use char overloads

src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs

@@ -133,7 +133,7 @@ private static string SingularizeAndDeduplicateOperationId(IList<string> operati
 
                 singularizedSegments.Add(segment);
             }
-            return string.Join(".", singularizedSegments);
+            return string.Join('.', singularizedSegments);
         }
 
         private static string RemoveHashSuffix(string operationId)
@@ -153,7 +153,7 @@ private static string RemoveKeyTypeSegment(string operationId, IList<OpenApiPara
                     segments.Remove(keyTypeExtension);
                 }
             }
-            return string.Join(".", segments);
+            return string.Join('.', segments);
         }
 
         private static IList<OpenApiParameter> ResolveFunctionParameters(IList<OpenApiParameter> parameters)

src/Microsoft.OpenApi.Readers/V2/OpenApiDocumentDeserializer.cs

@@ -213,7 +213,11 @@ private static string BuildUrl(string scheme, string host, string basePath)
 
             int? port = null;
 
-            if (!String.IsNullOrEmpty(host) && host.Contains(":"))
+#if NETSTANDARD2_1_OR_GREATER
+            if (!String.IsNullOrEmpty(host) && host.Contains(':', StringComparison.OrdinalIgnoreCase))
+#else
+            if (!String.IsNullOrEmpty(host) && host.Contains(':'))
+#endif
             {
                 var pieces = host.Split(':');
                 host = pieces.First();

test/Microsoft.OpenApi.SmokeTests/ApiGurus.cs

@@ -93,7 +93,7 @@ public async Task EnsureThatICouldParse(string url)
             if (diagnostic.Errors.Count > 0)
             {
                 _output.WriteLine($"Errors parsing {url}");
-                _output.WriteLine(String.Join("\n", diagnostic.Errors));
+                _output.WriteLine(String.Join('\n', diagnostic.Errors));
                 //               Assert.True(false);  // Uncomment to identify descriptions with errors.
             }
 

test/Microsoft.OpenApi.SmokeTests/GraphTests.cs

@@ -43,7 +43,7 @@ public GraphTests(ITestOutputHelper output)
             if (diagnostic.Errors.Count > 0)
             {
                 _output.WriteLine($"Errors parsing");
-                _output.WriteLine(String.Join("\n", diagnostic.Errors));
+                _output.WriteLine(String.Join('\n', diagnostic.Errors));
                 //               Assert.True(false);  // Uncomment to identify descriptions with errors.
             }
 

test/Microsoft.OpenApi.Tests/StringExtensions.cs

@@ -17,7 +17,7 @@ public static class StringExtensions
         public static string MakeLineBreaksEnvironmentNeutral(this string input)
         {
             return input.Replace("\r\n", "\n")
-                .Replace("\r", "\n")
+                .Replace('\r', '\n')
                 .Replace("\n", Environment.NewLine);
         }
     }