Validating user's input before continuing program execution

If you want to prevent closing the form you can handle the event

bool cancel = true;
protected override void OnFormClosing(FormClosingEventArgs e)
{
    e.Cancel = cancel;
    base.OnFormClosing(e);
}

Remember to change cancel to false when it's done to close the form.

There is an event named "FormClosing" on forms.

A quick sample:

 private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show("Are you sure you want to quit?", MessageBoxButtons.YesNo) == DialogResult.No)
            {
                e.Cancel = true;
            }

        }

I am assuming that you have an OpenfileDialog (to allow the user to select a file),and a button probably named Open File to pass the filename to next form.If this is the case,then you can try to to disable the open button if no file has been selected.

Consider the code below as the function where all logic takes place;

private void BrowseFile()
{
//dlgopenfile is the name of Openfiledialog that allows the user to browse for a file.
//string filename is the name of selected file if any.
//Form2 is the next form.

try
{

switch (dlgopenfile.ShowDialog())
{
case DialogResult.OK://If Ok(Yes) button is pressed on Openfiledialog.
filename = dlgopenfile.FileName;
break;

case DialogResult.Cancel://If Cancel button is pressed on Openfiledialog.
filename = "";
break;
}

if (filename.Length >= 1)
{
if (File.Exists(filename) == true)
{
ButtonOpenFile.Enabled = true;
}
else
{
ButtonOpenFile.Enabled = false;
throw new FileNotFoundException("The file you selected does not exist.");
}
}
}
catch (FileNotFoundException ex)
{
MessageBox.Show(ex.Message, "Form1", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
}

The next function occurs if user tries to close the form in mid session.

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
try
{
switch (MessageBox.Show("Do you want to exit ?", "Form1", MessageBoxButtons.YesNo, MessageBoxIcon.Asterisk))
{
case DialogResult.Yes:
this.Close();
break;

case DialogResult.No:
e.Cancel = true;
break;
}
}
catch (Exception)
{
//Exception handling code goes here.
}
}

Finally the function below calls Form2's constructor with the selected file as argument.

private void ButtonOpenFile_Click(object sender, EventArgs e)
{
//This Button is enabled only if the file has been selected and if its exists.
Form2 form2 = new Form2(filename);//filename is the name of selected file,as decided in function BrowseFile().
this.Close();//Close Form1.
form2.ShowDialog();//Show Form2 as modal dialog.
}

Hope it would help you achieve what you need.Anything more,please let me know.

Proposally you start you form with form.ShowDialog(), which returns DialogResult. You should check whether it is DialogResult.Ok or whether form.DialogResult != DialogResult.None. In the form, if the user inserts the file you can set the form.DialogResult explicitely to DialogResult.Ok

Just you can handle the close event with your own logic

private void Form1_FormClosing_1(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show(text:"Are you sure you want to quit?",caption:string.Empty,buttons: MessageBoxButtons.YesNo) == DialogResult.No)
            {
                e.Cancel = true;
            }
        }