Hi @anh.nguyen Welcome to the forum.
I would suggest start reading here …
Welcome to community
Have a look on the document for filtering.
To sum the column Try this below expression
(From d in dtData.AsEnumerable Where Not (isNothing(d(1)) OrElse String.IsNullorEmpty(d(1).toString.Trim)) Select v = CDbl(d(1).toString.Trim)).Sum(Function (x) x)
Here, d(1) → ColumnIndex, You can also use the column name with Double quotes