diff --git a/src/Avalonia.Controls/SystemDialog.cs b/src/Avalonia.Controls/SystemDialog.cs
index e74b950f23..d81926ecc7 100644
--- a/src/Avalonia.Controls/SystemDialog.cs
+++ b/src/Avalonia.Controls/SystemDialog.cs
@@ -4,30 +4,65 @@ using System.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls.Platform;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Base class for system file dialogs.
+    /// </summary>
     public abstract class FileDialog : FileSystemDialog
     {
+        /// <summary>
+        /// Gets or sets a collection of filters which determine the types of files displayed in an
+        /// <see cref="OpenFileDialog"/> or an <see cref="SaveFileDialog"/>.
+        /// </summary>
         public List<FileDialogFilter> Filters { get; set; } = new List<FileDialogFilter>();
-        public string InitialFileName { get; set; }        
+
+        /// <summary>
+        /// Gets or sets initial file name that is displayed when the dialog is opened.
+        /// </summary>
+        public string? InitialFileName { get; set; }        
     }
 
+    /// <summary>
+    /// Base class for system file and directory dialogs.
+    /// </summary>
     public abstract class FileSystemDialog : SystemDialog
     {
         [Obsolete("Use Directory")]
-        public string InitialDirectory
+        public string? InitialDirectory
         {
             get => Directory;
             set => Directory = value;
         }
-        public string Directory { get; set; }
+
+        /// <summary>
+        /// Gets or sets the initial directory that will be displayed when the file system dialog
+        /// is opened.
+        /// </summary>
+        public string? Directory { get; set; }
     }
 
+    /// <summary>
+    /// Represents a system dialog that prompts the user to select a location for saving a file.
+    /// </summary>
     public class SaveFileDialog : FileDialog
     {
-        public string DefaultExtension { get; set; }
+        /// <summary>
+        /// Gets or sets the default extension to be used to save the file (including the period ".").
+        /// </summary>
+        public string? DefaultExtension { get; set; }
 
-        public async Task<string> ShowAsync(Window parent)
+        /// <summary>
+        /// Shows the save file dialog.
+        /// </summary>
+        /// <param name="parent">The parent window.</param>
+        /// <returns>
+        /// A task that on completion contains the full path of the save location, or null if the
+        /// dialog was canceled.
+        /// </returns>
+        public async Task<string?> ShowAsync(Window parent)
         {
             if(parent == null)
                 throw new ArgumentNullException(nameof(parent));
@@ -37,11 +72,25 @@ namespace Avalonia.Controls
         }
     }
 
+    /// <summary>
+    /// Represents a system dialog that allows the user to select one or more files to open.
+    /// </summary>
     public class OpenFileDialog : FileDialog
     {
+        /// <summary>
+        /// Gets or sets a value indicating whether the user can select multiple files.
+        /// </summary>
         public bool AllowMultiple { get; set; }
 
-        public Task<string[]> ShowAsync(Window parent)
+        /// <summary>
+        /// Shows the open file dialog.
+        /// </summary>
+        /// <param name="parent">The parent window.</param>
+        /// <returns>
+        /// A task that on completion returns an array containing the full path to the selected
+        /// files, or null if the dialog was canceled.
+        /// </returns>
+        public Task<string[]?> ShowAsync(Window parent)
         {
             if(parent == null)
                 throw new ArgumentNullException(nameof(parent));
@@ -49,15 +98,27 @@ namespace Avalonia.Controls
         }
     }
 
+    /// <summary>
+    /// Represents a system dialog that allows the user to select a directory.
+    /// </summary>
     public class OpenFolderDialog : FileSystemDialog
     {
         [Obsolete("Use Directory")]
-        public string DefaultDirectory
+        public string? DefaultDirectory
         {
             get => Directory;
             set => Directory = value;
         }
-        public Task<string> ShowAsync(Window parent)
+
+        /// <summary>
+        /// Shows the open folder dialog.
+        /// </summary>
+        /// <param name="parent">The parent window.</param>
+        /// <returns>
+        /// A task that on completion returns the full path of the selected directory, or null if the
+        /// dialog was canceled.
+        /// </returns>
+        public Task<string?> ShowAsync(Window parent)
         {
             if(parent == null)
                 throw new ArgumentNullException(nameof(parent));
@@ -65,14 +126,32 @@ namespace Avalonia.Controls
         }
     }
 
+
+    /// <summary>
+    /// Base class for system dialogs.
+    /// </summary>
     public abstract class SystemDialog
     {
-        public string Title { get; set; }
+        /// <summary>
+        /// Gets or sets the dialog title.
+        /// </summary>
+        public string? Title { get; set; }
     }
 
+    /// <summary>
+    /// Represents a filter in an <see cref="OpenFileDialog"/> or an <see cref="SaveFileDialog"/>.
+    /// </summary>
     public class FileDialogFilter
     {
-        public string Name { get; set; }
+        /// <summary>
+        /// Gets or sets the name of the filter, e.g. ("Text files (.txt)").
+        /// </summary>
+        public string? Name { get; set; }
+
+        /// <summary>
+        /// Gets or sets a list of file extensions matched by the filter (e.g. "txt" or "*" for all
+        /// files).
+        /// </summary>
         public List<string> Extensions { get; set; } = new List<string>();
     }
 }