[DefaultValue(false)]
public virtual bool MultiValue { get; set; }

<DefaultValue(False)>
Public Overridable Property MultiValue As Boolean

using DevExpress.XtraReports.Parameters;
// ...
// Create a parameter and set up its properties. Set the
// parameter's MultiValue property to true to make it multi-value.
var parameter1 = new Parameter() {
    Name = "categories",
    Description = "Categories: ",
    Type = typeof(System.Int32),
    MultiValue = true
};

// Create a DynamicListLookUpSettings class instance and
// configure its properties to retrieve the parameter's predefined values
// from the report's data source.
var lookupSettings = new DynamicListLookUpSettings() {
    DataSource = report.DataSource,
    DataMember = "Categories",
    DisplayMember = "CategoryName",
    ValueMember = "CategoryId"
};

// Assign the setting to the parameter's LookUpSettings property.
parameter1.LookUpSettings = lookupSettings;

// Enable the parameter's SelectAllValues property to use
// all predefined values as defaults.
parameter1.SelectAllValues = true;

Imports DevExpress.XtraReports.Parameters
' ...
' Create a parameter and set up its properties. Set the
' parameter's MultiValue property to true to make it multi-value.
Dim parameter1 = New Parameter() With {
    .Name = "categories",
    .Description = "Categories: ",
    .Type = GetType(System.Int32),
    .MultiValue = True
}

' Create a DynamicListLookUpSettings class instance and
' configure its properties to retrieve the parameter's predefined values
' from the report's data source.
Dim lookupSettings = New DynamicListLookUpSettings() With {
    .DataSource = report.DataSource,
    .DataMember = "Categories",
    .DisplayMember = "CategoryName",
    .ValueMember = "CategoryId"
}

' Assign the setting to the parameter's LookUpSettings property.
parameter1.LookUpSettings = lookupSettings

' Enable the parameter's SelectAllValues property to use
' all predefined values as defaults.
parameter1.SelectAllValues = True

parameter1.Type = typeof(System.Int32);
parameter1.MultiValue = true;
parameter1.Description = "Categories: ";