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C# LoadFile throwing unhandled exception when loading .txt, .rtf

I'm learning C# and went through a text editor tutorial. The final result works pretty good, except there is something strange happening I do not understand.
When I write/save/load files all in the text editor they work fine. But whenever I write a file in a different editor/download a text file from the internet somewhere, the file fails to load.
When I load the file, I get
"An unhandled exception of type 'System.ArgumentException' occurred in System.Windows.Forms.dll"
And when I look at "View Details" is says
"File format is not valid."
Even though there is text in the file (when viewed in a different text editor), the text property has nothing in it, a result of the file format being incorrect.
I'm pretty confused why it would load files made in the text editor itself (with the same extension) but not from somewhere else. I'm really not sure how to begin debugging this one. My save file/open file methods are listed below.
Open File
private void Open()
{
openFileDialog1.Filter = "RTF|*.rtf|Text Files|*.txt|VB Files|*.vb|C# Files|*.cs|All Files|*.*";
if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK && openFileDialog1.FileName.Length > 0)
{
GetCurrentDocument.LoadFile(openFileDialog1.FileName, RichTextBoxStreamType.RichText);
}
}
Save File
private void Save()
{
saveFileDialog1.FileName = tabControl1.SelectedTab.Name;
saveFileDialog1.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
saveFileDialog1.Filter = "RTF|.rtf";
saveFileDialog1.Title = "Save";
if (saveFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
if (saveFileDialog1.FileName.Length > 0)
{
GetCurrentDocument.SaveFile(saveFileDialog1.FileName, RichTextBoxStreamType.RichText);
}
}
}
Help would be much appreciated, thanks!

It's not just the extension of the file that determines it's type. This version of the method allows loading both "regular" RTF files and also ASCII files.
The RichTextBoxStreamType Enumeration provides a few different possibilities. If you are trying to load a file created using a different editor, you might need to use RichTextBoxStreamType.PlainText instead of RichTextBoxStreamType.RichText.

Delete .doc duplication during PDF conversion c#

I currently have a program that merges a folder consisting of word docs into one combined file via user input with a FileBrowserDialog. Once files are selected, a 'combine' button applies the code shown below which sources the folder containing the documents, output location and name of the file created.
string fileDate = DateTime.Now.ToString("dd-MM-yy");
string fileTime = DateTime.Now.ToString("HH.mm.ss");
string outcomeFolder = outputFolder;
string outputFileType = ".docx";
string outputFile = "Combined Folder " + fileDate + " # " + fileTime + outputFileType;
string outputFileName = Path.Combine(outcomeFolder, outputFile);
// Combines the file name, output path selected and the yes / no for pagebreaks.
MsWord.Merge(sourceFiles, outputFileName, pageBreaker);
// Message displaying how many files are combined.
MessageBox.Show("A total of " + sourceFiles.Length.ToString() + " documents have been merged", "Success", MessageBoxButtons.OK, MessageBoxIcon.Information);}
The MsWord referenced calls a separate .CS file which combines the folder components, output name and a boolean to enable page-breaks. The MsWord also automatically saves the word .doc to the user specified location once the contents of the folder are successfully combined. MsWord.Merge(sourceFiles, outputFileName, pageBreaker);
The issue i'm wanting to address is, when I enable this check box:
if (convert2PDFBox.Checked)
Microsoft.Office.Interop.Word.Application officeApp = new Microsoft.Office.Interop.Word.Application();
officeApp.Documents.Open(outputFileName);
outputFileType = ".pdf";
officeApp.ActiveDocument.SaveAs(outputFileName + outputFileType, WdSaveFormat.wdFormatPDF);
officeApp.Quit();
I want the program to solely create a PDF of the combined folder and not 2 seperate .doc and .PDF files, which it currently does. Since the MsWord.save function is called separately and is essential to the overall function of my program, I was wondering is there a possibility of deleting the initially combined file once conversion of the PDF takes place? e.g. "combinedDocument".Delete - Essentially allowing the copy to take place however not presenting the user with the initial .doc - only the .PDF
Though the issue is small, I would love to get it addressed and welcome any suggestions or advice with this manner. I can also provide any additional information if needed, thank you.
tl;dr - merging program creates an amalgamated Word .doc, which i want to change solely to a PDF when a checkbox is enabled instead of creating a .doc and PDF.

I finally resolved my issue - What I decided to do was manipulate my existing MsWord.cs and create a separate PDF.cs call for my main form:
Rather than save the Word .doc when being merged, I instead used: wordApplication.ActiveDocument.SaveAs(outputFile, Word.WdSaveFormat.wdFormatPDF);
which saved the merged content thus far as a .pdf
This however presented errors with Microsoft Word as I was then prompted to 'Save File As' due to the merged file never actually being saved in a .Doc / .Docx format
I then altered the closing statement of the call,
// Close Word application
wordApplication.Quit(
false, // save changes
By setting the 'Save Changes' setting to False, it removed the 'Save As' prompt which allowed the Word doc. to be dismissed without needing to be saved, thus leaving only the initial PDF created. I then applied the two separate File type calls to each checkbox presented, which allowed the user to enable the outcome format of the merged files.
Thank you for the suggestions regarding the issue.

