What is the difference between Single and FirstOrDefault?

Question

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LINQ Single vs First

Im new to Linq and want to learn it the best way, I have here 2 working update events for linq, thay do the same, but what way is the best and do i need to add something to make it better !?

Solution 1

Protected Sub Button2_Click(sender As Object, e As System.EventArgs) Handles Button2.Click
    Using db As New ThedatabaseconnectionDataContext()
        Try
            Dim TheUpdateID = DirectCast(FindControl("Textbox5"), TextBox).Text
            Dim getEditing As testtable = (From c In db.testtables Where c.test_id = TheUpdateID Select c).FirstOrDefault()
            If getEditing IsNot Nothing Then
                getEditing.test_cat = DirectCast(FindControl("Textbox1"), TextBox).Text
                getEditing.test_info = DirectCast(FindControl("Textbox2"), TextBox).Text
                getEditing.test_number = DirectCast(FindControl("Textbox3"), TextBox).Text
                getEditing.test_datetime = DirectCast(FindControl("Textbox4"), TextBox).Text
                db.SubmitChanges()
                'textBox1.Text = "Contact updated."
            End If
        Catch ex As Exception
            'Me.lblMsg.Text = ex.Message
        End Try
    End Using
End Sub

Solution 2

Protected Sub Button2_Click(sender As Object, e As System.EventArgs) Handles Button2.Click
    Using db As New ThedatabaseconnectionDataContext()
        Try
            Dim tbltest As Table(Of testtable) = db.GetTable(Of testtable)()
            Dim TheUpdateID = DirectCast(FindControl("Textbox5"), TextBox).Text
            Dim getEditing As testtable = tbltest.Single(Function(c) c.test_id = TheUpdateID)
            If getEditing IsNot Nothing Then
                getEditing.test_cat = DirectCast(FindControl("Textbox1"), TextBox).Text
                getEditing.test_info = DirectCast(FindControl("Textbox2"), TextBox).Text
                getEditing.test_number = DirectCast(FindControl("Textbox3"), TextBox).Text
                getEditing.test_datetime = DirectCast(FindControl("Textbox4"), TextBox).Text
                db.SubmitChanges()
                'textBox1.Text = "Contact updated."
            End If
        Catch ex As Exception
            'Me.lblMsg.Text = ex.Message
        End Try
    End Using
End Sub

Answer 1

First off, you should be able to write your first or default as

Dim getEditing As testtable = tbltest.FirstOrDefault(Function(c) c.test_id = TheUpdateID)

Untested but more to point out that first or default handles lambdas

As for which to use, it depends on your data. To break down what happens

Single - Expects exactly one match, A exception is thrown if no results are found OR multiple results are found

SingleOrDefault - Expects 0 or 1 match. A exception is thrown if multiple matches are found

First - Expects 1 or many match. Exception is thrown if no matches are found. Any results after the first result are ignored.

FirstOrDefault - handles 0, 1 or multiple matches. Any results after the first result are ignored.

If you are picking based on a ID from a listbox (I.e. it's unique and is definatly in the database) then single is a safe choice.

If the user is entering a ID (again unique) that may or may not be in the DB single or default is safe.

If searching based on a possible duplicate value, like surname, then first or firstordefault is what you should use depending if it is guaranteed to exist in the database or not.

Personally, regardless of the data, I would stick to either first or firstordefault as it handles more scenarios.

Answer 2

Where to even start on this one...

For a start, I know this is likely a test project, so apologies if you're already doing this, but please make sure you're using acceptable data tier hierarchies - your DBML should be in a separate project from your Presentation layer.

But, to the question in hand. My preferred way of doing this is getting the data object and updating it on an object level. Such as (pseudo code / I'm a C# kinda guy!):

private MyObject object;

protected void Page_Load(object sender, EventArgs e)
{
    // Select usually be ID
    object = DataLayer.GetObject();

    if(!IsPostBack)
    {
         // Load object details for editing into presentation layer
         TextboxObjectName.Text = object.Name;
    }
}

protected void Button_Click(object sender, EventArgs e)
{
     // Button click event - update object and send it to database
     object.Name = TextboxObjectName.Text;

     DataLayer.UpdateObject(object);
}

This makes use of object tracking, and the Daya Layer can then look like this:

function void UpdateObject(MyObject obj)
{
    using (TestDataContext db = new TestDataContext ())
    {

       db.MyObjects.Attach(obj);

       db.Refresh(RefreshMode.KeepCurrentValues, obj);

       db.SubmitChanges();
    }
}