Fix OpenApiEncoding explode property serialization

Author: kammerjaeger

src/Microsoft.OpenApi/Models/OpenApiEncoding.cs

@@ -13,6 +13,10 @@ namespace Microsoft.OpenApi.Models
     /// </summary>
     public class OpenApiEncoding : IOpenApiSerializable, IOpenApiExtensible
     {
+        /// <summary>
+        /// Explode backing variable
+        /// </summary>
+        private bool? _explode;
         /// <summary>
         /// The Content-Type for encoding a specific property.
         /// The value can be a specific media type (e.g. application/json),
@@ -37,7 +41,11 @@ public class OpenApiEncoding : IOpenApiSerializable, IOpenApiExtensible
         /// For all other styles, the default value is false.
         /// This property SHALL be ignored if the request body media type is not application/x-www-form-urlencoded.
         /// </summary>
-        public bool? Explode { get; set; }
+        public bool? Explode
+        {
+            get => _explode ?? Style == ParameterStyle.Form;
+            set => _explode = value;
+        }
 
         /// <summary>
         /// Determines whether the parameter value SHOULD allow reserved characters,
@@ -89,7 +97,10 @@ public void SerializeAsV3(IOpenApiWriter writer)
             writer.WriteProperty(OpenApiConstants.Style, Style?.GetDisplayName());
 
             // explode
-            writer.WriteProperty(OpenApiConstants.Explode, Explode, false);
+            if (_explode.HasValue)
+            {
+                writer.WriteProperty(OpenApiConstants.Explode, Explode);
+            }
 
             // allowReserved
             writer.WriteProperty(OpenApiConstants.AllowReserved, AllowReserved, false);

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

@@ -78,5 +78,55 @@ public void SerializeAdvanceEncodingAsV3YamlWorks()
             expected = expected.MakeLineBreaksEnvironmentNeutral();
             actual.Should().Be(expected);
         }
+
+        [Theory]
+        [InlineData(ParameterStyle.Form, true)]
+        [InlineData(ParameterStyle.SpaceDelimited, false)]
+        [InlineData(null, false)]
+        public void WhenStyleIsFormTheDefaultValueOfExplodeShouldBeTrueOtherwiseFalse(ParameterStyle? style, bool expectedExplode)
+        {
+            // Arrange
+            var parameter = new OpenApiEncoding
+            {
+                Style = style
+            };
+
+            // Act & Assert
+            parameter.Explode.Should().Be(expectedExplode);
+        }
+
+        [Theory]
+        [InlineData(true, true)]
+        [InlineData(false, true)]
+        [InlineData(null, false)]
+        public void WhenExplodeIsSetOutputShouldHaveExplode(bool? expectedExplode, bool hasExplode)
+        {
+            // Arrange
+            OpenApiEncoding parameter = new()
+            {
+                ContentType = "multipart/form-data",
+                Style = ParameterStyle.Form,
+                Explode = expectedExplode,
+            };
+
+            var expected =
+                $"""
+                contentType: multipart/form-data
+                style: form
+                """;
+
+            if (hasExplode)
+            {
+                expected = expected + $"\nexplode: {expectedExplode.ToString().ToLower()}";
+            }
+
+            // Act
+            var actual = parameter.SerializeAsYaml(OpenApiSpecVersion.OpenApi3_0);
+
+            // Assert
+            actual = actual.MakeLineBreaksEnvironmentNeutral();
+            expected = expected.MakeLineBreaksEnvironmentNeutral();
+            actual.Should().Be(expected);
+        }
     }
 }