Fastest way to fill DataTable from LINQ query using DataContext

Question

I am trying to run a linq query but I need the result as a datatable as I am using that to store records from different queries in the same viewstate object.

The 2 v

Answer 1

Your problem is this:

as IEnumerable<DataRow>

The as keyword performs a safe cast, not a conversion, which it seems like you might think that it's doing. The as keyword is semantically the same as doing this:

IEnumerable<DataRow> queryProjects = 
    (IEnumerable<DataRow>)(from DataRow p in db.STREAM_PROJECTs.AsEnumerable()
    where p.Field<int>("STREAM_ID") == StreamID
    select new
    {
        PROJECT_ID = p.Field<int>("PROJECT_ID"),
        PROJECT_NAME = p.Field<int>("PROJECT_NAME")
    });

Except the version with as won't throw an exception when it fails to cast your query object (which is an IQueryable<T>, where T is an anonymous type) to an IEnumerable<DataRow> (which it isn't).

Unfortunately, there is no built-in method that I'm aware of that will take an enumerable of a concrete type (like your anonymous type in this example) and turn it into a DataTable. Writing one wouldn't be too complicated, as you'd essentially need to get the properties reflectively then iterate over the collection and use those properties to create columns in a DataTable. I'll post an example in a few.

Something like this, placed in a static class within a namespace that you're using, should provide an extension method that will do what you want:

public static DataTable ToDataTable<T>(this IEnumerable<T> source)
{
    PropertyInfo[] properties = typeof(T).GetProperties();

    DataTable output = new DataTable();

    foreach(var prop in properties)
    {
        output.Columns.Add(prop.Name, prop.PropertyType);
    }

    foreach(var item in source)
    {
        DataRow row = output.NewRow();

        foreach(var prop in properties)
        {
            row[prop.Name] = prop.GetValue(item, null);
        }

        output.Rows.Add(row);
    }

    return output;
}