Update GetHashCode implementations to use HashCode.Combine

docs/csharp/programming-guide/concepts/covariance-contravariance/using-variance-in-interfaces-for-generic-collections.md

@@ -81,10 +81,7 @@ class PersonComparer : IEqualityComparer<Person>
     public int GetHashCode(Person person)  
     {  
         if (Object.ReferenceEquals(person, null)) return 0;  
-        int hashFirstName = person.FirstName == null  
-            ? 0 : person.FirstName.GetHashCode();  
-        int hashLastName = person.LastName.GetHashCode();  
-        return hashFirstName ^ hashLastName;  
+        return HashCode.Combine(person.FirstName, person.LastName);  
     }  
 }  
 

docs/fundamentals/runtime-libraries/snippets/System/Object/Equals/csharp/equals2.cs

@@ -30,7 +30,7 @@ public override bool Equals(Object obj)
 
     public override int GetHashCode()
     {
-        return (x << 2) ^ y;
+        return HashCode.Combine(x, y);
     }
 
     public override string ToString()
@@ -59,7 +59,7 @@ public override bool Equals(Object obj)
 
     public override int GetHashCode()
     {
-        return (base.GetHashCode() << 2) ^ z;
+        return HashCode.Combine(base.GetHashCode(), z);
     }
 
     public override String ToString()

docs/fundamentals/runtime-libraries/snippets/System/Object/Equals/fsharp/Project.fsproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="equals2.fs" />
+    <Compile Include="equals3.fs" />
+    <Compile Include="equals4.fs" />
+    <Compile Include="equals_ref.fs" />
+    <Compile Include="equals_val1.fs" />
+    <Compile Include="equals_val2.fs" />
+    <Compile Include="equalsoverride.fs" />
+    <Compile Include="equalssb1.fs" />
+  </ItemGroup>
+
+</Project>

docs/fundamentals/runtime-libraries/snippets/System/Object/Equals/fsharp/equals2.fs

@@ -15,7 +15,7 @@ type Point(x, y) =
             false
 
     override _.GetHashCode() =
-        (x <<< 2) ^^^ y
+        System.HashCode.Combine(x, y)
 
     override _.ToString() =
         $"Point({x}, {y})"
@@ -32,7 +32,7 @@ type Point3D(x, y, z) =
             false
 
     override _.GetHashCode() =
-        (base.GetHashCode() <<< 2) ^^^ z
+        System.HashCode.Combine(base.GetHashCode(), z)
 
     override _.ToString() =
         $"Point({x}, {y}, {z})"

docs/fundamentals/runtime-libraries/snippets/System/Object/Equals/vb/equals2.vb

@@ -23,7 +23,7 @@ Class Point1
     End Function
 
     Public Overrides Function GetHashCode() As Integer
-        Return (x << 2) Xor y
+        Return HashCode.Combine(x, y)
     End Function
 
     Public Overrides Function ToString() As String
@@ -49,7 +49,7 @@ Class Point3D : Inherits Point1
     End Function
 
     Public Overrides Function GetHashCode() As Integer
-        Return (MyBase.GetHashCode() << 2) Xor z
+        Return HashCode.Combine(MyBase.GetHashCode(), z)
     End Function
 
     Public Overrides Function ToString() As String

docs/fundamentals/runtime-libraries/snippets/System/Object/GetHashCode/csharp/xor2.cs

@@ -27,7 +27,7 @@ public override bool Equals(Object obj)
 
     public override int GetHashCode()
     {
-        return Tuple.Create(x, y).GetHashCode();
+        return HashCode.Combine(x, y);
     }
 }
 
@@ -42,7 +42,8 @@ public static void Main()
         Console.WriteLine(pt.GetHashCode());
    }
 }
-// The example displays the following output:
-//       173
-//       269
+// The example displays output similar to the following.
+// Note: HashCode.Combine results are not stable across .NET versions.
+//       185727722
+//       -363254492
 // </Snippet3>

docs/fundamentals/runtime-libraries/snippets/System/Object/GetHashCode/fsharp/Project.fsproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="direct1.fs" />
+    <Compile Include="shift1.fs" />
+    <Compile Include="xor1.fs" />
+    <Compile Include="xor2.fs" />
+  </ItemGroup>
+
+</Project>

docs/fundamentals/runtime-libraries/snippets/System/Object/GetHashCode/fsharp/xor2.fs

