diff --git a/src/ACadSharp.Tests/IO/IOTests.cs b/src/ACadSharp.Tests/IO/IOTests.cs
index 515c44ad..00aec809 100644
--- a/src/ACadSharp.Tests/IO/IOTests.cs
+++ b/src/ACadSharp.Tests/IO/IOTests.cs
@@ -3,6 +3,7 @@
 using ACadSharp.Tests.TestModels;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -100,6 +101,11 @@ public void DwgEntitiesToDxfFile(FileModel test)
 			List<Entity> entities = new List<Entity>(doc.Entities);
 			foreach (var item in entities)
 			{
+				if (item.ExtendedData.Any())
+				{
+
+				}
+
 				Entity e = doc.Entities.Remove(item);
 				transfer.Entities.Add(e);
 			}
diff --git a/src/ACadSharp.Tests/XData/ExtendedDataTests.cs b/src/ACadSharp.Tests/XData/ExtendedDataTests.cs
new file mode 100644
index 00000000..7483a8f1
--- /dev/null
+++ b/src/ACadSharp.Tests/XData/ExtendedDataTests.cs
@@ -0,0 +1,35 @@
+using ACadSharp.Entities;
+using ACadSharp.Tables;
+using ACadSharp.XData;
+using System.Linq;
+using Xunit;
+
+namespace ACadSharp.Tests.XData
+{
+	public class ExtendedDataTests
+	{
+		[Fact]
+		public void AddObjectWithXData()
+		{
+			CadDocument doc = new CadDocument();
+
+			string appName = "my_custom_app";
+			AppId app = new AppId(appName);
+			Line line = new Line();
+			ExtendedData extendedData = new ExtendedData();
+			ExtendedDataRecord extendedDataRecord = new ExtendedDataString("extended data record");
+			extendedData.Records.Add(extendedDataRecord);
+
+			line.ExtendedData.Add(app, extendedData);
+
+			doc.Entities.Add(line);
+
+			Assert.Contains(app, doc.AppIds);
+			Assert.False(app.Handle == 0);
+
+			Assert.NotEmpty(line.ExtendedData);
+			Assert.NotEmpty(line.ExtendedData.Get(appName).Records);
+			Assert.Equal(extendedDataRecord, line.ExtendedData.Get(appName).Records.First());
+		}
+	}
+}
diff --git a/src/ACadSharp/CadObject.cs b/src/ACadSharp/CadObject.cs
index b5a4335b..8d7ec842 100644
--- a/src/ACadSharp/CadObject.cs
+++ b/src/ACadSharp/CadObject.cs
@@ -7,6 +7,7 @@
 using System;
 using ACadSharp.XData;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace ACadSharp
 {
@@ -143,7 +144,21 @@ internal virtual void AssignDocument(CadDocument doc)
 			this.Document = doc;
 
 			if (this.XDictionary != null)
+			{
 				doc.RegisterCollection(this.XDictionary);
+			}
+
+			if (this.ExtendedData.Any())
+			{
+				//Reset existing collection
+				var entries = this.ExtendedData.ToArray();
+				this.ExtendedData.Clear();
+
+				foreach (var item in entries)
+				{
+					this.ExtendedData.Add(item.Key, item.Value);
+				}
+			}
 		}
 
 		internal virtual void UnassignDocument()
@@ -153,6 +168,18 @@ internal virtual void UnassignDocument()
 
 			this.Handle = 0;
 			this.Document = null;
+
+			if (this.ExtendedData.Any())
+			{
+				//Reset existing collection
+				var entries = this.ExtendedData.ToArray();
+				this.ExtendedData.Clear();
+
+				foreach (var item in entries)
+				{
+					this.ExtendedData.Add(item.Key.Clone() as AppId, item.Value);
+				}
+			}
 		}
 
 		protected T updateTable<T>(T entry, Table<T> table)
diff --git a/src/ACadSharp/IO/DWG/DwgStreamWriters/DwgObjectWriter.Common.cs b/src/ACadSharp/IO/DWG/DwgStreamWriters/DwgObjectWriter.Common.cs
index 0b9d83c3..690ef0fc 100644
--- a/src/ACadSharp/IO/DWG/DwgStreamWriters/DwgObjectWriter.Common.cs
+++ b/src/ACadSharp/IO/DWG/DwgStreamWriters/DwgObjectWriter.Common.cs
@@ -334,7 +334,7 @@ private void writeExtendedData(ExtendedDataDictionary data)
 			if (this.WriteXData)
 			{
 				//EED size BS size of extended entity data, if any
-				foreach (var item in data.Entries)
+				foreach (var item in data)
 				{
 					writeExtendedDataEntry(item.Key, item.Value);
 				}
diff --git a/src/ACadSharp/IO/DXF/DxfStreamWriter/DxfSectionWriterBase.cs b/src/ACadSharp/IO/DXF/DxfStreamWriter/DxfSectionWriterBase.cs
index bd877292..aa76c6b0 100644
--- a/src/ACadSharp/IO/DXF/DxfStreamWriter/DxfSectionWriterBase.cs
+++ b/src/ACadSharp/IO/DXF/DxfStreamWriter/DxfSectionWriterBase.cs
@@ -95,7 +95,7 @@ protected void writeExtendedData(ExtendedDataDictionary xdata)
 				return;
 			}
 
