winFormsJsonSave.cs

public static void WriteToFile(string writeData, string fileExtension)
{
    string defaultExtension, extensionFilter;
    switch (fileExtension)
    {
        case "json":
            defaultExtension = "json";
            extensionFilter = "json files (*.json)|*.json";
            break;
        default:
            defaultExtension = "txt";
            extensionFilter = "text files (*.txt)|*.txt";
            break;
    }

    System.Windows.Forms.SaveFileDialog saveFileDialog = new System.Windows.Forms.SaveFileDialog
    {
        Title = "Select Destination File",
        DefaultExt = defaultExtension,
        Filter = extensionFilter
    };

    if (saveFileDialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
    {
        string fileName = saveFileDialog.FileName;
        using (System.IO.StreamWriter file = new System.IO.StreamWriter(fileName))
        {
            file.WriteLine(writeData);
        }
    }
}