fix: add meaningful exception message during validation

src/Microsoft.OpenApi.Readers/OpenApiYamlReader.cs

@@ -11,6 +11,7 @@
 using SharpYaml.Serialization;
 using Microsoft.OpenApi.Models;
 using System;
+using System.Linq;
 using System.Text;
 
 namespace Microsoft.OpenApi.Readers
@@ -123,8 +124,12 @@ static JsonNode LoadJsonNodesFromYamlDocument(TextReader input)
         {
             var yamlStream = new YamlStream();
             yamlStream.Load(input);
-            var yamlDocument = yamlStream.Documents[0];
-            return yamlDocument.ToJsonNode();
+            if (yamlStream.Documents.Any())
+            {
+                return yamlStream.Documents[0].ToJsonNode();
+            }
+
+            throw new InvalidOperationException("No documents found in the YAML stream.");
         }
     }
 }

src/Microsoft.OpenApi/Reader/OpenApiModelFactory.cs

@@ -278,10 +278,13 @@
             {
                 throw new InvalidOperationException("Loading external references are not supported when using synchronous methods.");
             }
+            if (input.Length == 0 || input.Position == input.Length)
+            {
+                throw new ArgumentException($"Cannot parse the stream: {nameof(input)} is empty or contains no elements.");
+            }
 
             var reader = OpenApiReaderRegistry.GetReader(format);
             var readResult = reader.Read(input, settings);
-
             return readResult;
         }
 
@@ -298,7 +301,7 @@
                     var response = await _httpClient.GetAsync(url, token).ConfigureAwait(false);
                     var mediaType = response.Content.Headers.ContentType.MediaType;
                     var contentType = mediaType.Split(";".ToCharArray(), StringSplitOptions.RemoveEmptyEntries)[0];
                     format = contentType.Split('/').Last().Split('+').Last().Split('-').Last();
                     // for non-standard MIME types e.g. text/x-yaml used in older libs or apps
 #if NETSTANDARD2_0
                     stream = await response.Content.ReadAsStreamAsync();

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDocumentTests.cs

@@ -11,8 +11,8 @@
 using Microsoft.OpenApi.Writers;
 using Xunit;
 using VerifyXunit;
-using VerifyTests;
 using Microsoft.OpenApi.Models.Interfaces;
+using System;
 
 namespace Microsoft.OpenApi.Readers.Tests.V31Tests
 {
@@ -539,5 +539,11 @@ public async Task ParseDocumentWith31PropertiesWorks()
             // Assert
             await Verifier.Verify(actual);
         }
+
+        [Fact]
+        public void ParseEmptyMemoryStreamThrowsAnArgumentException()
+        {
+            Assert.Throws<ArgumentException>(() => OpenApiDocument.Load(new MemoryStream()));
+        }
     }
 }