Simple file and folder picker for ImGui.Net

FilePicker.cs

using ImGuiNET;
using Microsoft.Xna.Framework;
using System;
using System.Collections.Generic;
using System.IO;
using Num = System.Numerics;

namespace Nez.ImGuiTools
{
	public class FilePicker
	{
		static readonly Dictionary<object, FilePicker> _filePickers = new Dictionary<object, FilePicker>();

		public string RootFolder;
		public string CurrentFolder;
		public string SelectedFile;
		public List<string> AllowedExtensions;
		public bool OnlyAllowFolders;

		public static FilePicker GetFolderPicker(object o, string startingPath)
			=> GetFilePicker(o, startingPath, null, true);

		public static FilePicker GetFilePicker(object o, string startingPath, string searchFilter = null, bool onlyAllowFolders = false)
		{
			if (File.Exists(startingPath))
			{
				startingPath = new FileInfo(startingPath).DirectoryName;
			}
			else if (string.IsNullOrEmpty(startingPath) || !Directory.Exists(startingPath))
			{
				startingPath = Environment.CurrentDirectory;
				if (string.IsNullOrEmpty(startingPath))
					startingPath = AppContext.BaseDirectory;
			}

			if (!_filePickers.TryGetValue(o, out FilePicker fp))
			{
				fp = new FilePicker();
				fp.RootFolder = startingPath;
				fp.CurrentFolder = startingPath;
				fp.OnlyAllowFolders = onlyAllowFolders;

				if (searchFilter != null)
				{
					if (fp.AllowedExtensions != null)
						fp.AllowedExtensions.Clear();
					else
						fp.AllowedExtensions = new List<string>();
					
					fp.AllowedExtensions.AddRange(searchFilter.Split(new char[] { '|' }, StringSplitOptions.RemoveEmptyEntries));
				}

				_filePickers.Add(o, fp);
			}

			return fp;
		}

		public static void RemoveFilePicker(object o) => _filePickers.Remove(o);

		public bool Draw()
		{
			ImGui.Text("Current Folder: " + Path.GetFileName(RootFolder) + CurrentFolder.Replace(RootFolder, ""));
			bool result = false;

			if (ImGui.BeginChildFrame(1, new Num.Vector2(400, 400)))
			{
				var di = new DirectoryInfo(CurrentFolder);
				if (di.Exists)
				{
					if (di.Parent != null && CurrentFolder != RootFolder)
					{
						ImGui.PushStyleColor(ImGuiCol.Text, Color.Yellow.PackedValue);
						if (ImGui.Selectable("../", false, ImGuiSelectableFlags.DontClosePopups))
							CurrentFolder = di.Parent.FullName;
						
						ImGui.PopStyleColor();
					}

					var fileSystemEntries = GetFileSystemEntries(di.FullName);
					foreach (var fse in fileSystemEntries)
					{
						if (Directory.Exists(fse))
						{
							var name = Path.GetFileName(fse);
							ImGui.PushStyleColor(ImGuiCol.Text, Color.Yellow.PackedValue);
							if (ImGui.Selectable(name + "/", false, ImGuiSelectableFlags.DontClosePopups))
								CurrentFolder = fse;
							ImGui.PopStyleColor();
						}
						else
						{
							var name = Path.GetFileName(fse);
							bool isSelected = SelectedFile == fse;
							if (ImGui.Selectable(name, isSelected, ImGuiSelectableFlags.DontClosePopups))
								SelectedFile = fse;

							if (ImGui.IsMouseDoubleClicked(0))
							{
								result = true;
								ImGui.CloseCurrentPopup();
							}
						}
					}
				}
			}
			ImGui.EndChildFrame();


			if (ImGui.Button("Cancel"))
			{
				result = false;
				ImGui.CloseCurrentPopup();
			}

			if (OnlyAllowFolders)
			{
				ImGui.SameLine();
				if (ImGui.Button("Open"))
				{
					result = true;
					SelectedFile = CurrentFolder;
					ImGui.CloseCurrentPopup();
				}
			}
			else if (SelectedFile != null)
			{
				ImGui.SameLine();
				if (ImGui.Button("Open"))
				{
					result = true;
					ImGui.CloseCurrentPopup();
				}
			}

			return result;
		}

		bool TryGetFileInfo(string fileName, out FileInfo realFile)
		{
			try
			{
				realFile = new FileInfo(fileName);
				return true;
			}
			catch
			{
				realFile = null;
				return false;
			}
		}

		List<string> GetFileSystemEntries(string fullName)
		{
			var files = new List<string>();
			var dirs = new List<string>();

			foreach (var fse in Directory.GetFileSystemEntries(fullName, ""))
			{
				if (Directory.Exists(fse))
				{
					dirs.Add(fse);
				}
				else if (!OnlyAllowFolders)
				{
					if (AllowedExtensions != null)
					{
						var ext = Path.GetExtension(fse);
						if (AllowedExtensions.Contains(ext))
							files.Add(fse);
					}
					else
					{
						files.Add(fse);
					}
				}
			}
			
			var ret = new List<string>(dirs);
			ret.AddRange(files);

			return ret;
		}
	
	}
}

FilePickerUsage.cs

			var isOpen = true;
			if (ImGui.BeginPopupModal("save-file", ref isOpen, ImGuiWindowFlags.NoTitleBar))
			{
				var picker = FilePicker.GetFolderPicker(this, Path.Combine(Environment.CurrentDirectory));
				if (picker.Draw())
				{
					Console.WriteLine(picker.SelectedFile);
					FilePicker.RemoveFilePicker(this);
				}
				ImGui.EndPopup();
			}

Very slow and inefficient, especially for directories with many entries.
With every draw() (i.e. frame) the directory is read in again, which massively reduces the framerate. It is better to read in the directory only once when creating and again when changing the directory.