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 versions below compile, but return an empty set. The exact error is "Value cannot be null. Parameter name: source". (and yes I have checked there is data):

MyDatabaseDataContext db = new MyDatabaseDataContext(conn); 
IEnumerable<DataRow> queryProjects = 
    (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<string>("PROJECT_NAME")
    }) as IEnumerable<DataRow>;
DataTable results = queryProjects.CopyToDataTable<DataRow>();

...

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

The examples in this thread don't seem to work in this situation either.

I guess I could just run a sql query command the old-fashioned way, but isn't linq supposed to be quicker?

Adam Robinson · Accepted Answer · 2011-02-10T04:24:18.670

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;
}

I see - so it is just hiding the invalidcastexception? How do I get it into the datatable then? — JumpingJezza, Feb 10 '11 at 04:23
@JumpingJezza: Yes; `as` is to be used in cases where you know that the object might not actually be the type you're expecting and isn't an "exceptional" case. The `as` keyword evaluates to `null` if the cast is invalid, and (as a result) can only be used with reference types and `Nullable`. See the edit I just posted for a sample extension method that can turn any `IEnumerable` to a `DataTable`. — Adam Robinson, Feb 10 '11 at 04:25
I had to built in a check for Nullable Types since the output.Columns.Add call will fail if type is nullable — PBMe_HikeIt, Jan 12 '15 at 22:27

Fastest way to fill DataTable from LINQ query using DataContext

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