Question

I have this class…

public class MyDTO
{
    public int Id { get; set; }

    public bool Selected { get; set; }
}

and then use dapper to attempt to create a list like this…

            var list = this.db.OpenConnection().Query<MyDTO>(
            @"SELECT T1.id, T2.id IS NOT NULL AS selected
            FROM     table1 T1
            LEFT
            JOIN     table2 T2 
            ON   T2.id = T1.id
            AND  Tl.id = @Id",
            new { Id = id });

which returns a result set like this….

id  selected
 9         0
10         1
11         1
12         0

But when code above is executed, i get an error

[InvalidCastException: Specified cast is not valid.]
Deserialize3d3c9260-abcb-4964-97c1-4a4e66b786d3(IDataReader ) +354

[DataException: Error parsing column 2 (selected=0 - Int64)]
Dapper.SqlMapper.ThrowDataException(Exception ex, Int32 index, IDataReader reader) in     C:\Projects\Web\SqlMapper.cs:1685
Deserialize3d3c9260-abcb-4964-97c1-4a4e66b786d3(IDataReader ) +432
Dapper.<QueryInternal>d__13`1.MoveNext() in C:\Projects\Web\Source\SqlMapper.cs:608
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +327
System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
Dapper.SqlMapper.Query(IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Boolean buffered, Nullable`1 commandTimeout, Nullable`1 commandType) in C:\Projects\Web\Source\SqlMapper.cs:538

I’m going to create a “Translating” property for now, but is this an unusual use case?

Answer 1

I’m not sure if this is the best way to do it, but it should do the trick:

SELECT T1.id, CAST(CASE WHEN T2.id IS NULL THEN 0 ELSE 1 END AS BIT) AS selected
FROM ...

(To be honest, I’m not convinced that your T2.id IS NOT NULL AS selected clause is legal T-SQL in the first place, but if you say that it’s working then I’ll take your word for it!)