source: Common/Dialogs.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Windows.Forms;

namespace Common
{
    public class Dialogs
    {
        public const string AnyFile = "Any file";
        public const string AnyFileExt = "*";

        public static bool OpenFileDialog(string fileName, string[] fileTypes, string defaultExt, out string newFileName)
        {
            string filter = string.Join(@"|", fileTypes) + @"|" + AnyFile + @"|*." + AnyFileExt;
            OpenFileDialog openFileDialog = new OpenFileDialog { Filter = filter, DefaultExt = defaultExt };
            if (!string.IsNullOrEmpty(fileName))
            {
                openFileDialog.FileName = Path.GetFileName(fileName);
                openFileDialog.InitialDirectory = Path.GetDirectoryName(fileName);
            }

            if (openFileDialog.ShowDialog() == DialogResult.OK)
            {
                newFileName = openFileDialog.FileName;
                return true;
            }
            else
            {
                newFileName = null;
                return false;
            }
        }

        public static bool OpenFileDialogMulti(string fileName, string fileType, string fileExtension, out string[] newFileNames)
        {
            if (fileExtension.StartsWith(".", StringComparison.Ordinal)) fileExtension = fileExtension.Substring(1);
            OpenFileDialog openFileDialog = new OpenFileDialog { Filter = fileType + @"|*." + fileExtension + @"|" + AnyFile + @"|*." + AnyFileExt, DefaultExt = fileExtension };
            if (!string.IsNullOrEmpty(fileName))
            {
                openFileDialog.FileName = Path.GetFileName(fileName);
                openFileDialog.InitialDirectory = Path.GetDirectoryName(fileName);
            }

            openFileDialog.Multiselect = true;

            if (openFileDialog.ShowDialog() == DialogResult.OK)
            {
                newFileNames = openFileDialog.FileNames;
                return true;
            }
            else
            {
                newFileNames = null;
                return false;
            }
        }

        public static bool SaveFileDialog(string fileName, string fileType, string fileExtension, out string newFileName)
        {
            if (fileExtension.StartsWith(".", StringComparison.Ordinal)) fileExtension = fileExtension.Substring(1);
            SaveFileDialog saveFileDialog = new SaveFileDialog { Filter = fileType + @"|*." + fileExtension + @"|" + AnyFile + @"|*." + AnyFileExt, DefaultExt = fileExtension };
            if (!string.IsNullOrEmpty(fileName))
            {
                saveFileDialog.FileName = Path.GetFileName(fileName);
                saveFileDialog.InitialDirectory = Path.GetDirectoryName(fileName);
            }

            if (saveFileDialog.ShowDialog() == DialogResult.OK)
            {
                newFileName = saveFileDialog.FileName;
                return true;
            }
            else
            {
                newFileName = null;
                return false;
            }
        }
    }
}