Small fixes.

Author: mikasoukhov

BusinessEntities/ExchangeBoard.cs

@@ -180,19 +180,14 @@ protected override bool OnEquals(ExchangeBoard other)
 		return Code == other.Code && Exchange == other.Exchange;
 	}
 
-	private int _hashCode;
+	private int? _hashCode;
 
 	/// <summary>
 	/// Get the hash code of the object <see cref="ExchangeBoard"/>.
 	/// </summary>
 	/// <returns>A hash code.</returns>
 	public override int GetHashCode()
-	{
-		if (_hashCode == 0)
-			_hashCode = Code.GetHashCode() ^ (Exchange == null ? 0 : Exchange.GetHashCode());
-
-		return _hashCode;
-	}
+		=> _hashCode ??= Code.GetHashCode() ^ (Exchange == null ? 0 : Exchange.GetHashCode());
 
 	/// <summary>
 	/// Create a copy of <see cref="ExchangeBoard"/>.

BusinessEntities/SecurityExternalId.cs

@@ -238,9 +238,7 @@ public override string ToString()
 	/// <param name="other">Another value with which to compare.</param>
 	/// <returns><see langword="true" />, if the specified object is equal to the current object, otherwise, <see langword="false" />.</returns>
 	public override bool Equals(object other)
-	{
-		return Equals((SecurityExternalId)other);
-	}
+		=> other is SecurityExternalId typedOther && Equals(typedOther);
 
 	/// <inheritdoc />
 	public bool Equals(SecurityExternalId other)

Messages/SecurityIdMapping.cs

@@ -58,8 +58,8 @@ public override string ToString()
 	/// <param name="storage">Settings storage.</param>
 	public void Load(SettingsStorage storage)
 	{
-		StockSharpId = storage.GetValue<SettingsStorage>(nameof(StockSharpId)).Load<SecurityId>() ?? default;
-		AdapterId = storage.GetValue<SettingsStorage>(nameof(AdapterId)).Load<SecurityId>() ?? default;
+		StockSharpId = storage.GetValue<SettingsStorage>(nameof(StockSharpId)).Load<SecurityId>();
+		AdapterId = storage.GetValue<SettingsStorage>(nameof(AdapterId)).Load<SecurityId>();
 	}
 
 	/// <summary>