Mapping dictionaries with AutoMapper

The problem you are having is because AutoMapper is struggling to map the contents of the Dictionary. You have to think what it is a store of - in this case KeyValuePairs.

If you try create a mapper for the KeyValuePair combination you will quickly work out that you can't directly as the Key property doesn't have a setter.

AutoMapper gets around this though by allowing you to Map using the constructor.

/* Create the map for the base object - be explicit for good readability */
Mapper.CreateMap<TestClass, TestClassDto>()
      .ForMember( x => x.Name, o => o.MapFrom( y => y.Name ) );

/* Create the map using construct using rather than ForMember */
Mapper.CreateMap<KeyValuePair<string, TestClass>, KeyValuePair<string, TestClassDto>>()
      .ConstructUsing( x => new KeyValuePair<string, TestClassDto>( x.Key, 
                                                                    x.Value.MapTo<TestClassDto>() ) );

var testDict = new Dictionary<string, TestClass>();
var testValue = new TestClass()
{
    Name = "value1"
};
testDict.Add( "key1", testValue );

/* Mapped Dict will have your new KeyValuePair in there */
var mappedDict = Mapper.Map<Dictionary<string, TestClass>,
Dictionary<string, TestClassDto>>( testDict );

AutoMapper has changed a bit so it looks more like:

CreateMap<Thing, ThingDto>()
     .ReverseMap();
CreateMap<Thing, KeyValuePair<int, ThingDto>>()
     .ConstructUsing((t, ctx) => new KeyValuePair<int, ThingDto>(t.id, ctx.Mapper.Map<ThingDto>(t)));