Dynamic Predicate (WHERE clause) for a LINQ Query

I love LINQ! It really has been a godsend and anyone still using datatable.select seriously needs to learn it. One thing I didn’t like too much though was I was having to write each LINQ query out instead of being able to pass it off to a function as a parameter. I haven’t gotten all the way there yet but this was a start. The idea was when we have a source of data that we refer to often that we pass the predicate (WHERE statement) off to a function instead of having to rewrite the whole query each time. While this example uses a datatable, you can use it with any collection that can have LINQ written against it.

So how is it done? First we identify the single element type we need to use ( Of TRow As DataRow) and then identify the “source” we are using and tie the identifier to that source ((source As TypedTableBase(Of TRow)). Then we must specify the predicate, or the WHERE clause that is going to be passed (predicate As Func(Of TRow, Boolean)) which will either be returned as true or false. Then we identify how we want the returned information ordered (OrderByField As String). Our function will then return a EnumerableRowCollection(Of TRow), our collection of datarows that have met the conditions of our predicate(EnumerableRowCollection(Of TRow)). This is a basic example. Of course you must make sure your order field doesn’t contain nulls, or have handled that situation properly and make sure your column names (if you are using a strongly typed datasource never mind this, it will rename the columns for you) are standard.

What is left // TO DO is to accomplish this with a table join, which is what my next step will be. It won’t be hard. Hopefully this code helps you and I hope you all have a great Christmas!

VB

USAGE:

 Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim da As New DataSet1TableAdapters.OrdersTableAdapter
        da.Fill(ds.Orders)
        Dim MyRet = LINQ_Where(ds.Orders, Function(row As DataSet1.OrdersRow) row.Order_ID < 200 And row.Order_ID > 1, "Order ID")
        If MyRet Is Nothing Then
            MessageBox.Show("NO ROWS")
        Else
            DataGridView1.DataSource = MyRet.CopyToDataTable
        End If
    End Sub

 Function LINQ_Where(Of TRow As DataRow)(source As TypedTableBase(Of TRow), predicate As Func(Of TRow, Boolean), OrderByField As String) As EnumerableRowCollection(Of TRow)

        Try

            Dim ReturnedRows = From row In source
                               Where predicate(row)
                               Order By row.Item(OrderByField)
                               Select row
            If ReturnedRows Is DBNull.Value Then
                Return Nothing
            Else
                Return ReturnedRows
            End If
            If ReturnedRows.Any = True Then
                Return ReturnedRows
            Else
                Return Nothing
            End If

        Catch ex As Exception
            Return Nothing
        End Try

    End Function

C#

USAGE:

private void Form1_Load(object sender, EventArgs e)
{
        DataSet1TableAdapters.OrdersTableAdapter da = new DataSet1TableAdapters.OrdersTableAdapter();
        da.Fill(ds.Orders);
        var MyRet = LINQ_Where(ds.Orders, (DataSet1.OrdersRow row) => row.Order_ID < 200 && row.Order_ID > 1, "Order ID");
        if (MyRet == null)
        {
            MessageBox.Show("NO ROWS");
        }
        else
        {
            DataGridView1.DataSource = MyRet.CopyToDataTable;
        }
    }

public EnumerableRowCollection<TRow> LINQ_Where<TRow>(TypedTableBase<TRow> source, Func<TRow, bool> predicate, string OrderByField) where TRow: DataRow
{

        try
        {

            var ReturnedRows = from row in source
                where predicate(row)
                orderby row.Item(OrderByField)
                select row;
            if (ReturnedRows == DBNull.Value)
            {
                return null;
            }
            else
            {
                return ReturnedRows;
            }
            if (ReturnedRows.Any() == true)
            {
                return ReturnedRows;
            }
            else
            {
                return null;
            }

        }
        catch (Exception ex)
        {
            return null;
        }

    }