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How can I show Text in ComboBox when no item selected in Windows Application using LINQ C#
Here is my code how I get all rooms.... in Combobox.

private void LoadRoom()

        {



            try

            {

                db = new HotelEntities();



                // cmbProvince.Text = "";



                var Room = (from u in db.Room



                            select new { u.RoomId, u.RoomNumber }).ToList();



                cmbRoom.Text = ".. Select.."; // This one do not working.

                cmbRoom.DisplayMember = "RoomNumber";

                cmbRoom.ValueMember = "RoomId";

                cmbRoom.DataSource = Room;



            }

            catch (Exception ex)

            {

                MessageBox.Show(ex.Message);

            }

        }

Thank you!

If you set the DataSource of a combobox, a currencymanager is used on the background and its position is selected (the first item).

Instead of setting DataSource, try adding the items:

cmbRoom.Items.AddRange(Room);

NB, setting Text as a placeholder will not work if an item is chosen and cleared later, unless you add an extra check (in TextChanged or SelectedIndexChanged)

Fast solution

Create a source class for your ComboItems, with (at least) the properties to Display and the inner value of the property. If you create a generic class you can use it for all your combo boxes.

class ComboDisplay<TSource>

{

     public string Display {get; set;}

     public TSource Value {get; set;}

}



cmbRoom.DisplayMember = nameof(ComboDisplay.Display);

cmbRoom.ValueMember = nameof(ComboDisplay.Value);

When you create the data source for your combobox, make sure you add a default value. In the example below I assume that you want to select items of type Room in your combo:

IEnumerable<Room> availableRooms = myDbContext.Rooms

    .Where(room => room.IsAvailable) 

    .Select(room => new ComboDisplay<Room>

    {

         Display = room.Name,

         Value = new Room

         {

             Id = room.Id,

             ...

         },

    })

    // add a dummy value if nothing is selected

    .Concat(new Room

    {

        Display = "Please select a room",

        Value = null, // meaning: nothing selected

    });

After selection, use comboBox1.SelectedValue to get the selected Room, or null if nothing is selected.

Create a Special Combobox class

If you have to use this regularly, consider creating a generic sub class of ComboBox that can display items of a certain TSource, and will return null if nothing is selected:

class MyComboBox<TSource> : ComboBox

{

    public MyComboBox() : base()

    {

        base.DataSource = this.EmptyList;

        base.DisplayMember = nameof(ComboDisplay.Display);

        base.ValueMember = nameof(ComboDisplay.Value);

    }



    private static readonly EmptyItem = new ComboDisplay

    {

        Display = "Please select a value",

        Value = null,

    }

Make a property that returns the available combo items. Make sure that the EmptyItem is always in the collection:

public IReadonlyCollection<TSource> ComboItems

{

    get {return (IReadOnlyCollection<TSource>)base.DataSource;}

    set 

    {

        // TODO: check if the empty element is in your list; if not add it

        base.DataSource = value;

    }

}

Finally: the function to get the Selected value, or null if nothing is selected:

public TSource SelectedValue

{

     get => return (TSource)base.SelectedValue;

     set

     {

         // TODO: check if value is in ComboItems

         base.SelectedValue = value;

     }

}