because its implements IDataErrorInfo which is used on app for validation.