Populate Combobox2 according to Combobox1 content, LINQ to Entities in Windows Forms

As this thread is too long, it is hard to add more posts.
To be continued here: http://www.c-sharpcorner.com/Forums/Thread/166792/initialize-combo-box-with-a-blank-item-linq-C-Sharp.aspx

MessageBox to show the value of comboBox1.SelectedValue:

MessageBox.Show("Convert.ToInt32(comboBox1.SelectedValue).ToString() = " + Convert.ToInt32(comboBox1.SelectedValue).ToString());
comboBox2.DataSource = (from t in Tsk where t.ID == Convert.ToInt32(comboBox1.SelectedValue) select new { t.ID, t.TaskName }).ToList();

The messageBox returns the right value, but at comboBox2.Datasource level get the following error:

LINQ to Entities does not recognize the method 'Int32 ToInt32 (System.Object) ", and it can not be translated into a store expression.

Conversion could not be done inside the LINQ query

Finally this code works:

  int val = Convert.ToInt32(comboBox1.SelectedValue.ToString());
  comboBox2.DataSource = (from t in Tsk where t.ID == val select new { t.ID, t.TaskName }).ToList();
  comboBox2.ValueMember = "ID";
  comboBox2.DisplayMember = "TaskName";

Now the final code is as the next:

I realize that we only want the ID property but the problem is that (unless things have changed since the 11th post of this thread) we need to somehow extract it from the anonymous type which has been assigned to comboBox1's  SelectedValue property and we can't do that unless we know exactly how that looks when ToString() is applied to it.

If we're getting 0 when the range of values is 1 to 5, then there seems to be something wrong somewhere. 

What is needed for 'val' is only: ID

Table Employee Fields are: ID, LastName, FirstName, BirthDate.
  Records: IDs from 1 to 3
  First record ID = 1 LastName = "FONDA"

Table Tasks Fields are: ID, TaskName, EmployeeID
  Records: IDs from 1 to 5
  Retated records: ID = 4, TaskName = Task8, EmployeeID = 1
  ID = 5, TaskName = Task3, EmployeeID = 1

Employee.ID is related to Tasks.EmployeeID (1 Employee to many Tasks)

I admit that "Tasks" should have been called "TasksPerEmployee".

Is that what you expecting 'val' to be for the current selected item in comboBox1.

In other words does the SelectedValue look something like this:

 { ID = 0, LastName = "FONDA" }

Even if it is, I don't really understand the error because 'while' is just a filter - if it's false for all rows, the query should just return values for all the rows. 

val = 0, and Tasks.ID values are from 1 to 5

int val = Convert.ToInt32(comboBox1.SelectedValue.ToString().Split(' ')[3].Replace(",", ""));


the index is outside the limits of Table

Hmm, I wonder then whether this will now work:

int val = Convert.ToInt32(comboBox1.SelectedValue.ToString().Split(' ')[3].Replace(",", ""));

comboBox2.DataSource = (from t in Tsk where t.ID ==  val select new { t.ID, t.TaskName }).ToList();

t.EmployeeID is an int and is in relation with u.ID (1 u.ID to Many t.t.EmployeeID).

although I use this:

comboBox2.DataSource = (from t in Tsk select new { t.EmloyeeID, t.TaskName }).ToList();

I get the following error:

An error occurred while executing the command definition. For details, see the inner exception.

a first idea on the issue:

I think I made a mistake as I used EmployeeID as ValueMember of ComboBox2, ValueMember should be unique, I had to put ID (of Tasks table) instead. Code works if used as next (without link to ComboBox1.SelectedValue):

  comboBox2.DataSource = (from t in Tsk  select new { t.ID, t.TaskName }).ToList();
  label2.Text = comboBox2.DataSource.ToString();
  comboBox2.ValueMember = "ID";
  comboBox2.DisplayMember = "TaskName";

What is the type of t.EmployeeID?

Is it a string already or is it an int?

Sorry for the delay,

Got this: LINQ to Entities does not recognize the method 'System.String ToString () ", and it can not be translated into a store expression.

The code below is filling well ComboBox1:

private void Form1_Load(object sender, EventArgs e)
{
  using (LINQtoEntitiesEntities MyEntities = new LINQtoEntitiesEntities())
  {
 ObjectQuery Emp = MyEntities.Employee;
 comboBox1.DataSource = (from u in Emp select new { u.ID, u.LastName }).ToList();
 comboBox1.ValueMember = "ID";
 comboBox1.DisplayMember = "LastName";
  }
}

also we have to change this:

if (comboBox1.SelectedIndex == -1) return;

to this:

  if (comboBox1.SelectedIndex.ToString() == "0") return;

Because comboBox1.SelectedIndex gets the value 0 each time there is a new ligne, and this a change Event for ComboBox2.

