<dxwschs:ASPxScheduler ID="ASPxScheduler1" runat="server" ClientIDMode="AutoID" 
            Start="2014-06-26" GroupType="Resource" Width="100%" 
            AppointmentDataSourceID="AppointmentSqlDataSource" 
            ResourceDataSourceID="ResSqlDataSource" 
            ClientInstanceName="ASPxScheduler1" OnFetchAppointments="ASPxScheduler1_FetchAppointments">
....
</dxwschs:ASPxScheduler>

<asp:SqlDataSource ID="AppointmentSqlDataSource" runat="server" 
        ConnectionString="<%$ ConnectionStrings:fbcs %>" 
        ProviderName="<%$ ConnectionStrings:fbcs.ProviderName %>" 
        SelectCommand="SELECT * FROM APPOINTMENTS 
                        WHERE  (actualstart >= @START_DATE) AND (actualstart <= @END_DATE) OR
                               (actualfinish >= @START_DATE) AND (actualfinish <= @END_DATE)">
        <SelectParameters>
            <asp:Parameter Name="START_DATE" />
            <asp:Parameter Name="END_DATE" />
        </SelectParameters>
    </asp:SqlDataSource>

    <asp:SqlDataSource ID="ResSqlDataSource" runat="server" 
            ConnectionString="<%$ ConnectionStrings:fbcs %>" 
            ProviderName="FirebirdSql.Data.FirebirdClient" 
            SelectCommand="select * from RESOURCE">
    </asp:SqlDataSource>  

public partial class Test : System.Web.UI.Page
{
    TimeInterval lastFetchedInterval = new TimeInterval();

    protected void Page_Load(object sender, EventArgs e)
    {
    }

    protected void ASPxScheduler1_FetchAppointments(object sender, DevExpress.XtraScheduler.FetchAppointmentsEventArgs e)
    {
        DateTime start = e.Interval.Start;
        DateTime end = e.Interval.End;

        TimeSpan padding = TimeSpan.FromDays(100);

        if (start <= lastFetchedInterval.Start || end >= lastFetchedInterval.End)
        {
            lastFetchedInterval = new TimeInterval(start - padding, end + padding);

            AppointmentSqlDataSource.SelectParameters["START_DATE"].DefaultValue = lastFetchedInterval.Start.ToString();
            AppointmentSqlDataSource.SelectParameters["END_DATE"].DefaultValue = lastFetchedInterval.End.ToString();
        }
    }
}

 <dx:ASPxComboBox ID="CustomerNameASPxComboBox" runat="server" ClientIDMode="AutoID" 
                    ClientInstanceName="CustomerNameASPxComboBox"  Width="100%"
                    CssFilePath="~/App_Themes/BlackGlass/{0}/styles.css" CssPostfix="BlackGlass" 
                    SpriteCssFilePath="~/App_Themes/BlackGlass/{0}/sprite.css" 
                    ValueType="System.Int32"
                    DropDownStyle="DropDown" ValueField="ID"
                    TextFormatString="{0} {1} {2}" EnableCallbackMode="True"  CallbackPageSize="10"
                   OnItemsRequestedByFilterCondition="ASPxComboBox_OnItemsRequestedByFilterCondition_SQL"
                    OnItemRequestedByValue="ASPxComboBox_OnItemRequestedByValue_SQL">
                    <Columns>
                        <dx:ListBoxColumn Caption="Фамилия" FieldName="LAST_NAME" />
                        <dx:ListBoxColumn Caption="Имя" FieldName="FIRST_NAME" />
                        <dx:ListBoxColumn Caption="Отчество" FieldName="MIDDLE_NAME" />
                        <dx:ListBoxColumn Caption="Адрес" FieldName="ADDRESS" Width="250px" />
                        <dx:ListBoxColumn Caption="Телефон" FieldName="PHONE" />
                        <dx:ListBoxColumn FieldName="ID" Width="0px" />
                    </Columns>
                    <LoadingPanelImage Url="~/App_Themes/BlackGlass/Editors/Loading.gif">
                    </LoadingPanelImage>
                    <ButtonStyle Width="11px">
                    </ButtonStyle>
                    <ValidationSettings ErrorText="Error has occurred">
                        <ErrorFrameStyle ImageSpacing="4px">
                            <ErrorTextPaddings PaddingLeft="4px" />
<ErrorTextPaddings PaddingLeft="4px"></ErrorTextPaddings>
                        </ErrorFrameStyle>
                    </ValidationSettings>
                    <ReadOnlyStyle BackColor="#D5DCE8">
                    </ReadOnlyStyle>
                </dx:ASPxComboBox>

<asp:SqlDataSource ID="ClientsSqlDataSource" runat="server" 
                            ConnectionString="<%$ ConnectionStrings:fbcs %>" 
                            ProviderName="<%$ ConnectionStrings:fbcs.ProviderName %>" >
</asp:SqlDataSource>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Windows.Forms;
using System.Data;
using System.Globalization;
using System.Drawing;
using DevExpress.XtraScheduler;
using DevExpress.Web.ASPxScheduler;
using DevExpress.Web.ASPxClasses;
using DevExpress.Web.ASPxEditors;

public partial class Test2_Cb : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }

    protected void ASPxComboBox_OnItemRequestedByValue_SQL(object source, ListEditItemRequestedByValueEventArgs e)
    {
        long value = 0;
        if (e.Value == null || !Int64.TryParse(e.Value.ToString(), out value))
            return;
        ASPxComboBox comboBox = (ASPxComboBox)source;
        ClientsSqlDataSource.SelectCommand = @"SELECT *  
                                               FROM CLIENT c 
                                               WHERE (ID = @ID) ORDER BY full_name";

        ClientsSqlDataSource.SelectParameters.Clear();
        ClientsSqlDataSource.SelectParameters.Add("ID", TypeCode.Int64, e.Value.ToString());
        comboBox.DataSource = ClientsSqlDataSource;
        comboBox.DataBind();
    }

    protected void ASPxComboBox_OnItemsRequestedByFilterCondition_SQL(object source, ListEditItemsRequestedByFilterConditionEventArgs e)
    {
     
        ASPxComboBox comboBox = (ASPxComboBox)source;   
 
        ClientsSqlDataSource.SelectCommand =
               @"SELECT FIRST @rec_cnt SKIP @skip_rec
                    ID, FIRST_NAME, MIDDLE_NAME, LAST_NAME, ADDRESS, PHONE
                 FROM client c
                 where c.full_name CONTAINING @filter
                 order by c.full_name";

        ClientsSqlDataSource.SelectParameters.Clear();
        ClientsSqlDataSource.SelectParameters.Add("filter", TypeCode.String, string.IsNullOrEmpty(e.Filter) ? " " : e.Filter );
        ClientsSqlDataSource.SelectParameters.Add("skip_rec", TypeCode.Int64, e.BeginIndex.ToString());
        ClientsSqlDataSource.SelectParameters.Add("rec_cnt", TypeCode.Int64, (e.EndIndex - e.BeginIndex + 1).ToString());

        comboBox.DataSource = ClientsSqlDataSource;
        comboBox.DataBind();
    }
}