@@ -14,14 +14,15 @@ type Point(x: int, y: int) =
             false
 
     override _.GetHashCode() =
-        (x, y).GetHashCode()
+        System.HashCode.Combine(x, y)
 
 let pt = Point(5, 8)
 printfn $"{pt.GetHashCode()}"
 
 let pt2 = Point(8, 5)
 printfn $"{pt2.GetHashCode()}"
-// The example displays the following output:
-//       173
-//       269
+// The example displays output similar to the following.
+// Note: HashCode.Combine results are not stable across .NET versions.
+//       185727722
+//       -363254492
 // </Snippet3>

docs/fundamentals/runtime-libraries/snippets/System/Object/GetHashCode/vb/xor2.vb

@@ -16,7 +16,7 @@ Public Structure Point
     End Function
 
     Public Overrides Function GetHashCode() As Integer 
-        Return Tuple.Create(x, y).GetHashCode()
+        Return HashCode.Combine(x, y)
     End Function 
 End Structure 
 
@@ -29,7 +29,8 @@ Public Module Example
         Console.WriteLine(pt.GetHashCode())
     End Sub 
 End Module         
-' The example displays the following output:
-'       173
-'       269
+' The example displays output similar to the following.
+' Note: HashCode.Combine results are not stable across .NET versions.
+'       185727722
+'       -363254492
 ' </Snippet3>

docs/fundamentals/runtime-libraries/system-object-gethashcode.md

@@ -58,13 +58,13 @@ Frequently, a type has multiple data fields that can participate in generating t
 :::code language="fsharp" source="./snippets/System/Object/GetHashCode/fsharp/xor1.fs" id="Snippet2":::
 :::code language="vb" source="./snippets/System/Object/GetHashCode/vb/xor1.vb" id="Snippet2":::
 
-The previous example returns the same hash code for (n1, n2) and (n2, n1), and so may generate more collisions than are desirable. A number of solutions are available so that hash codes in these cases are not identical. One is to return the hash code of a `Tuple` object that reflects the order of each field. The following example shows a possible implementation that uses the <xref:System.Tuple`2> class. Note, though, that the performance overhead of instantiating a `Tuple` object may significantly impact the overall performance of an application that stores large numbers of objects in hash tables.
+The previous example returns the same hash code for (n1, n2) and (n2, n1), and so may generate more collisions than are desirable. On .NET 5+, the recommended solution is to use <xref:System.HashCode.Combine%2A?displayProperty=nameWithType>. It avoids the symmetry problem and produces a well-distributed hash code without the overhead of creating a `Tuple` object.
 
 :::code language="csharp" source="./snippets/System/Object/GetHashCode/csharp/xor2.cs" id="Snippet3":::
 :::code language="fsharp" source="./snippets/System/Object/GetHashCode/fsharp/xor2.fs" id="Snippet3":::
 :::code language="vb" source="./snippets/System/Object/GetHashCode/vb/xor2.vb" id="Snippet3":::
 
-A second alternative solution involves weighting the individual hash codes by left-shifting the hash codes of successive fields by two or more bits. Optimally, bits shifted beyond bit 31 should wrap around rather than be discarded. Since bits are discarded by the left-shift operators in both C# and Visual Basic, this requires creating a left shift-and-wrap method like the following:
+In .NET Framework, an alternative is to weight the individual hash codes by left-shifting the hash codes of successive fields by two or more bits. Optimally, bits shifted beyond bit 31 should wrap around rather than be discarded. Since bits are discarded by the left-shift operators in both C# and Visual Basic, this requires creating a left shift-and-wrap method like the following:
 
 :::code language="csharp" source="./snippets/System/Object/GetHashCode/csharp/shift1.cs" id="Snippet4":::
 :::code language="fsharp" source="./snippets/System/Object/GetHashCode/fsharp/shift1.fs" id="Snippet4":::

docs/visual-basic/programming-guide/concepts/covariance-contravariance/using-variance-in-interfaces-for-generic-collections.md

@@ -82,11 +82,7 @@ Class PersonComparer
         Implements IEqualityComparer(Of Person).GetHashCode
 
         If person Is Nothing Then Return 0
-        Dim hashFirstName =
-            If(person.FirstName Is Nothing,
-            0, person.FirstName.GetHashCode())
-        Dim hashLastName = person.LastName.GetHashCode()
-        Return hashFirstName Xor hashLastName
+        Return HashCode.Combine(person.FirstName, person.LastName)
     End Function
 End Class