I think the best way to select all items in this case (with Store) would be using .AddScript method.
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (!X.IsAjaxRequest)
{
Store store = this.MultiSelect1.Store.Primary;
store.DataSource = new object[] {
new object[] {"Text1", "Value1" },
new object[] {"Text2", "Value2" },
new object[] {"Text3", "Value3" }
};
store.DataBind();
}
}
protected void SelectAll(object sender, DirectEventArgs e)
{
string script = "var values = MultiSelect1.getValues()," +
" s = [];" +
"for (var i=0; i < values.length; i++) {" +
" s.push(values[i].value)" +
"}" +
"MultiSelect1.setValue(s);";
MultiSelect1.AddScript(script);
}
</script>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Ext.Net Example</title>
</head>
<body>
<form runat="server">
<ext:ResourceManager runat="server" />
<ext:MultiSelect ID="MultiSelect1" runat="server">
<Store>
<ext:Store runat="server">
<Reader>
<ext:ArrayReader>
<Fields>
<ext:RecordField Name="text" />
<ext:RecordField Name="value" />
</Fields>
</ext:ArrayReader>
</Reader>
</ext:Store>
</Store>
</ext:MultiSelect>
<ext:Button runat="server" Text="Select all">
<DirectEvents>
<Click OnEvent="SelectAll" />
</DirectEvents>
</ext:Button>
</form>
</body>
</html>