C# issue with user selected docs being merged into one

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C#

currently having an issue with individual files being selected via an 'openFileInput'. The idea is that a user selects between 1-5 docs using the open-file, the combine button then merges these into one document depending on the user output folder selection.

private string[] selectedDocs;
    string selectedFile1 = @"";
    private void browseFileButton1_Click(object sender, EventArgs e)
    {
        OpenFileDialog openFileInput1 = new OpenFileDialog();
        openFileInput1.Filter = "Word Documents|*.docx;*.doc";
        openFileInput1.Title = "Select a Word Document";

        // Default file, altered when the user selects file of choice 
        openFileInput1.FileName = selectedFile1;

        // initial file path display
        filePath1.Text = openFileInput1.FileName;

        // 'OK' button being confirmed on the popup menu
        if (openFileInput1.ShowDialog() == DialogResult.OK)
        {
            selectedFile1 = openFileInput1.FileName;
            filePath1.Text = openFileInput1.FileName;
        }
    }

  // ** INPUT FILE PROCESS AS SHOWN ABOVE REPEATED 4 MORE TIMES
  // this allows user to insert 4 more files **


    // Output Destination - for separate files
    private string outputFolder2 = @"";
    private void browseButtonOut2_Click(object sender, EventArgs e)
    {
        FolderBrowserDialog diagBrowserOutput2 = new FolderBrowserDialog();
       diagBrowserOutput2.Description = "Select a folder location to save the document...";

        // Default folder, altered when the user selects folder of choice 
        diagBrowserOutput2.SelectedPath = outputFolder2;

        // output file path display
        outputPath2.Text = diagBrowserOutput2.SelectedPath;

        if (DialogResult.OK == diagBrowserOutput2.ShowDialog())
        {
            outputFolder2 = diagBrowserOutput2.SelectedPath;
            outputPath2.Text = diagBrowserOutput2.SelectedPath;
        }
    }

    // combine files in folder selected using MsWord.cs, 
    private void combineButton2_Click(object sender, EventArgs e)
    {
        string mixedFolder = Path.Combine(selectedFile1, selectedFile2, selectedFile3, selectedFile4, selectedFile5);
        selectedDocs = Directory.GetFiles(mixedFolder, "*.doc");
        string outcomeFolder2 = outputFolder2;
        string outputFile2 = "Combined-files.docx";
        string outputFileName2 = Path.Combine(outcomeFolder2, outputFile2);

        MsWord.Merge(selectedDocs, outputFileName2, true);
    }

i currently having this working if the user selects a folder containing a list of documents, however i'm trying to enable this to work with individual file selection as opposed to folder selection. The issue seems to be flagged up here,

selectedDocs = Directory.GetFiles(mixedFolder, "*.doc");

any advice or suggestions are welcomed, thank you

What I have tried:

combining the list of individual user selections into one path for merging