dashboardDesigner.BeginUpdateParameters();
dashboardDesigner.Parameters["fromDate"].Value = new DateTime(2015, 4, 1);
dashboardDesigner.Parameters["toDate"].Value = new DateTime(2015, 10, 1);
dashboardDesigner.EndUpdateParameters();

dashboardDesigner.BeginUpdateParameters()
dashboardDesigner.Parameters("fromDate").Value = New DateTime(2015, 4, 1)
dashboardDesigner.Parameters("toDate").Value = New DateTime(2015, 10, 1)
dashboardDesigner.EndUpdateParameters()

private void DashboardDesigner_CustomParameters(object sender, CustomParametersEventArgs e) {
    // The countryParameter parameter value is changed to `Brazil`.
    var countryParam = e.Parameters.FirstOrDefault(p => p.Name == "countryParameter");
    if (countryParam != null) {
        countryParam.Value = "Brazil";
    }
}

Private Sub DashboardDesigner_CustomParameters(ByVal sender As Object, ByVal e As CustomParametersEventArgs)
    ' The countryParameter parameter value is changed to `Brazil`.
    Dim countryParam = e.Parameters.FirstOrDefault(Function(p) p.Name = "countryParameter")
    If countryParam IsNot Nothing Then
        countryParam.Value = "Brazil"
    End If
End Sub

private void DashboardDesigner_CustomParameters(object sender, CustomParametersEventArgs e) {
    // A new dashboard parameter is created with the `Brazil` string as a value.
    e.Parameters.Add(new DashboardParameter("countryParameter", typeof(string), "Brazil"));
}

Private Sub DashboardDesigner_CustomParameters(ByVal sender As Object, ByVal e As CustomParametersEventArgs)
    ' A new dashboard parameter is created with the `Brazil` string as a value.
    e.Parameters.Add(New DashboardParameter("countryParameter", GetType(String), "Brazil"))
End Sub

using System.Collections.Generic;
using DevExpress.DashboardCommon;
using DevExpress.DashboardWin;
using DevExpress.DataAccess.ConnectionParameters;

namespace WinViewer_DefaultParameterValues {
    public partial class Form1 : DevExpress.XtraEditors.XtraForm {
        public Form1() {
            InitializeComponent();
        }
        private void dashboardViewer1_SetInitialDashboardState(object sender, SetInitialDashboardStateEventArgs e) {
            DashboardState state = new DashboardState();
            DashboardParameterState parameters = new DashboardParameterState();
            parameters.Name = "customerIdParameter";
            parameters.Value = new List<string>() { "ALFKI", "AROUT", "BONAP" };
            state.Parameters.Add(parameters);
            e.InitialState = state;
        }
        private void dashboardViewer1_ConfigureDataConnection(object sender, DashboardConfigureDataConnectionEventArgs e) {
            if (e.DataSourceName == "SQL Data Source 1")
                e.ConnectionParameters = new Access97ConnectionParameters(@"..\..\Data\nwind.mdb", "", "");
        }
    }
}

Imports System.Collections.Generic
Imports DevExpress.DashboardCommon
Imports DevExpress.DashboardWin
Imports DevExpress.DataAccess.ConnectionParameters

Namespace WinViewer_DefaultParameterValues
    Partial Public Class Form1
        Inherits DevExpress.XtraEditors.XtraForm

        Public Sub New()
            InitializeComponent()
        End Sub

        Private Sub dashboardViewer1_SetInitialDashboardState(ByVal sender As Object, ByVal e As SetInitialDashboardStateEventArgs)
            Dim state As New DashboardState()
            Dim parameters As New DashboardParameterState()
            parameters.Name = "customerIdParameter"
            parameters.Value = New List(Of String)() From {"ALFKI", "AROUT", "BONAP"}
            state.Parameters.Add(parameters)
            e.InitialState = state
        End Sub

        Private Sub dashboardViewer1_ConfigureDataConnection(ByVal sender As Object, ByVal e As DashboardConfigureDataConnectionEventArgs) Handles dashboardViewer1.ConfigureDataConnection
            If e.DataSourceName = "SQL Data Source 1" Then
                e.ConnectionParameters = New Access97ConnectionParameters("..\..\Data\nwind.mdb", "", "")
            End If
        End Sub
    End Class
End Namespace

using DevExpress.DashboardCommon;
//...
public DashboardState CreateDashboardState() {
    DashboardState state = new DashboardState();
    //...
    state.Parameters.Add(new DashboardParameterState() {
        Name = "ParameterCountry",
        Value = "UK",
        Type = typeof(string)
    });
    return state;
}

Imports DevExpress.DashboardCommon
'...
Public Function CreateDashboardState() As DashboardState
        Dim state As New DashboardState()
        '...
        state.Parameters.Add(New DashboardParameterState() With {.Name = "ParameterCountry", .Value = "UK", .Type = GetType(String)})
        Return state
    End Function