Refactor ElicitRequestParams and McpServerExtensions. #630

- Simplified "type" property handling in Converter.
- Changed s_lazyElicitAllowedProperties to nullable type.
- Updated ElicitAsync to use static lambda for clarity.
- Refactored CreatePrimitiveSchema to handle nullable types.
- Initialized allowed properties in TryValidateElicitationPrimitiveSchema.
- Updated ElicitationTypedTests for naming conventions and added tests for nullable properties.
- Enforced required properties in SampleForm and introduced NullablePropertyForm.
- Added JSON source generation context for NullablePropertyForm.

Author: mehrandvd

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -126,14 +126,7 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                     switch (propertyName)
                     {
                         case "type":
-                            if (reader.TokenType == JsonTokenType.String)
-                            {
-                                type = reader.GetString();
-                            }
-                            else
-                            {
-                                throw new JsonException("The 'type' property must be a string.");
-                            }
+                            type = reader.GetString();
                             break;
 
                         case "title":

src/ModelContextProtocol.Core/Server/McpServerExtensions.cs

@@ -21,12 +21,7 @@ public static class McpServerExtensions
     /// </summary>
     private static readonly ConditionalWeakTable<JsonSerializerOptions, ConcurrentDictionary<Type, ElicitRequestParams.RequestSchema>> s_elicitResultSchemaCache = new();
 
-    private static Lazy<Dictionary<string, HashSet<string>>> s_lazyElicitAllowedProperties = new(()=> new()
-    {
-        ["string"]  = ["type", "title", "description", "minLength", "maxLength", "format", "enum", "enumNames"],
-        ["number"]  = ["type", "title", "description", "minimum", "maximum"],
-        ["boolean"] = ["type", "title", "description", "default"]
-    });
+    private static Dictionary<string, HashSet<string>>? s_lazyElicitAllowedProperties = null;
 
     /// <summary>
     /// Requests to sample an LLM via the client using the specified request parameters.
@@ -279,7 +274,7 @@ public static ValueTask<ElicitResult> ElicitAsync(
         var dict = s_elicitResultSchemaCache.GetValue(serializerOptions, _ => new());
 
 #if NET
-        var schema = dict.GetOrAdd<JsonSerializerOptions>(typeof(T), (t, s) => BuildRequestSchema(t, s), serializerOptions);
+        var schema = dict.GetOrAdd(typeof(T), static (t, s) => BuildRequestSchema(t, s), serializerOptions);
 #else
         var schema = dict.GetOrAdd(typeof(T), type => BuildRequestSchema(type, serializerOptions));
 #endif
@@ -344,19 +339,21 @@ private static ElicitRequestParams.RequestSchema BuildRequestSchema(Type type, J
     /// <exception cref="McpException">Thrown when the type is not supported.</exception>
     private static ElicitRequestParams.PrimitiveSchemaDefinition CreatePrimitiveSchema(Type type, JsonSerializerOptions serializerOptions)
     {
-        var originalType = type;
-        var underlyingType = Nullable.GetUnderlyingType(originalType) ?? originalType;
-        
-        var typeInfo = serializerOptions.GetTypeInfo(underlyingType);
+        if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
+        {
+            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests. Nullable types are not supported.");
+        }
+
+        var typeInfo = serializerOptions.GetTypeInfo(type);
 
         if (typeInfo.Kind != JsonTypeInfoKind.None)
         {
-            throw new McpException($"Type '{originalType.FullName}' is not a supported property type for elicitation requests.");
+            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
         }
 
-        var jsonElement = AIJsonUtilities.CreateJsonSchema(underlyingType, serializerOptions: serializerOptions);
+        var jsonElement = AIJsonUtilities.CreateJsonSchema(type, serializerOptions: serializerOptions);
 
-        if (!TryValidateElicitationPrimitiveSchema(jsonElement, originalType, out var error))
+        if (!TryValidateElicitationPrimitiveSchema(jsonElement, type, out var error))
         {
             throw new McpException(error);
         }
@@ -365,7 +362,7 @@ private static ElicitRequestParams.PrimitiveSchemaDefinition CreatePrimitiveSche
             jsonElement.Deserialize(McpJsonUtilities.JsonContext.Default.PrimitiveSchemaDefinition);
 
         if (primitiveSchemaDefinition is null)
-            throw new McpException($"Type '{originalType.FullName}' is not a supported property type for elicitation requests.");
+            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
 
         return primitiveSchemaDefinition;
     }
@@ -421,7 +418,14 @@ private static bool TryValidateElicitationPrimitiveSchema(JsonElement schema, Ty
         if (typeKeyword == "integer")
             typeKeyword = "number";
 
