mline nullable

ACadSharp/Entities/MLine.cs

@@ -34,7 +34,18 @@ public partial class MLine : Entity
 		/// Do not modify this field without also updating the associated entry in the MLINESTYLE dictionary
 		/// </remarks>
 		[DxfCodeValue(DxfReferenceType.Handle | DxfReferenceType.Name, 340)]
-		public MLStyle MLStyle { get; set; }
+		public MLStyle MLStyle
+		{
+			get { return _style; }
+			set
+			{
+				if (value == null)
+				{
+					throw new System.ArgumentNullException(nameof(value), "Multi line style cannot be null");
+				}
+				this._style = value;
+			}
+		}
 
 		/// <summary>
 		/// Scale factor
@@ -72,6 +83,8 @@ public partial class MLine : Entity
 		[DxfCodeValue(DxfReferenceType.Count, 72)]
 		public List<Vertex> Vertices { get; set; } = new List<Vertex>();
 
+		private MLStyle _style = MLStyle.Default;
+
 		public MLine() : base() { }
 
 		public override CadObject Clone()

ACadSharp/IO/DXF/DxfStreamWriter/DxfSectionWriterBase.Entities.cs

@@ -532,7 +532,11 @@ private void writeMLine(MLine mLine)
 			this._writer.Write(70, (short)mLine.Justification);
 			this._writer.Write(71, (short)mLine.Flags);
 			this._writer.Write(72, (short)mLine.Vertices.Count);
-			this._writer.Write(73, (short)mLine.MLStyle.Elements.Count);
+
+			if (mLine.MLStyle != null)
+			{
+				this._writer.Write(73, (short)mLine.MLStyle.Elements.Count);
+			}
 
 			this._writer.Write(10, mLine.StartPoint, map);
 

ACadSharp/Objects/MLStyle.cs

@@ -14,6 +14,16 @@ namespace ACadSharp.Objects
 	[DxfSubClass(DxfSubclassMarker.MLineStyle)]
 	public partial class MLStyle : CadObject, INamedCadObject
 	{
+		/// <summary>
+		/// Default multiline style name
+		/// </summary>
+		public const string DefaultName = "Standard";
+
+		/// <summary>
+		/// Gets the default MLine style
+		/// </summary>
+		public static MLStyle Default { get { return new MLStyle(DefaultName); } }
+
 		/// <inheritdoc/>
 		public override ObjectType ObjectType => ObjectType.MLINESTYLE;
 
@@ -67,5 +77,12 @@ public partial class MLStyle : CadObject, INamedCadObject
 		/// </summary>
 		[DxfCodeValue(DxfReferenceType.Count, 71)]
 		public List<MLStyle.Element> Elements { get; } = new List<Element>();
+
+		internal MLStyle() { }
+
+		public MLStyle(string name)
+		{
+			this.Name = name;
+		}
 	}
 }

ACadSharp/Tables/Layer.cs

@@ -16,12 +16,12 @@ namespace ACadSharp.Tables
 	public class Layer : TableEntry
 	{
 		/// <summary>
-		/// Default layer 0, it will always exist in a file.
+		/// Default layer 0, it will always exist in a file
 		/// </summary>
 		public const string DefaultName = "0";
 
 		/// <summary>
-		/// Default layer in all cad formats, it will always exist in a file.
+		/// Default layer in all cad formats, it will always exist in a file
 		/// </summary>
 		public static Layer Default { get { return new Layer(DefaultName); } }