[OpenAPI] Get default value with [FromQuery]

Get the default value from the associated object's property when `[FromQuery]` is applied to a property of an object used as a `[FromQuery]` parameter.

Resolves #61934.

Author: martincostello

src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs

@@ -309,6 +309,11 @@ internal static void ApplyParameterInfo(this JsonNode schema, ApiParameterDescri
             var attributes = validations.OfType<ValidationAttribute>();
             schema.ApplyValidationAttributes(attributes);
         }
+        if (parameterDescription.ModelMetadata is Mvc.ModelBinding.Metadata.DefaultModelMetadata { Attributes.PropertyAttributes.Count: > 0 } metadata &&
+            metadata.Attributes.PropertyAttributes.OfType<DefaultValueAttribute>().LastOrDefault() is { } metadataDefaultValueAttribute)
+        {
+            schema.ApplyDefaultValue(metadataDefaultValueAttribute.Value, jsonTypeInfo);
+        }
         if (parameterDescription.ParameterDescriptor is IParameterInfoParameterDescriptor { ParameterInfo: { } parameterInfo })
         {
             if (parameterInfo.HasDefaultValue)

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs

@@ -522,8 +522,7 @@ public async Task GetOpenApiParameters_HandlesAsParametersParametersWithDescript
         await VerifyOpenApiDocument(builder, document =>
         {
             var operation = document.Paths["/api"].Operations[HttpMethod.Get];
-            var parameter = Assert.Single(operation.Parameters);
-            Assert.Equal("The ID of the entity", parameter.Description);
+            Assert.Contains(operation.Parameters, actualMemory => actualMemory.Name == "id" && actualMemory.Description == "The ID of the entity");
         });
     }
 
@@ -537,8 +536,56 @@ public async Task GetOpenApiParameters_HandlesFromQueryParametersWithDescription
         await VerifyOpenApiDocument(actionDescriptor, document =>
         {
             var operation = document.Paths["/"].Operations[HttpMethod.Get];
-            var parameter = Assert.Single(operation.Parameters);
-            Assert.Equal("The ID of the entity", parameter.Description);
+            Assert.Contains(operation.Parameters, actualMemory => actualMemory.Name == "id" && actualMemory.Description == "The ID of the entity");
+        });
+    }
+
+    [Fact]
+    public async Task GetOpenApiParameters_HandlesAsParametersParametersWithDefaultValueAttribute()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapGet("/api", ([AsParameters] FromQueryModel model) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[HttpMethod.Get];
+            Assert.Contains(
+                operation.Parameters,
+                actualMemory =>
+                {
+                    return actualMemory.Name == "limit" &&
+                           actualMemory.Schema != null &&
+                           actualMemory.Schema.Default != null &&
+                           actualMemory.Schema.Default.GetValueKind() == JsonValueKind.Number &&
+                           actualMemory.Schema.Default.GetValue<int>() == 20;
+                });
+        });
+    }
+
+    [Fact]
+    public async Task GetOpenApiParameters_HandlesFromQueryParametersWithDefaultValueAttribute()
+    {
+        // Arrange
+        var actionDescriptor = CreateActionDescriptor(nameof(TestFromQueryController.GetWithFromQueryDto), typeof(TestFromQueryController));
+
+        // Assert
+        await VerifyOpenApiDocument(actionDescriptor, document =>
+        {
+            var operation = document.Paths["/"].Operations[HttpMethod.Get];
+            Assert.Contains(
+                operation.Parameters,
+                actualMemory =>
+                {
+                    return actualMemory.Name == "limit" &&
+                           actualMemory.Schema != null &&
+                           actualMemory.Schema.Default != null &&
+                           actualMemory.Schema.Default.GetValueKind() == JsonValueKind.Number &&
+                           actualMemory.Schema.Default.GetValue<int>() == 20;
+                });
         });
     }
 
@@ -860,5 +907,10 @@ private record FromQueryModel
         [Description("The ID of the entity")]
         [FromQuery(Name = "id")]
         public int Id { get; set; }
+
+        [Description("The maximum number of results")]
+        [FromQuery(Name = "limit")]
+        [DefaultValue(20)]
+        public int Limit { get; set; }
     }
 }