A diagram can be saved to an XML file and then loaded from this file at design time and runtime.
At design time, use the dedicated Save, Save As and Open menu buttons provided by the Diagram Designer Control.
In code, a diagram can be saved/loaded to/from a file or stream with the DiagramControl.SaveDocument and DiagramControl.LoadDocument methods.
You can implement custom logic for loading and saving diagrams by handling the DiagramControl.CustomSaveDocument and DiagramControl.CustomLoadDocument events.
The DiagramContentItem.Content property that defines items with custom content is serialized by default, provided that its content supports serialization. Simple types are supported out of the box. Custom types should be marked with the SerializableAttribute. See the Value types article in MSDN.
To enable serialization for custom item types, use the DevExpress.Diagram.Core.DiagramItemTypeRegistrator.Register method. To modify the list of serialized properties, use the DiagramControl.CustomGetSerializableItemProperties event. Alternatively, mark the additional properties of a custom item type with the DevExpress.Utils.Serializing.XtraSerializableProperty attribute.