using System;
using System.Windows.Forms;
using System.IO;
using DevExpress.XtraPrinting;
using DevExpress.XtraPrinting.Preview;
//...

// Create a MemoryStream instance.
MemoryStream stream = new MemoryStream();

private void button1_Click(object sender, EventArgs e) {
    // Create a report instance.
    XtraReport1 report = new XtraReport1();

    // Generate a report document.
    report.CreateDocument();
    
    // Save a report document to a stream.
    report.PrintingSystem.SaveDocument(stream);
}

private void button2_Click(object sender, EventArgs e) {
    // Create a PrintingSystem instance.
    PrintingSystem ps = new PrintingSystem();
    
    // Load the document from a stream.
    ps.LoadDocument(stream);

    // Create an instance of the preview dialog.
    PrintPreviewFormEx preview = new PrintPreviewFormEx();

    // Load the report document into it.
    preview.PrintingSystem = ps;

    // Show the preview dialog.
    preview.ShowDialog();
}

Imports System
Imports System.Windows.Forms
Imports System.IO
Imports DevExpress.XtraPrinting
Imports DevExpress.XtraPrinting.Preview
'...

' Create a MemoryStream instance.
Private stream As New MemoryStream()

Private Sub button1_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles button1.Click
    ' Create a report instance.
    Dim report As New XtraReport1()

    ' Generate a report document.
    report.CreateDocument()

    ' Save a report document to a stream.
    report.PrintingSystem.SaveDocument(stream)
End Sub

Private Sub button2_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles button2.Click
    ' Create a PrintingSystem instance.
    Dim ps As New PrintingSystem()

    ' Load the document from a stream.
    ps.LoadDocument(stream)

    ' Create an instance of the preview dialog.
    Dim preview As New PrintPreviewFormEx()

    ' Load the report document into it.
    preview.PrintingSystem = ps

    ' Show the preview dialog.
    preview.ShowDialog()
End Sub