Fix ColumnInfoRef bug with implicint convertion from null

Author: SergeiPavlov

Orm/Xtensive.Orm/Comparison/ComparisonRule.cs

@@ -16,7 +16,7 @@ namespace Xtensive.Comparison
   /// Describes how to compare values of comparable objects.
   /// </summary>
   [Serializable]
-  public struct ComparisonRule :
+  public readonly struct ComparisonRule :
     IEquatable<ComparisonRule>,
     ISerializable
   {
@@ -75,12 +75,8 @@ public ComparisonRule Combine(ComparisonRule rule)
     #region Equals, GetHashCode
 
     /// <inheritdoc/>
-    public bool Equals(ComparisonRule other)
-    {
-      if (Direction != other.Direction)
-        return false;
-      return Equals(Culture, other.Culture);
-    }
+    public bool Equals(ComparisonRule other) =>
+      Direction == other.Direction && Equals(Culture, other.Culture);
 
     /// <inheritdoc/>
     public override bool Equals(object obj) =>

Orm/Xtensive.Orm/Orm/Model/ColumnInfoRef.cs

@@ -51,12 +51,6 @@ public ColumnInfo Resolve(DomainModel model) =>
           ? throw new InvalidOperationException(string.Format(Strings.ExCouldNotResolveXYWithinDomain, "field", FieldName))
           : field.Column ?? throw new InvalidOperationException(string.Format(Strings.ExCouldNotResolveXYWithinDomain, "column", ColumnName));
 
-    /// <summary>
-    /// Creates reference for <see cref="ColumnInfo"/>.
-    /// </summary>
-    public static implicit operator ColumnInfoRef(ColumnInfo columnInfo) => 
-      new ColumnInfoRef(columnInfo);
-
     #region Equality members, ==, !=
 
     /// <summary>

Orm/Xtensive.Orm/Orm/Rse/MappedColumn.cs

@@ -51,7 +51,7 @@ public override Column Clone(string newName)
     /// <param name="index"><see cref="Column.Index"/> property value.</param>
     /// <param name="type"><see cref="Column.Type"/> property value.</param>
     public MappedColumn(string name, int index, Type type)
-      : this(null, name, index, type)
+      : this(default, name, index, type)
     {
     }
 

Orm/Xtensive.Orm/Orm/Rse/Providers/Compilable/OrderProviderBase.cs

@@ -66,11 +66,10 @@ protected override void Initialize()
       var comparisonRules = new ComparisonRules[Order.Count];
       for (int i = 0; i < Order.Count; i++) {
         var orderItem = Order[i];
-        var culture = CultureInfo.InvariantCulture;
         var column = Header.Columns[orderItem.Key];
-        var mappedColumn = column as MappedColumn;
-        if (mappedColumn != null && mappedColumn.ColumnInfoRef != null)
-          culture = mappedColumn.ColumnInfoRef.CultureInfo;
+        var culture = column is MappedColumn mappedColumn && mappedColumn.ColumnInfoRef != default
+            ? mappedColumn.ColumnInfoRef.CultureInfo
+            : CultureInfo.InvariantCulture;
         comparisonRules[i] = new ComparisonRule(orderItem.Value, culture);
       }
 

Orm/Xtensive.Orm/Orm/Rse/RecordSetHeader.cs

@@ -251,7 +251,7 @@ private static RecordSetHeader CreateHeader(IndexInfo indexInfo)
         .ToArray(indexInfoKeyColumns.Count);
       var keyDescriptor = TupleDescriptor.Create(keyFieldTypes);
 
-      var resultColumns = indexInfoColumns.Select((c,i) => (Column) new MappedColumn(c,i,c.ValueType));
+      var resultColumns = indexInfoColumns.Select((c,i) => (Column) new MappedColumn(new ColumnInfoRef(c), i,c.ValueType));
       var resultGroups = new[]{indexInfo.Group};
 
       return new RecordSetHeader(