Dynamically Mapping a Custom Entity
This post from Chris Wallace
got me thinking about dynamically mapping data results to custom
entities. In his post Chris outlines a way to dynamically fill a
custom entity using a switch statement on the columns that are present
in the datareader. Towards the end, he mentions dynamically
creating a method (using the CodeDom presumably) and compiling and
executing the method on the fly.
I thought of a different way to accomplish the same goal, one that
requires no knowledge of the custom entity, by using some simple
reflection. Basically, you have a datareader (or datatable,
dataset, whatever), you loop through the columns of the datareader and
get the name of the current column. Using reflection, check to
see if the custom entity has a property with the same name, if it does
then fill the value, if not, continue looping.
I’m going to show a small sample below which is very simplistic, but
it gets the point across. There are a few caveats that I’m not
addressing:
- Null values (i.e. trying to set an int property to null)
- I’m assuming that a default parameterless constructor exists for the custom entity
- I’m not doing anything for indexed properties (like DataReader[int ordinal])
You can download a little sample winforms application here. Just change the “DB” value in the app.config to point to the sample access db and you should be all set.
So, heres the code. The method below takes in a datareader and
the Type of the custom entity that we want to fill and returns a custom
entity with its properties filled in with the data from the datareader.
public static object MapEntity(IDataRecord record, Type customType)
{
object customEntity = Activator.CreateInstance(customType);
PropertyInfo[] properties = customType.GetProperties();
for(int i=0;i < record.FieldCount;i++)
{
string columnName = record.GetName(i);
foreach(PropertyInfo prop in properties)
{
if (string.Compare(prop.Name, columnName) == 0)
{
prop.SetValue(customEntity, record.GetValue(i), null);
break;
}
}
}
return customEntity;
}
This works pretty slick for most cases, but there are situations
that you would run into where this break down. If you wanted to
have property names that didn’t match the database schema for instance,
but it could certainly handle a large amount of the data access,
so long as you address the issues that I mentioned above.
[Edit: Fixed typo]
. So I think the floors are all set to be finished
