Modify textbox from class other than Form1

[ezrollers]

I'm still learning C# and this is really annoying me.

I need to modify the contents of a text box on a windows program.

(Using windows app template on VS 2005)

I have created a property on Form1 (SourcePath is a textbox)

public string SourceDir
        {
            get { return SourcePath.Text; }
            set { SourcePath.Text = value; }
        }
 

I then have this method on a class called dirinfo.

public void dirreader(FolderBrowserDialog folderBrowserDialog, ListBox listBox)
        {
            folderBrowserDialog.ShowDialog();
 
            Form1.SourceDir = folderBrowserDialog.SelectedPath;
 
            listBox.Items.Clear();
 
            DirectoryInfo dir = new DirectoryInfo(folderBrowserDialog.SelectedPath);
 
            DirectoryInfo[] subdirectories = dir.GetDirectories();
 
            FileInfo[] directoryFiles = dir.GetFiles();
 
            foreach (FileInfo str in directoryFiles)
            { listBox.Items.Add(str); }
        }
 
 

This will not work, I get this

Error An object reference is required for the nonstatic field, method, or property 'Ras.Form1.SourceDir.get'

The thing is that I don't know what the instance for Form1, as far as I can tell there does not appear to an instance of Form1, which does not sound right.

I tried creating an instance of Form1, substituted bold line by this

Form1 newform = new Form1();
newform.SourceDir = folderBrowserDialog.SelectedPath;
 

but unsurprisingly while it compiles, it does not show the path on the textbox.

I know i can make it work by putting

SourceDir = folderBrowserDialog.SelectedPath;

on the Form1 class, but I would like to learn how to do it without resorting to this.

Any ideas?

TIA