Dapper .NET: Customize Mapping

Question

Model

public class ErrorReport{
    public int? Id {get;set;}
    public ExceptionReport Exception {get;set;}
    public ExceptionReport InnerException {get;set;}
}

public class ExceptionReport{
    public string Type {get; set;}
    public string Message {get; set;}
}

Database

This is the table i want to query from

ErrorReports

Id: int
ExceptionType: varchar
ExceptionMessage: varchar
InnerExceptionType: varchar
InnerExceptionMessage: varchar

The problem

So, what i want to do is querying the database and map the results into my model properties. But this doesn't work:

using (var con = ConnectionFactory.CreateConnection(_connectionString))
{
     IEnumerable<ErrorReport> reports = con.Query<ErrorReport>("Select * from ErrorReports");
}

I understand that i have to explicitly say which columns map to which property, so, how can i do that?

David L · Accepted Answer · 2015-05-27 19:29:00Z

2

You can return your query as dynamic and then map each property to your corresponding complex object.

using (var con = ConnectionFactory.CreateConnection(_connectionString))
{
    List<ErrorReport> reports = 
        con.Query<dynamic>("Select * from ErrorReports")
            .Select(x => new ErrorReport 
            { 
                Id = x.Id, 
                Exception = new ExceptionReport
                {
                    Type = x.ExceptionType,
                    Messsage = x.ExceptionMessage
                },
                InnerException = new ExceptionReport
                {
                    Type = x.InnerExceptionType,
                    Messsage = x.InnerExceptionMessage
                }
            }).ToList();
}

edited May 27, 2015 at 19:29

answered May 27, 2015 at 18:51

David L

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4 Comments

ascherman Over a year ago

But does not have an overhead? I mean... you map all the items to a model, and then re-execute the foreach again...

David L Over a year ago

@ArielScherman The overhead is negligible and mostly tied up in the use of dynamic unless you're bulk reading hundreds of thousands of records (and even then, profiling might not show much of a performance gap). In addition, IEnumerable is returned, meaning that we will only be iterating once..

ascherman Over a year ago

Is it really be iterated just once? Isn't a two step process? (first from records to dynamic, then from dynamic to object)

David L Over a year ago

@ArielScherman Correct, it is a two step process in terms of object transformation, but because we select to a new object off of IEnumerable<dynamic>, that transformation occurs during a single iteration. The objects do change twice, but the rows returned are iterated once. Now, if you called ToList() on your collection BEFORE selecting new objects, yes, you would be iterating twice.

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