Show combo on AspxGridview

I work on northwind database. In my AspxGridview I want to show comboBox. I fill grid on back end C# I also want my combo will fill back end.

 <dxwgv:ASPxGridView ID="ASPxGridView1" runat="server" 
            AutoGenerateColumns="False" KeyFieldName="CategoryID" 
            oncelleditorinitialize="ASPxGridView1_CellEditorInitialize">
            <Columns>
                <dxwgv:GridViewCommandColumn VisibleIndex="0" Width="80px">
                    <EditButton Visible="True">
                    </EditButton>
                    <NewButton Visible="True">
                    </NewButton>
                    <DeleteButton Visible="True">
                    </DeleteButton>
                </dxwgv:GridViewCommandColumn>
                <dxwgv:GridViewDataTextColumn Caption="CategoryID" FieldName="CategoryID" 
                    VisibleIndex="1">
                </dxwgv:开发者_如何学PythonGridViewDataTextColumn>
                <dxwgv:GridViewDataComboBoxColumn Caption="CategoryName" 
                    FieldName="CategoryName" VisibleIndex="2">
                    <PropertiesComboBox TextField="Value" ValueField="key" ValueType="System.Int32">
                        <ClientSideEvents SelectedIndexChanged="function(s, e) { OnBankChanged(s); }" />
                    </PropertiesComboBox>
                </dxwgv:GridViewDataComboBoxColumn>
                <dxwgv:GridViewDataTextColumn Caption="Description" FieldName="Description" 
                    VisibleIndex="3">
                </dxwgv:GridViewDataTextColumn>
            </Columns>
        </dxwgv:ASPxGridView>

To fill grid i use the bellow C# syntax.

  DataClasses1DataContext db = new DataClasses1DataContext();
            var r = from p in db.Categories
                    select p;
            ASPxGridView1.DataSource = r;
            ASPxGridView1.DataBind();

To fill gridview cell of combo i use Bellow C# syntax

 protected void ASPxGridView1_CellEditorInitialize(object sender, DevExpress.Web.ASPxGridView.ASPxGridViewEditorEventArgs e)
        {
            if (!ASPxGridView1.IsEditing || e.Column.FieldName != "CategoryID") return;
            ASPxComboBox combo = e.Editor as ASPxComboBox;

            if (!(e.KeyValue == DBNull.Value || e.KeyValue == null)) //return;
            {
                object val = ASPxGridView1.GetRowValuesByKeyValue(e.KeyValue, "CategoryID");
                if (val == DBNull.Value) return;
                Int32 CategoryID = (Int32)val;
                FillCityCombo(combo, CategoryID);
            }

            combo.Callback += new CallbackEventHandlerBase(cmbBranch_OnCallback);

        }

        private void cmbBranch_OnCallback(object source, CallbackEventArgsBase e)
        {
            FillCityCombo(source as ASPxComboBox, Convert.ToInt16(e.Parameter));
        }

        protected void FillCityCombo(ASPxComboBox cmb, Int32 CategoryID)
        {
            //cmb.Items.Clear();
            //cmb.DataSourceID = "";
            DataClasses1DataContext db = new DataClasses1DataContext();
            var r = from p in db.Categories
                    select new { p.CategoryID,p.CategoryName};
            cmb.DataSource = r;
            cmb.DataBind();
        }

When I run the code AspxGridview fill well but when I click on Edit or New Command on left side of my grid shows me error message below:

**Object reference not set to an instance of an object.**

1. What's the problem?
2. How to solve this problem?
3. How to bind cell combo on aspx gridview?

This is the cause of this issue. You are checking for the "CategoryID" field name. But the ComboBox column is created for the "CategoryName" field:

if (!ASPxGridView1.IsEditing || e.Column.FieldName != "CategoryID") return;
ASPxComboBox combo = e.Editor as ASPxComboBox;

We've posted a sample project showing how to implement dependent combos in insert mode at:

http://www.devexpress.com/Support/Center/ViewIssue.aspx?issueid=Q102130

I hope, this project will be helpful to you.

Bind in the OnRowDataBound event (or AspxGridview equivalent)