Ensure Clear(T) also have a safety check to ensure no listener is attached.

Move related methods next to each other.
Change Clear(T) to use Array.Fill.

OpenRA.Game/Map/CellLayer.cs

@@ -25,24 +25,33 @@ namespace OpenRA
 		public CellLayer(MapGridType gridType, Size size)
 			: base(gridType, size) { }
 
-		public override void Clear()
-		{
-			if (CellEntryChanged != null)
-				throw new InvalidOperationException(
-					"Cannot clear values when there are listeners attached to the CellEntryChanged event.");
-
-			base.Clear();
-		}
-
 		public override void CopyValuesFrom(CellLayerBase<T> anotherLayer)
 		{
 			if (CellEntryChanged != null)
 				throw new InvalidOperationException(
-					"Cannot copy values when there are listeners attached to the CellEntryChanged event.");
+					$"Cannot copy values when there are listeners attached to the {nameof(CellEntryChanged)} event.");
 
 			base.CopyValuesFrom(anotherLayer);
 		}
 
+		public override void Clear()
+		{
+			if (CellEntryChanged != null)
+				throw new InvalidOperationException(
+					$"Cannot clear values when there are listeners attached to the {nameof(CellEntryChanged)} event.");
+
+			base.Clear();
+		}
+
+		public override void Clear(T clearValue)
+		{
+			if (CellEntryChanged != null)
+				throw new InvalidOperationException(
+					$"Cannot clear values when there are listeners attached to the {nameof(CellEntryChanged)} event.");
+
+			base.Clear(clearValue);
+		}
+
 		// Resolve an array index from cell coordinates
 		int Index(CPos cell)
 		{

OpenRA.Game/Map/CellLayerBase.cs

@@ -35,11 +35,6 @@ namespace OpenRA
 			entries = new T[size.Width * size.Height];
 		}
 
-		public virtual void Clear()
-		{
-			Array.Clear(entries, 0, entries.Length);
-		}
-
 		public virtual void CopyValuesFrom(CellLayerBase<T> anotherLayer)
 		{
 			if (Size != anotherLayer.Size || GridType != anotherLayer.GridType)
@@ -48,11 +43,16 @@ namespace OpenRA
 			Array.Copy(anotherLayer.entries, entries, entries.Length);
 		}
 
-		/// <summary>Clears the layer contents with a known value</summary>
-		public void Clear(T clearValue)
+		/// <summary>Clears the layer contents with their default value</summary>
+		public virtual void Clear()
 		{
-			for (var i = 0; i < entries.Length; i++)
-				entries[i] = clearValue;
+			Array.Clear(entries, 0, entries.Length);
+		}
+
+		/// <summary>Clears the layer contents with a known value</summary>
+		public virtual void Clear(T clearValue)
+		{
+			Array.Fill(entries, clearValue);
 		}
 
 		public IEnumerator<T> GetEnumerator()

OpenRA.Game/Map/Map.cs

@@ -309,6 +309,7 @@ namespace OpenRA
 			Resources = new CellLayer<ResourceTile>(Grid.Type, size);
 			Height = new CellLayer<byte>(Grid.Type, size);
 			Ramp = new CellLayer<byte>(Grid.Type, size);
+			Tiles.Clear(terrainInfo.DefaultTerrainTile);
 			if (Grid.MaximumTerrainHeight > 0)
 			{
 				Height.CellEntryChanged += UpdateProjection;
@@ -316,8 +317,6 @@ namespace OpenRA
 				Tiles.CellEntryChanged += UpdateRamp;
 			}
 
-			Tiles.Clear(terrainInfo.DefaultTerrainTile);
-
 			PostInit();
 		}