Fix Initialization of StreamReader in OpenApiStreamReader throws an ArgumentNullException

src/Microsoft.OpenApi.Readers/OpenApiStreamReader.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.IO;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.OpenApi.Interfaces;
@@ -41,7 +42,7 @@ public OpenApiStreamReader(OpenApiReaderSettings settings = null)
         /// <returns>Instance of newly created OpenApiDocument.</returns>
         public OpenApiDocument Read(Stream input, out OpenApiDiagnostic diagnostic)
         {
-            using var reader = new StreamReader(input, default, true, -1, _settings.LeaveStreamOpen);
+            using var reader = new StreamReader(input, Encoding.UTF8, true, 4096, _settings.LeaveStreamOpen);
             return new OpenApiTextReaderReader(_settings).Read(reader, out diagnostic);
         }
 
@@ -54,6 +55,7 @@ public OpenApiDocument Read(Stream input, out OpenApiDiagnostic diagnostic)
         public async Task<ReadResult> ReadAsync(Stream input, CancellationToken cancellationToken = default)
         {
             MemoryStream bufferedStream;
+            int bufferSize = 4096;
             if (input is MemoryStream stream)
             {
                 bufferedStream = stream;
@@ -63,11 +65,12 @@ public async Task<ReadResult> ReadAsync(Stream input, CancellationToken cancella
                 // Buffer stream so that OpenApiTextReaderReader can process it synchronously
                 // YamlDocument doesn't support async reading.
                 bufferedStream = new();
-                await input.CopyToAsync(bufferedStream, 81920, cancellationToken);
+                bufferSize = 81920;
+                await input.CopyToAsync(bufferedStream, bufferSize, cancellationToken);
                 bufferedStream.Position = 0;
             }
 
-            using var reader = new StreamReader(bufferedStream, default, true, -1, _settings.LeaveStreamOpen);
+            using var reader = new StreamReader(bufferedStream, Encoding.UTF8, true, bufferSize, _settings.LeaveStreamOpen);
             return await new OpenApiTextReaderReader(_settings).ReadAsync(reader, cancellationToken);
         }
 
@@ -80,7 +83,7 @@ public async Task<ReadResult> ReadAsync(Stream input, CancellationToken cancella
         /// <returns>Instance of newly created OpenApiDocument</returns>
         public T ReadFragment<T>(Stream input, OpenApiSpecVersion version, out OpenApiDiagnostic diagnostic) where T : IOpenApiReferenceable
         {
-            using var reader = new StreamReader(input, default, true, -1, _settings.LeaveStreamOpen);
+            using var reader = new StreamReader(input, Encoding.UTF8, true, 4096, _settings.LeaveStreamOpen);
             return new OpenApiTextReaderReader(_settings).ReadFragment<T>(reader, version, out diagnostic);
         }
     }

test/Microsoft.OpenApi.Readers.Tests/OpenApiReaderTests/OpenApiStreamReaderTests.cs

@@ -1,7 +1,9 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using System.IO;
+using System.Net.Http;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -44,5 +46,20 @@ public async Task StreamShouldNotBeDisposedIfLeaveStreamOpenSettingIsTrue()
             stream.Seek(0, SeekOrigin.Begin); // does not throw an object disposed exception
             Assert.True(stream.CanRead);
         }
+
+        [Fact]
+        public async Task StreamShouldReadWhenInitialized()
+        {
+            var httpClient = new HttpClient
+            {
+                BaseAddress = new Uri("https://raw.githubusercontent.com/OAI/OpenAPI-Specification/")
+            };
+
+            var stream = await httpClient.GetStreamAsync("master/examples/v3.0/petstore.yaml");
+
+            // Read V3 as YAML
+            var openApiDocument = new OpenApiStreamReader().Read(stream, out var diagnostic);
+            Assert.NotNull(openApiDocument);
+        }
     }
 }