-        var allowed = s_lazyElicitAllowedProperties.Value[typeKeyword];
+        s_lazyElicitAllowedProperties ??= new()
+        {
+            ["string"] = ["type", "title", "description", "minLength", "maxLength", "format", "enum", "enumNames"],
+            ["number"] = ["type", "title", "description", "minimum", "maximum"],
+            ["boolean"] = ["type", "title", "description", "default"]
+        };
+
+        var allowed = s_lazyElicitAllowedProperties[typeKeyword];
 
         foreach (JsonProperty prop in schema.EnumerateObject())
         {

tests/ModelContextProtocol.Tests/Protocol/ElicitationTypedTests.cs

@@ -35,7 +35,7 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
                 Assert.Equal(SampleRole.Admin, result.Content!.Role);
                 Assert.Equal(99.5, result.Content!.Score);
             }
-            else if (request.Params!.Name == "TestElicitationTypedCamel")
+            else if (request.Params!.Name == "TestElicitationCamelForm")
             {
                 var result = await request.Server.ElicitAsync<CamelForm>(
                     message: "Please provide more information.",
@@ -48,6 +48,19 @@ protected override void ConfigureServices(ServiceCollection services, IMcpServer
                 Assert.Equal(90210, result.Content!.ZipCode);
                 Assert.False(result.Content!.IsAdmin);
             }
+            else if (request.Params!.Name == "TestElicitationNullablePropertyForm")
+            {
+                var result = await request.Server.ElicitAsync<NullablePropertyForm>(
+                    message: "Please provide more information.",
+                    serializerOptions: ElicitationNullablePropertyJsonContext.Default.Options,
+                    cancellationToken: CancellationToken.None);
+
+                // Should be unreachable
+                return new CallToolResult
+                {
+                    Content = [new TextContentBlock { Text = "unexpected" }],
+                };
+            }
             else if (request.Params!.Name == "TestElicitationUnsupportedType")
             {
                 await request.Server.ElicitAsync<UnsupportedForm>(
@@ -222,7 +235,7 @@ public async Task Elicit_Typed_Respects_NamingPolicy()
             },
         });
 
-        var result = await client.CallToolAsync("TestElicitationTypedCamel", cancellationToken: TestContext.Current.CancellationToken);
+        var result = await client.CallToolAsync("TestElicitationCamelForm", cancellationToken: TestContext.Current.CancellationToken);
         Assert.Equal("success", (result.Content[0] as TextContentBlock)?.Text);
     }
 
@@ -251,6 +264,29 @@ public async Task Elicit_Typed_With_Unsupported_Property_Type_Throws()
         Assert.Contains(typeof(UnsupportedForm.Nested).FullName!, ex.Message);
     }
 
+    [Fact]
+    public async Task Elicit_Typed_With_Nullable_Property_Type_Throws()
+    {
+        await using IMcpClient client = await CreateMcpClientForServer(new McpClientOptions
+        {
+            Capabilities = new()
+            {
+                Elicitation = new()
+                {
+                    // Handler should never be invoked because the exception occurs before the request is sent.
+                    ElicitationHandler = async (req, ct) =>
+                    {
+                        Assert.Fail("Elicitation handler should not be called for unsupported schema test.");
+                        return new ElicitResult { Action = "cancel" };
+                    },
+                },
+            },
+        });
+
+        var ex = await Assert.ThrowsAsync<McpException>(async () =>
+            await client.CallToolAsync("TestElicitationNullablePropertyForm", cancellationToken: TestContext.Current.CancellationToken));
+    }
+
     [Fact]
     public async Task Elicit_Typed_With_NonObject_Generic_Type_Throws()
     {
@@ -286,9 +322,9 @@ public enum SampleRole
 
     public sealed class SampleForm
     {
-        public string? Name { get; set; }
+        public required string Name { get; set; }
         public int Age { get; set; }
-        public bool? Active { get; set; }
+        public bool Active { get; set; }
         public SampleRole Role { get; set; }
         public double Score { get; set; }
 
@@ -297,6 +333,13 @@ public sealed class SampleForm
     }
 
     public sealed class CamelForm
+    {
+        public required string FirstName { get; set; }
+        public int ZipCode { get; set; }
+        public bool IsAdmin { get; set; }
+    }
+
+    public sealed class NullablePropertyForm
     {
         public string? FirstName { get; set; }
         public int ZipCode { get; set; }
@@ -313,6 +356,12 @@ internal partial class ElicitationTypedDefaultJsonContext : JsonSerializerContex
     [JsonSerializable(typeof(JsonElement))]
     internal partial class ElicitationTypedCamelJsonContext : JsonSerializerContext;
 
+
+    [JsonSourceGenerationOptions(PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase)]
+    [JsonSerializable(typeof(NullablePropertyForm))]
+    [JsonSerializable(typeof(JsonElement))]
+    internal partial class ElicitationNullablePropertyJsonContext : JsonSerializerContext;
+
     public sealed class UnsupportedForm
     {
         public string? Name { get; set; }