What we really need is to retrieve the current "u.ID" from the first query (ComboBox1) to use it in the second query "EmployeeID = u.ID" as condition to display it in comboBox2

OK, having thought about it some more, I think it would be better to use a named class rather than a Tuple so I'd try making the highlighted changes. I'm hoping that the SelectedValue property will now work correctly but, if it doesn't, then there will be a n easy way to deal with it:

Yes .NET 4.0 (VS 2010)

Hmm, it looks like the SelectedValue property is returning the full anonymous type instance.

As the property is of type object, that makes it difficult to deal with an anonymous type as casting is not possible.

We could apply the ToString() method and then parse out the value of the ID field but that's error prone.

It's probably better to use either a Tuple or a named type instead.

Are you using .NET 4.0 (VS 2010)?

comboBox1.SelectedValue = { ID = 1, LastName = "FONDA" }

We have 2 combos

ComboEmployee (ComboBox1) should have ID and LastName, and should display only LastName

ComboTasks (ComboBox2) should have EmployeeID and TaskName, and should display only TaskName

When we change Employee in ComboEmployee its related tasks should change in ComboTasks

I don't understand how that is possible when you have this line in your other method:

comboBox1.ValueMember = "ID";

If it is a combination of ID and LastName, then how are they being combined - in other words what does comboBox1.SelectedValue.ToString() typically look like?

Here
comboBox1.SelectedValue return 2 args: "ID" and "LastName", we need to retrieve only "ID" to make comparison with EmployeeID. So I think we have to use something other than .SelectedValue

Sorry, that's a mistake on my part.

This line:

comboBox2.DataSource = (from t in Tsk where t.EmloyeeID.ToString() = comboBox1.SelectedValue.ToString() select new { t.EmloyeeID, t.TaskName }).ToList();

should be:

comboBox2.DataSource = (from t in Tsk where t.EmloyeeID.ToString() == comboBox1.SelectedValue.ToString() select new { t.EmloyeeID, t.TaskName }).ToList();

@Vulpes

Sorry for this inconvenience,


the used code is as next:

using System;
using System.Windows.Forms;
using System.Data.Objects;
using System.Linq;


namespace MyNameSpace
{
public partial class Form1 : Form
{

public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
using (LINQtoEntitiesEntities MyEntities = new LINQtoEntitiesEntities())
{
ObjectQuery Emp = MyEntities.Employee;
comboBox1.DataSource = (from u in Emp select new { u.ID, u.LastName }).ToList();
comboBox1.ValueMember = "ID";
comboBox1.DisplayMember = "LastName";
}
}
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
if (comboBox1.SelectedIndex == -1) return;

using (LINQtoEntitiesEntities MyEntities = new LINQtoEntitiesEntities())
{
ObjectQuery Tsk = MyEntities.Tasks;
comboBox2.DataSource = (from t in Tsk where t.EmloyeeID.ToString() = comboBox1.SelectedValue.ToString() select new { t.EmloyeeID, t.TaskName }).ToList();
comboBox2.ValueMember = "EmployeeID";
comboBox2.DisplayMember = "TaskName";
}
}
}
}

The above code returned:

-Delegate 'System.Func' does not take 1 arguments
-Cannot convert lambda expression to type 'string' because it is not a delegate type
-The left-hand side of an assignment must be a variable, property or indexer
-Cannot implicitly convert type 'string' to 'bool'

You're getting that error because you haven't followed the 'using' statement with a block i.e. a group of statements enclosed in curly braces:

using (.....)
{
    // statements in block
}

Check how I've done it in my first post to this thread.

That's right, I've removed the semi-colon, and now I have an error at this level:

            using (LINQtoEntitiesEntities MyEntities = new LINQtoEntitiesEntities())
            ObjectQuery Emp = MyEntities.Employee;

Error: Embedded statement cannot be a declaration or labeled statement

Hi Satyapriya,

Sorry, but I am forced to use LINQ to Entities for this case.

Thank you anyway.

@Vulpes

Got the next :


- The name 'MyEntities' does not exist in the current context

- Delegate 'System.Func' does not take 1 arguments

Try:

Hi Samio,

Try this...


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
namespace Combobox
{
    public partial class Form1 : Form
    {
        string ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
        OleDbCommand com;
        OleDbDataAdapter oda;
        DataSet ds;
        string str;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.Add("Choose CourseID");
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Courses";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox1.Items.Add(reader["CourseID"]); 
            }
          
           
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            comboBox2.Items.Clear();
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Students where CourseID='" + comboBox1.Text.Trim() + "'";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox2.Items.Add(reader["FirstName"].ToString());
            }
            con.Close();
            reader.Close();
           
        }
    }
}



Thanks
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