LINQ query on a DataTable

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable<T>. You need to use the AsEnumerable() extension for DataTable. Like so:

var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;

And as @Keith says, you'll need to add a reference to System.Data.DataSetExtensions

AsEnumerable() returns IEnumerable<DataRow>. If you need to convert IEnumerable<DataRow> to a DataTable, use the CopyToDataTable() extension.

Below is query with Lambda Expression,

var result = myDataTable
    .AsEnumerable()
    .Where(myRow => myRow.Field<int>("RowNo") == 1);

var results = from DataRow myRow in myDataTable.Rows
    where (int)myRow["RowNo"] == 1
    select myRow

It's not that they were deliberately not allowed on DataTables, it's just that DataTables pre-date the IQueryable and generic IEnumerable constructs on which Linq queries can be performed.

Both interfaces require some sort type-safety validation. DataTables are not strongly typed. This is the same reason why people can't query against an ArrayList, for example.

For Linq to work you need to map your results against type-safe objects and query against that instead.

As @ch00k said:

using System.Data; //needed for the extension methods to work

...

var results = 
    from myRow in myDataTable.Rows 
    where myRow.Field<int>("RowNo") == 1 
    select myRow; //select the thing you want, not the collection

You also need to add a project reference to System.Data.DataSetExtensions

You can use LINQ to objects on the Rows collection, like so:

var results = from myRow in myDataTable.Rows where myRow.Field("RowNo") == 1 select myRow;

Example on how to achieve this provided below:

DataSet dataSet = new DataSet(); //Create a dataset
dataSet = _DataEntryDataLayer.ReadResults(); //Call to the dataLayer to return the data

//LINQ query on a DataTable
var dataList = dataSet.Tables["DataTable"]
              .AsEnumerable()
              .Select(i => new
              {
                 ID = i["ID"],
                 Name = i["Name"]
               }).ToList();

you can try this, but you must be sure the type of values for each Column

List<MyClass> result = myDataTable.AsEnumerable().Select(x=> new MyClass(){
     Property1 = (string)x.Field<string>("ColumnName1"),
     Property2 = (int)x.Field<int>("ColumnName2"),
     Property3 = (bool)x.Field<bool>("ColumnName3"),    
});

I propose following solution:

DataView view = new DataView(myDataTable); 
view.RowFilter = "RowNo = 1";
DataTable results = view.ToTable(true);

Looking at the DataView Documentation, the first thing we can see is this:

Represents a databindable, customized view of a DataTable for sorting, filtering, searching, editing, and navigation.

What I am getting from this is that DataTable is meant to only store data and DataView is there enable us to "query" against the DataTable.

Here is how this works in this particular case:

You try to implement the SQL Statement

SELECT *
FROM myDataTable
WHERE RowNo = 1

in "DataTable language". In C# we would read it like this:

FROM myDataTable
WHERE RowNo = 1
SELECT *

which looks in C# like this:

DataView view = new DataView(myDataTable);  //FROM myDataTable
view.RowFilter = "RowNo = 1";  //WHERE RowNo = 1
DataTable results = view.ToTable(true);  //SELECT *