Detect when OpenFileDialog returns a downloaded URL/URI

I'm using OpenFileDialog (.Net Framework 4, Windows 10) and I've noticed that it will allow the user to specify a URL as the file name (e.g., http://somewebsite/picture.jpg). This is very useful for my application, so I don't intend to disable it. The way it works is downloading the file into the user's temp directory and returning the temporary file name in the dialog's Filename property. This is nice, except for the fact that the user starts to build up garbage in his/her temp directory.
I would like to tell when a file was downloaded by the OpenFileDialog class (as opposed to a previously existing file), so I can clean up by deleting the file after use. I could check if the file's directory is the temp directory, but that's not very good since the user might have downloaded the file him/herself.
I've tried intercepting the FileOK event and inspect the Filename property to see if it is an HTTP/FTP URI, but despite what the documentation says ("Occurs when the user selects a file name by either clicking the Open button of the OpenFileDialog") it is fired after the file is downloaded, so I don't get access to the URL: the Filename property already has the temporary file name.
EDIT: This is an example of what I'like to do:
Dim dlgOpenFile As New System.Windows.Forms.OpenFileDialog
If dlgOpenFile.ShowDialog(Me) <> Windows.Forms.DialogResult.OK Then Return
''//do some stuff with dlgOpenFile.Filename
If dlgOpenFile.WasAWebResource Then
Dim finfo = New IO.FileInfo(dlgOpenFile.Filename)
finfo.Delete()
End If
In this example, I've imagined a property to dlgOpenFile "WasAWebResource" that would tell me if the file was downloaded or originally local. If it's the first case, I'll delete it.

There's no obvious way to do this, but as a workaround, how about checking where the file lives? It looks like by default this dialog downloads files to the users Temporary Internet Files directory, so you could introduce some code that looks something like this:
FileDialog dialog = new OpenFileDialog();
if (dialog.ShowDialog() == DialogResult.OK)
{
string temporaryInternetFilesDir = Environment.GetFolderPath(System.Environment.SpecialFolder.InternetCache);
if (!string.IsNullOrEmpty(temporaryInternetFilesDir) &&
dialog.FileName.StartsWith(temporaryInternetFilesDir, StringComparison.InvariantCultureIgnoreCase))
{
// the file is in the Temporary Internet Files directory, very good chance it has been downloaded...
}
}

C# WPF - Only allow certain file extensions

I have an OpenFileDialog and I only want to allow .txt as a valid file for the users.
I know I can add a Filter to the OpenFileDialog like so:
var dialog = new OpenFileDialog();
dialog.DefaultExt = ".txt";
dialog.Filter = "Text Files (*.txt)|*.txt";
var result = dialog.ShowDialog();
// Do something with the result
The problem however, is that I can still directly say something like "test.jpg" in the OpenFileDialog and then it opens this uploads this .jpg file. (Obviously it goes wrong somewhere later, but that doesn't matter for now.) I just want to know how I can restrict the user to only add ".txt" files, nothing else? (By directly validation it inside the OpenFileDialog, instead of doing it somewhere later.)

You cant do that only in OpenFileDialog and even if you could its a bad limitation.
Using the *.txt example there are multiple files extensions that are plain text inside, *.bat or all the codding file extensions *.cs, *.js, etc...
You should not limit the user on what file he can put on it.
For more complex file types if your program cant handle the file passed by the user you should show an error not prevent the user from passing the file.

Where are paths previously used in the program stored (e.g. FileDialog paths)?

According to this msdn article one of the sources for InitialDirectory property used in FileDialog is:
A path that was previously used in the program, perhaps retained from the last directory or file operation.
...
So if you selected your first file from folder x, the next time you try to select a file it will open up the FileDialog with with folder x selected (saving you having to navigate there).
Playing around with notepad this seems to carry across opening a file, saving a file, opening a file and even when printing with "Microsoft XPS Document Writer" which brings up it's own dialog.
So my question is where is this value stored between dialogs? I would like to be able to see what it is and potentially change it? The specific area i would like to change it is in the "Microsoft XPS Document Writer" printer which brings up it's own dialog. So it's not as simple as just setting the initalDirectory Value.

It's stored in the registry, somewhere in HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\ (LastVisitedPidlMRU).

You should take a look at this link:
MRU locations are what you are looking at!
Here's a way for accessing it:
var openFileDialog1 = new OpenFileDialog();
string path = openFileDialog1.InitialDirectory;
// you can change path if you want
openFileDialog1.InitialDirectory = path;
// after you are donw you can display you dialog
if (openFileDialog1.ShowDialog() == DialogResult.OK)
{
// do something
}
Another way is to use Directory.SetCurrentDirectory method which sets the application's current working directory
And from Microsoft website, it is stored at this location in the registry:
//The MRU lists for Windows Explorer-style dialog boxes are stored by file type for each user in the following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\OpenSaveMRU
hope this helps