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I have a WinForms project in which I am trying to implement the Passive View MVP pattern (meaning no business logic in my Views). Each form is a concrete View with an IView interface to which a Presenter is connected.

I do think i should handle some UI related logic in my View because I am otherwise adding needless complexity to try to handle this in the Presenter. Therefore, I have created a simple function in my View class which validates the result from the file dialog. I did not put this logic in the event handler of the button press because I wanted to avoid duplicate code since I have three of these buttons.

Please let me know if you think this is an appropriate way to implement this. I have posted the function, and the event (I have three button_Click events that each use this function) that utilizes this function below.

Button event that uses the GetFileName function:

// Gets the filename and fires the compliance standard adding event.
private void newComplianceStandardButton_Click(object sender, EventArgs e)
{
    string fileName = GetFileName();

    if (fileName != null)
    {
        AddingComplianceStandard?.Invoke(this, fileName);
    }

    Close();
}

Function that checks the dialog result and returns the filename:

// Opens the openfile dialog and checks the result. If the result is OK, the form is closed and the filename is returned.
public string GetFileName()
{
    DialogResult result = openFileDialog1.ShowDialog();
    string fileName = openFileDialog1.FileName;

    if (result == DialogResult.OK &&
        (Path.GetExtension(openFileDialog1.FileName) == ".txt" || Path.GetExtension(openFileDialog1.FileName) == ".csv"))
    {
        Close();
        return fileName;
    }
    else
    {
        MessageBox.Show("You have selected a file with an illegal extension. Please try again and select a *.txt or a *.csv file");
        return null;
    }
}
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In the big if you have the following:

Path.GetExtension(openFileDialog1.FileName) == ".txt" || Path.GetExtension(openFileDialog1.FileName) == ".csv")

Why? You already put openFileDialog1.FileName in a fileName variable. Just use that.

The next thing I would do is define a list of valid extensions:

public readonly string[] ValidExtensions => new string[] { "txt", "csv" }

Then simply:

if (result == DialogResult.OK && ValidExtensions.Contains(Path.GetExtension(fileName).Substring(1)))

We use the .Substring(1) to remove the period.

That way, if you add support for a different file type, you just add it to ValidExtensions.

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