use pattern matching in casts

src/Microsoft.OpenApi.Readers/ParseNodes/JsonPointerExtensions.cs

@@ -26,16 +26,13 @@ public static YamlNode Find(this JsonPointer currentPointer, YamlNode baseYamlNo
                 var pointer = baseYamlNode;
                 foreach (var token in currentPointer.Tokens)
                 {
-                    var sequence = pointer as YamlSequenceNode;
-
-                    if (sequence != null)
+                    if (pointer is YamlSequenceNode sequence)
                     {
                         pointer = sequence.Children[Convert.ToInt32(token)];
                     }
                     else
                     {
-                        var map = pointer as YamlMappingNode;
-                        if (map != null)
+                        if (pointer is YamlMappingNode map)
                         {
                             if (!map.Children.TryGetValue(new YamlScalarNode(token), out pointer))
                             {

src/Microsoft.OpenApi.Readers/ParseNodes/MapNode.cs

@@ -152,8 +152,7 @@ public override Dictionary<string, T> CreateSimpleMap<T>(Func<ValueNode, T> map)
                     try
                     {
                         Context.StartObject(key);
-                        YamlScalarNode scalarNode = n.Value as YamlScalarNode;
-                        if (scalarNode == null)
+                        if (n.Value is not YamlScalarNode scalarNode)
                         {
                             throw new OpenApiReaderException($"Expected scalar while parsing {typeof(T).Name}", Context);
                         }
@@ -205,8 +204,7 @@ public string GetReferencePointer()
 
         public string GetScalarValue(ValueNode key)
         {
-            var scalarNode = _node.Children[new YamlScalarNode(key.GetScalarValue())] as YamlScalarNode;
-            if (scalarNode == null)
+            if (_node.Children[new YamlScalarNode(key.GetScalarValue())] is not YamlScalarNode scalarNode)
             {
                 throw new OpenApiReaderException($"Expected scalar at line {_node.Start.Line} for key {key.GetScalarValue()}", Context);
             }

src/Microsoft.OpenApi.Readers/YamlHelper.cs

@@ -12,8 +12,7 @@ internal static class YamlHelper
     {
         public static string GetScalarValue(this YamlNode node)
         {
-            var scalarNode = node as YamlScalarNode;
-            if (scalarNode == null)
+            if (node is not YamlScalarNode scalarNode)
             {
                 throw new OpenApiException($"Expected scalar at line {node.Start.Line}");
             }

src/Microsoft.OpenApi/Validations/OpenApiValidator.cs

@@ -311,8 +311,7 @@ private void Validate(object item, Type type)
             }
 
             // Validate unresolved references as references
-            var potentialReference = item as IOpenApiReferenceable;
-            if (potentialReference != null && potentialReference.UnresolvedReference)
+            if (item is IOpenApiReferenceable potentialReference && potentialReference.UnresolvedReference)
             {
                 type = typeof(IOpenApiReferenceable);
             }

src/Microsoft.OpenApi/Validations/ValidationRuleSet.cs

@@ -176,8 +176,7 @@ private static ValidationRuleSet BuildDefaultRuleSet()
             foreach (var property in rules)
             {
                 var propertyValue = property.GetValue(null); // static property
-                ValidationRule rule = propertyValue as ValidationRule;
-                if (rule != null)
+                if (propertyValue is ValidationRule rule)
                 {
                     ruleSet.Add(rule);
                 }

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -142,8 +142,7 @@ public static void WriteOptionalObject<T>(
         {
             if (value != null)
             {
-                var values = value as IEnumerable;
-                if (values != null && !values.GetEnumerator().MoveNext())
+                if (value is IEnumerable values && !values.GetEnumerator().MoveNext())
                 {
                     return; // Don't render optional empty collections
                 }

test/Microsoft.OpenApi.SmokeTests/ApiGurus.cs

@@ -45,8 +45,7 @@ public static IEnumerable<object[]> GetSchemas()
             var json = JObject.Parse(listJsonStr);
             foreach (var item in json.Properties())
             {
-                var versions = GetProp(item.Value, "versions") as JObject;
-                if (versions == null)
+                if (GetProp(item.Value, "versions") is not JObject versions)
                     continue;
                 foreach (var prop in versions.Properties())
                 {