Hi dnnyobi,

Refer below sample code.

Namespaces

C#

using System.IO;

VB.Net

Imports System.IO

Code

C#

private void Form1_Load(object sender, EventArgs e)
{
    try
    {
        string[] allRTFDocument = Directory.GetFiles(AppDomain.CurrentDomain.BaseDirectory.Replace("bin\\Debug\\", "") + "Files", "*.rtf");
        Merge(allRTFDocument, AppDomain.CurrentDomain.BaseDirectory.Replace("bin\\Debug\\", "") + "Folder" + @"\Final.rtf", true, txtName.Text);
    }
    catch (Exception)
    {

        throw;
    }
}

public static void Merge(string[] filesToMerge, string outputFilename, bool insertPageBreaks, string name)
{
    object missing = System.Type.Missing;
    object pageBreak = Microsoft.Office.Interop.Word.WdBreakType.wdSectionBreakNextPage;
    object outputFile = outputFilename;
    Microsoft.Office.Interop.Word._Application wordApplication = new Microsoft.Office.Interop.Word.Application();
    try
    {
        Microsoft.Office.Interop.Word.Document wordDocument = wordApplication.Documents.Add(ref missing, ref missing, ref missing, ref missing);
        Microsoft.Office.Interop.Word.Selection selection = wordApplication.Selection;
        int documentCount = filesToMerge.Length;
        int breakStop = 0;
        foreach (string file in filesToMerge)
        {
            if (Path.GetFileName(file.ToLower()).StartsWith(name))
            {
                breakStop++;
                selection.InsertFile(file, ref missing, ref missing, ref missing, ref missing);
                if (insertPageBreaks && breakStop != documentCount)
                {
                    selection.InsertBreak(ref pageBreak);
                }
            }
        }
        wordDocument.SaveAs(ref outputFile, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
        wordDocument = null;
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        wordApplication.Quit(ref missing, ref missing, ref missing);
    }
}

VB.Net

Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
    Try
        Dim allRTFDocument As String() = Directory.GetFiles(AppDomain.CurrentDomain.BaseDirectory.Replace("bin\Debug\", "") & "Files", "*.rtf")
        Merge(allRTFDocument, AppDomain.CurrentDomain.BaseDirectory.Replace("bin\Debug\", "") & "Folder" & "\Final.rtf", True, txtName.Text)
    Catch __unusedException1__ As Exception
        Throw
    End Try
End Sub

Public Shared Sub Merge(ByVal filesToMerge As String(), ByVal outputFilename As String, ByVal insertPageBreaks As Boolean, ByVal name As String)
    Dim missing As Object = System.Type.Missing
    Dim pageBreak As Object = Microsoft.Office.Interop.Word.WdBreakType.wdSectionBreakNextPage
    Dim outputFile As Object = outputFilename
    Dim wordApplication As Microsoft.Office.Interop.Word._Application = New Microsoft.Office.Interop.Word.Application()

    Try
        Dim wordDocument As Microsoft.Office.Interop.Word.Document = wordApplication.Documents.Add(missing, missing, missing, missing)
        Dim selection As Microsoft.Office.Interop.Word.Selection = wordApplication.Selection
        Dim documentCount As Integer = filesToMerge.Length
        Dim breakStop As Integer = 0

        For Each file As String In filesToMerge

            If Path.GetFileName(file.ToLower()).StartsWith(name) Then
                breakStop += 1
                selection.InsertFile(file, missing, missing, missing, missing)

                If insertPageBreaks AndAlso breakStop <> documentCount Then
                    selection.InsertBreak(pageBreak)
                End If
            End If
        Next

        wordDocument.SaveAs(outputFile, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing, missing)
        wordDocument = Nothing
    Catch ex As Exception
        Throw ex
    Finally
        wordApplication.Quit(missing, missing, missing)
    End Try
End Sub