Make SqlNodeCloneContext readonly struct

Author: SergeiPavlov

Orm/Xtensive.Orm/Sql/Dml/Expressions/SqlExpression.cs

@@ -231,7 +231,7 @@ public static implicit operator SqlExpression(SqlSelect select)
 
     public sealed override bool Equals(object obj) => ReferenceEquals(this, obj);
 
-    public override SqlExpression Clone() => Clone(new SqlNodeCloneContext());
+    public override SqlExpression Clone() => Clone(new SqlNodeCloneContext(false));
 
     internal override abstract SqlExpression Clone(SqlNodeCloneContext context);
 

Orm/Xtensive.Orm/Sql/Internals/SqlNodeCloneContext.cs

@@ -6,9 +6,10 @@
 
 namespace Xtensive.Sql
 {
-  internal class SqlNodeCloneContext
+  internal readonly struct SqlNodeCloneContext
   {
-    public Dictionary<SqlNode, SqlNode> NodeMapping { get; } = new();
+    private readonly Dictionary<SqlNode, SqlNode> nodeMapping;
+    public Dictionary<SqlNode, SqlNode> NodeMapping => nodeMapping;
 
     public T TryGet<T>(T node) where T : SqlNode =>
       NodeMapping.TryGetValue(node, out var clone)
@@ -20,5 +21,10 @@ public T Add<T>(T node, T clone) where T : SqlNode
       NodeMapping[node] = clone;
       return clone;
     }
+
+    public SqlNodeCloneContext(bool _)
+    {
+      nodeMapping = new();
+    }
   }
 }

Orm/Xtensive.Orm/Sql/SqlNode.cs

@@ -24,7 +24,7 @@ public abstract class SqlNode : ISqlNode
     /// <returns>
     /// A new object that is a copy of this instance.
     /// </returns>
-    public virtual object Clone() => Clone(new SqlNodeCloneContext());
+    public virtual object Clone() => Clone(new SqlNodeCloneContext(false));
 
     internal abstract object Clone(SqlNodeCloneContext context);