-			foreach (var entry in xdata.Entries)
+			foreach (var entry in xdata)
 			{
 				this._writer.Write(DxfCode.ExtendedDataRegAppName, entry.Key.Name);
 
diff --git a/src/ACadSharp/XData/ExtendedData.cs b/src/ACadSharp/XData/ExtendedData.cs
index 8c4ac785..33ff5816 100644
--- a/src/ACadSharp/XData/ExtendedData.cs
+++ b/src/ACadSharp/XData/ExtendedData.cs
@@ -1,22 +1,25 @@
 using ACadSharp.Tables;
+using System;
 using System.Collections.Generic;
 
 namespace ACadSharp.XData
 {
 	public class ExtendedData
 	{
-		public AppId AppId { get; }
+		public List<ExtendedDataRecord> Records { get; } = new();
 
-		public List<ExtendedDataRecord> Records { get; } = new List<ExtendedDataRecord>();
-
-		public ExtendedData(AppId app)
+		public ExtendedData()
 		{
-			this.AppId = app;
 		}
 
-		public ExtendedData(AppId app, IEnumerable<ExtendedDataRecord> records) : this(app)
+		public ExtendedData(IEnumerable<ExtendedDataRecord> records) : this()
 		{
 			this.Records.AddRange(records);
 		}
+
+		public void AddControlStrings()
+		{
+			throw new NotImplementedException();
+		}
 	}
 }
diff --git a/src/ACadSharp/XData/ExtendedDataCollection.cs b/src/ACadSharp/XData/ExtendedDataCollection.cs
index a1b298dc..9b6d0a5f 100644
--- a/src/ACadSharp/XData/ExtendedDataCollection.cs
+++ b/src/ACadSharp/XData/ExtendedDataCollection.cs
@@ -1,14 +1,13 @@
 using ACadSharp.Tables;
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 namespace ACadSharp.XData
 {
-	public class ExtendedDataDictionary
+	public class ExtendedDataDictionary : IEnumerable<KeyValuePair<AppId, ExtendedData>>
 	{
-		public IEnumerable<KeyValuePair<AppId, ExtendedData>> Entries { get { return this._data; } }
-
 		public CadObject Owner { get; }
 
 		private Dictionary<AppId, ExtendedData> _data = new Dictionary<AppId, ExtendedData>();
@@ -20,8 +19,29 @@ public ExtendedDataDictionary(CadObject owner)
 
 		public void Add(AppId app)
 		{
-			this._data.Add(app, new ExtendedData(app));
+			this.Add(app, new ExtendedData());
+		}
+
+		public void Add(AppId app, ExtendedData extendedData)
+		{
+			if (this.Owner.Document != null)
+			{
+				if (this.Owner.Document.AppIds.TryGetValue(app.Name, out AppId existing))
+				{
+					this._data.Add(existing, extendedData);
+				}
+				else
+				{
+					this.Owner.Document.AppIds.Add(app);
+					this._data.Add(app, extendedData);
+				}
+			}
+			else
+			{
+				this._data.Add(app, extendedData);
+			}
 		}
+
 		/// <summary>
 		/// Add ExtendedData for a specific AppId to the Dictionary.
 		/// </summary>
@@ -29,7 +49,21 @@ public void Add(AppId app)
 		/// <param name="records">The ExtendedData records.</param>
 		public void Add(AppId app, IEnumerable<ExtendedDataRecord> records)
 		{
-			this._data.Add(app, new ExtendedData(app, records));
+			this._data.Add(app, new ExtendedData(records));
+		}
+
+		/// <summary>
+		/// Get the different extended data by it's name.
+		/// </summary>
+		/// <returns></returns>
+		public IDictionary<string, ExtendedData> GetExtendedDataByName()
+		{
+			return this._data.ToDictionary(x => x.Key.Name, x => x.Value);
+		}
+
+		public ExtendedData Get(string name)
+		{
+			return this.GetExtendedDataByName()[name];
 		}
 
 		/// <summary>
@@ -85,5 +119,15 @@ public void Clear()
 		{
 			this._data.Clear();
 		}
+
+		public IEnumerator<KeyValuePair<AppId, ExtendedData>> GetEnumerator()
+		{
+			return this._data.GetEnumerator();
+		}
+
+		IEnumerator IEnumerable.GetEnumerator()
+		{
+			return this._data.GetEnumerator();
+		}
 	}
 }