Multi-mapping to the same table twice - dapper

Question

How do you use Dapper's Multi-Mapping feature on two fields using the same table? i.e ClientInfo has two Address objects.
public class ClientInfo
{
public Guid Id => Guid.NewGuid();
public string FirstName { get; set; }
public string LastName { get; set; }
public Address PostalAddress { get; set; }
public Address BillingAddress { get; set; }
public int ContactNumber { get; set; }
}
public class Address
{
public Guid Id = Guid.NewGuid();
public string FirstLine { get; set; }
public string SecondLine { get; set; }
public string Town { get; set; }
public string PostCode { get; set; }
}
Tables
Relational - Address.Id used in ClientInfo.PostalAddress / BillingAddress
tbl.Address
|Id|FirstLine|SecondLine|Town|PostCode
tbl.ClientInfo
|Id|FirstName|LastName|PostalAddress|BillingAddress|etc..
Current implementation
Results only in all but PostalAddress being mapped.
var sql = #"select * from ClientInfo c left join Address as a on a.Id = c.PostalAddress left join Address as ad on ad.Id = c.BillingAddress";
var clients = connection.Query<ClientInfo, Address, Address, ClientInfo>(
sql,
(client, postal, billing) =>
{
client.PostalAddress = postal;
client.BillingAddress = billing;
return client;
},
splitOn: "PostalAddress,BillingAddress")
.Distinct()
.ToList();
return clients;

Answer 1

The splitOn parameter tells Dapper when/where to start mapping the next object, so you need to ensure that your SQL query returns the information in the correct order. Right now, you return 2 guids for PostalAddress and BillingAddress. Dapper doesn't know how to map them both.
select * from ... join ... will result in the Address data ordered AFTER the ClientInfo.PostalAddress and ClientInfo.BillingAddress columns.
Try: SELECT c.Id, c.FirstName, c.LastName, c.ContactNumber, a.*, ad.* FROM ClientInfo c LEFT JOIN Address AS a ON a.Id = c.PostalAddress JOIN Address AS ad ON ad.Id = c.BillingAddress
As you can see, removing the * effectively excludes the PostalAddress and BillingAddress guids from the results and we can now now splitOn: "Id,Id".

Answer 2

You will of course not have to provide the GUIDs in the select statement, this is just for the test to work.
[Test]
public void TestAbcd()
{
using (var dbConnection = new SqlConnection(_connectionString))
{
const string sql = #"WITH ClientInfo AS (
SELECT * FROM (
VALUES (#ci1, #adr1, #adr2), (#ci1, #adr3, #adr4)
) AS a (Id, PostalAddress, BillingAddress)
),
Address AS (
SELECT * FROM (
VALUES
(#adr1), (#adr2), (#adr3), (#adr4)
) AS a (Id)
)
select * from ClientInfo c left join Address as a on a.Id = c.PostalAddress left join Address as ad on ad.Id = c.BillingAddress";
dbConnection.Open();
var clients = dbConnection.Query<ClientInfo, Address, Address, ClientInfo>(
sql,
(client, postal, billing) =>
{
client.PostalAddress = postal;
client.BillingAddress = billing;
return client;
},
splitOn: "PostalAddress,BillingAddress", param: new {
ci1 = Guid.NewGuid(),
ci2 = Guid.NewGuid(),
adr1 = Guid.NewGuid(),
adr2 = Guid.NewGuid(),
adr3 = Guid.NewGuid(),
adr4 = Guid.NewGuid()
})
.Distinct()
.ToList();
}
}