Add support for querystring parameter location

src/Microsoft.OpenApi/Models/OpenApiParameter.cs

@@ -116,6 +116,20 @@ internal void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion versio
         {
             Utils.CheckArgumentNull(writer);
 
+            // Check for querystring restrictions
+            if (In == ParameterLocation.QueryString)
+            {
+                if (version < OpenApiSpecVersion.OpenApi3_2)
+                {
+                    throw new InvalidOperationException("Parameter location 'querystring' is only supported in OpenAPI 3.2.0 and above.");
+                }
+                // Only throw if forbidden properties are explicitly set (not just default values)
+                if ((_style.HasValue) || (_explode.HasValue && _explode.Value) || AllowReserved || Schema != null)
+                {
+                    throw new InvalidOperationException("When 'in' is 'querystring', 'style', 'explode', 'allowReserved', and 'schema' properties MUST NOT be used as per OpenAPI 3.2 specification.");
+                }
+            }
+
             writer.WriteStartObject();
 
             // name
@@ -252,6 +266,12 @@ public virtual void SerializeAsV2(IOpenApiWriter writer)
         {
             Utils.CheckArgumentNull(writer);
 
+            // Throw if 'querystring' is used in V2
+            if (In == ParameterLocation.QueryString)
+            {
+                throw new InvalidOperationException("Parameter location 'querystring' is not supported in OpenAPI 2.0.");
+            }
+
             writer.WriteStartObject();
 
             // in

src/Microsoft.OpenApi/Models/ParameterLocation.cs

@@ -27,6 +27,11 @@ public enum ParameterLocation
         /// <summary>
         /// Used to pass a specific cookie value to the API.
         /// </summary>
-        [Display("cookie")] Cookie
+        [Display("cookie")] Cookie,
+
+        /// <summary>
+        /// Parameters that are appended to the URL query string (OpenAPI 3.2+ only).
+        /// </summary>
+        [Display("querystring")] QueryString
     }
 }

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

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
@@ -406,5 +407,52 @@ public async Task SerializeParameterWithFormStyleAndExplodeTrueWorksAsync(bool p
             // Assert
             await Verifier.Verify(outputStringWriter).UseParameters(produceTerseOutput);
         }
+
+        [Fact]
+        public void SerializeQueryStringParameter_BelowV32_Throws()
+        {
+            var parameter = new OpenApiParameter
+            {
+                Name = "foo",
+                In = ParameterLocation.QueryString
+            };
+            var writer = new OpenApiJsonWriter(new StringWriter());
+            // Style, Explode, AllowReserved, and Schema must be unset for this test to throw as expected
+            Assert.Throws<InvalidOperationException>(() => parameter.SerializeAsV3(writer));
+            Assert.Throws<InvalidOperationException>(() => parameter.SerializeAsV31(writer));
+            Assert.Throws<InvalidOperationException>(() => parameter.SerializeAsV2(writer));
+        }
+
+        [Fact]
+        public void SerializeQueryStringParameter_WithForbiddenProperties_Throws()
+        {
+            var parameter = new OpenApiParameter
+            {
+                Name = "foo",
+                In = ParameterLocation.QueryString,
+                Style = ParameterStyle.Form,
+                Explode = true,
+                AllowReserved = true,
+                Schema = new OpenApiSchema { Type = JsonSchemaType.String }
+            };
+            var writer = new OpenApiJsonWriter(new StringWriter());
+            Assert.Throws<InvalidOperationException>(() => parameter.SerializeAsV32(writer));
+        }
+
+        [Fact]
+        public async Task SerializeQueryStringParameter_V32_Succeeds()
+        {
+            var parameter = new OpenApiParameter
+            {
+                Name = "foo",
+                In = ParameterLocation.QueryString,
+                Style = null,
+                AllowReserved = false,
+                Schema = null,
+                // Explode must be false (default) and not set
+            };
+            var json = await parameter.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_2);
+            Assert.Contains("querystring", json);
+        }
     }
 }

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -1736,6 +1736,8 @@ namespace Microsoft.OpenApi
         Path = 2,
         [Microsoft.OpenApi.Display("cookie")]
         Cookie = 3,
+        [Microsoft.OpenApi.Display("querystring")]
+        QueryString = 4,
     }
     public enum ParameterStyle
     {