Question

Why we need the new in the select statement on one of them?

var runs = from sampleDataTable in db.SampleData 
           from sampleInfoTable in db.SampleInfo
           where sampleDataTable.SampleInfo.SampleInfoId == sampleInfoTable.SampleInfoId
           select new {sampleDataTable.Timestamp, sampleDataTable.SampleDataId, sampleInfoTable.Lane} ;

and

var runs2 = from sampleDataTable in db.SampleData
            from sampleInfoTable in db.SampleInfo
            where sampleDataTable.SampleInfo.SampleInfoId == sampleInfoTable.SampleInfoId
               && sampleDataTable.Timestamp == timestamp
            select sampleInfoTable.Lane;

Answer 1

In the first example you want to select three properties but the return type must be IEnumberable<T> for some type T. With select you are only allowed to select one T per item in the collection you are selecting from. So you have to create an object that contains the three properties and select the reference to that object. This is what new does here – it creates an object of an anonymous type containing the three properties that you requested.

In the second example you only need to select a reference to one thing. The type T
can be the type of the property so you don’t need to wrap it in an anonymous type. The second example would work with new too, it just isn’t necessary.