How to use OpenFileDialog to select a folder?

Basically you need the FolderBrowserDialog class:

Prompts the user to select a folder. This class cannot be inherited.

Example:

using(var fbd = new FolderBrowserDialog())
{
    DialogResult result = fbd.ShowDialog();

    if (result == DialogResult.OK && !string.IsNullOrWhiteSpace(fbd.SelectedPath))
    {
        string[] files = Directory.GetFiles(fbd.SelectedPath);

        System.Windows.Forms.MessageBox.Show("Files found: " + files.Length.ToString(), "Message");
    }
}

If you work in WPF you have to add the reference to System.Windows.Forms.

you also have to add using System.IO for Directory class

As a note for future users who would like to avoid using FolderBrowserDialog, Microsoft once released an API called the WindowsAPICodePack that had a helpful dialog called CommonOpenFileDialog, that could be set into a IsFolderPicker mode. The API is available from Microsoft as a NuGet package.

This is all I needed to install and use the CommonOpenFileDialog. (NuGet handled the dependencies)

Install-Package Microsoft.WindowsAPICodePack-Shell

For the include line:

using Microsoft.WindowsAPICodePack.Dialogs;

Usage:

CommonOpenFileDialog dialog = new CommonOpenFileDialog();
dialog.InitialDirectory = "C:\\Users";
dialog.IsFolderPicker = true;
if (dialog.ShowDialog() == CommonFileDialogResult.Ok)
{
    MessageBox.Show("You selected: " + dialog.FileName);
}

There is a hackish solution using OpenFileDialog where ValidateNames and CheckFileExists are both set to false and FileName is given a mock value to indicate that a directory is selected.

I say hack because it is confusing to users about how to select a folder. They need to be in the desired folder and then just press Open while file name says "Folder Selection."

This is based on Select file or folder from the same dialog by Denis Stankovski.

OpenFileDialog folderBrowser = new OpenFileDialog();
// Set validate names and check file exists to false otherwise windows will
// not let you select "Folder Selection."
folderBrowser.ValidateNames = false;
folderBrowser.CheckFileExists = false;
folderBrowser.CheckPathExists = true;
// Always default to Folder Selection.
folderBrowser.FileName = "Folder Selection.";
if (folderBrowser.ShowDialog() == DialogResult.OK)
{
    string folderPath = Path.GetDirectoryName(folderBrowser.FileName);
    // ...
}