public CardViewCustomCommandButtonCollection CustomButtons { get; }

Public ReadOnly Property CustomButtons As CardViewCustomCommandButtonCollection

<dx:ASPxCardView ID="ASPxCardView1" runat="server" DataSourceID="AccessDataSource1" KeyFieldName="CustomerID" AutoGenerateColumns="False" OnCustomButtonCallback="ASPxCardView1_CustomButtonCallback">
<SettingsBehavior AllowFocusedCard="true" />
<Columns>
    <dx:CardViewTextColumn FieldName="CompanyName" VisibleIndex="1">
    </dx:CardViewTextColumn>
    <dx:CardViewTextColumn FieldName="ContactName" VisibleIndex="2">
    </dx:CardViewTextColumn>
    <dx:CardViewTextColumn FieldName="City" VisibleIndex="5">
    </dx:CardViewTextColumn>
    <dx:CardViewTextColumn FieldName="Region" VisibleIndex="6">
    </dx:CardViewTextColumn>
    <dx:CardViewTextColumn FieldName="Country" VisibleIndex="8">
    </dx:CardViewTextColumn>
</Columns>
<CardLayoutProperties>
    <Items>
        <dx:CardViewCommandLayoutItem HorizontalAlign="Right">
        </dx:CardViewCommandLayoutItem>
        <dx:CardViewColumnLayoutItem ColumnName="Company Name">
        </dx:CardViewColumnLayoutItem>
        <dx:CardViewColumnLayoutItem ColumnName="Contact Name">
        </dx:CardViewColumnLayoutItem>
        <dx:CardViewColumnLayoutItem ColumnName="City">
        </dx:CardViewColumnLayoutItem>
        <dx:CardViewColumnLayoutItem ColumnName="Region">
        </dx:CardViewColumnLayoutItem>
        <dx:CardViewColumnLayoutItem ColumnName="Country">
        </dx:CardViewColumnLayoutItem>
        <dx:EditModeCommandLayoutItem HorizontalAlign="Right">
        </dx:EditModeCommandLayoutItem>
    </Items>
</CardLayoutProperties>
</dx:ASPxCardView>  


<asp:AccessDataSource ID="AccessDataSource1" runat="server" DataFile="~/App_Data/nwind.mdb" SelectCommand="SELECT * FROM [Customers]" />

using DevExpress.Web;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e) {
        if (!IsPostBack)
        {
            (ASPxCardView1.CardLayoutProperties.Items[0] as CardViewCommandLayoutItem).CustomButtons.Add(CreateCustomButton());
        }
    }

    CardViewCustomCommandButton CreateCustomButton() {
        CardViewCustomCommandButton customBtn = new CardViewCustomCommandButton();
        customBtn.ID = "FilterBtn";
        customBtn.Text = "Filter";
        return customBtn;
    }
    protected void ASPxCardView1_CustomButtonCallback(object sender, ASPxCardViewCustomButtonCallbackEventArgs e) {
        if (e.ButtonID == "FilterBtn")
        {
            ASPxCardView cardview = sender as ASPxCardView;
            cardview.FilterExpression = "[Country] like '%Germany%'";
        }
    }
}

Imports DevExpress.Web

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
        If Not IsPostBack Then
            TryCast(ASPxCardView1.CardLayoutProperties.Items(0), CardViewCommandLayoutItem).CustomButtons.Add(CreateCustomButton())
        End If
    End Sub

    Private Function CreateCustomButton() As CardViewCustomCommandButton
        Dim customBtn As New CardViewCustomCommandButton()
        customBtn.ID = "FilterBtn"
        customBtn.Text = "Filter"
        Return customBtn
    End Function
    Protected Sub ASPxCardView1_CustomButtonCallback(ByVal sender As Object, ByVal e As ASPxCardViewCustomButtonCallbackEventArgs)
        If e.ButtonID = "FilterBtn" Then
            Dim cardview As ASPxCardView = TryCast(sender, ASPxCardView)
            cardview.FilterExpression = "[Country] like '%Germany%'"
        End If
    End Sub
End Class