V2 Serializer: Map custom type as string for parameter

Author: Shwetap05

src/Microsoft.OpenApi/Models/OpenApiParameter.cs

@@ -257,6 +257,12 @@ public void SerializeAsV2WithoutReference(IOpenApiWriter writer)
             {
                 writer.WriteOptionalObject(OpenApiConstants.Schema, Schema, (w, s) => s.SerializeAsV2(w));
             }
+            // In V2 parameter's type can't be a reference to a custom object schema or can't be of type object
+            // So in that case map the type as string.
+            else if (Schema?.Reference?.Type == ReferenceType.Schema || Schema?.Type == "object")
+            {
+                writer.WriteProperty(OpenApiConstants.Type, "string");
+            }
             else
             {
                 // type

test/Microsoft.OpenApi.Tests/Models/OpenApiParameterTests.cs

@@ -57,6 +57,58 @@ public class OpenApiParameterTests
             }
         };
 
+        public static OpenApiParameter AdvancedHeaderParameterWithSchemaReference = new OpenApiParameter
+        {
+            Name = "name1",
+            In = ParameterLocation.Header,
+            Description = "description1",
+            Required = true,
+            Deprecated = false,
+
+            Style = ParameterStyle.Simple,
+            Explode = true,
+            Schema = new OpenApiSchema
+            {
+                Reference = new OpenApiReference
+                {
+                    Type = ReferenceType.Schema,
+                    Id = "schemaObject1"
+                }
+            },
+            Examples = new List<OpenApiExample>
+            {
+                new OpenApiExample
+                {
+                    Summary = "summary3",
+                    Description = "description3"
+                }
+            }
+        };
+
+        public static OpenApiParameter AdvancedHeaderParameterWithSchemaTypeObject = new OpenApiParameter
+        {
+            Name = "name1",
+            In = ParameterLocation.Header,
+            Description = "description1",
+            Required = true,
+            Deprecated = false,
+
+            Style = ParameterStyle.Simple,
+            Explode = true,
+            Schema = new OpenApiSchema
+            {
+                Type = "object"
+            },
+            Examples = new List<OpenApiExample>
+            {
+                new OpenApiExample
+                {
+                    Summary = "summary3",
+                    Description = "description3"
+                }
+            }
+        };
+
         private readonly ITestOutputHelper _output;
 
         public OpenApiParameterTests(ITestOutputHelper output)
@@ -203,5 +255,57 @@ public void SerializeReferencedParameterAsV2JsonWithoutReferenceWorks()
             expected = expected.MakeLineBreaksEnvironmentNeutral();
             actual.Should().Be(expected);
         }
+
+        [Fact]
+        public void SerializeParameterWithSchemaReferenceAsV2JsonWorks()
+        {
+            // Arrange
+            var outputStringWriter = new StringWriter();
+            var writer = new OpenApiJsonWriter(outputStringWriter);
+            var expected =
+                @"{
+  ""in"": ""header"",
+  ""name"": ""name1"",
+  ""description"": ""description1"",
+  ""required"": true,
+  ""type"": ""string""
+}";
+
+            // Act
+            AdvancedHeaderParameterWithSchemaReference.SerializeAsV2(writer);
+            writer.Flush();
+            var actual = outputStringWriter.GetStringBuilder().ToString();
+
+            // Assert
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
+        }
+
+        [Fact]
+        public void SerializeParameterWithSchemaTypeObjectAsV2JsonWorks()
+        {
+            // Arrange
+            var outputStringWriter = new StringWriter();
+            var writer = new OpenApiJsonWriter(outputStringWriter);
+            var expected =
+                @"{
+  ""in"": ""header"",
+  ""name"": ""name1"",
+  ""description"": ""description1"",
+  ""required"": true,
+  ""type"": ""string""
+}";
+
+            // Act
+            AdvancedHeaderParameterWithSchemaTypeObject.SerializeAsV2(writer);
+            writer.Flush();
+            var actual = outputStringWriter.GetStringBuilder().ToString();
+
+            // Assert
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
+        }
     }
 }