Fastest way to fill DataTable from LINQ query using DataContext

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;}