How do I only show CSV files when I open file dialog

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C#

WinForms

CSV

I'm trying to only display CSV files when I open the file dialog instead of CSV and XML. I have attempted to do this like this:

private void button_OpenImportFile_Click(object sender, EventArgs e)
        {            
            OpenFileDialog OpenFileDialog_Import = new OpenFileDialog();

            OpenFileDialog_Import.Filter = "CSV files (*.csv)|*.csv";

            if (openFileDialog_Import.ShowDialog() == DialogResult.OK)
            {
                if (Path.GetExtension(openFileDialog_Import.FileName) != ".csv")
                {
                    WoodstarHelpers.cGeneralHelpers.ShowErrorMessage("Only CSV files are permitted for this import type", "Invalid File Format");
                }
                else
                {
                    OpenFileData(openFileDialog_Import.FileName);
                }
            }                       
        }

This should check that they have selected a CSV however ideally i'd only like them to have the option to choose CSV.

What I have tried:

I've tried doing it with a using but that doesn't seem to do the trick either:

private void button_OpenImportFile_Click(object sender, EventArgs e)
        {            
            using (OpenFileDialog OpenFileDialog_Import = new OpenFileDialog())
            {
                OpenFileDialog_Import.Filter = "CSV files (*.csv)|*.csv";

                if (openFileDialog_Import.ShowDialog() == DialogResult.OK)
                {
                    if (Path.GetExtension(openFileDialog_Import.FileName) != ".csv")
                    {
                        WoodstarHelpers.cGeneralHelpers.ShowErrorMessage("Only CSV files are permitted for this import type", "Invalid File Format");
                    }
                    else
                    {
                        OpenFileData(openFileDialog_Import.FileName);
                    }
                }
            }                      
        }