Fix: Allow calling methods with only output parameters

Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs

@@ -3691,11 +3691,9 @@ protected ServiceResult ValidateCallRequestItem(
                 return StatusCodes.BadMethodInvalid;
             }
 
-            // check input arguments
-            if (callMethodRequest.InputArguments == null)
-            {
-                return StatusCodes.BadStructureMissing;
-            }
+            // Initialize input arguments to empty collection if null.
+            // Methods with only output parameters (no input parameters) are valid.
+            callMethodRequest.InputArguments ??= new VariantCollection();
 
             return StatusCodes.Good;
         }

Tests/Opc.Ua.Core.Tests/Types/BuiltIn/CallMethodRequestTests.cs

@@ -0,0 +1,133 @@
+/* ========================================================================
+ * Copyright (c) 2005-2024 The OPC Foundation, Inc. All rights reserved.
+ *
+ * OPC Foundation MIT License 1.00
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following
+ * conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * The complete license agreement can be found here:
+ * http://opcfoundation.org/License/MIT/1.00/
+ * ======================================================================*/
+
+using System.IO;
+using NUnit.Framework;
+using Opc.Ua.Tests;
+using Assert = NUnit.Framework.Legacy.ClassicAssert;
+
+namespace Opc.Ua.Core.Tests.Types.BuiltIn
+{
+    /// <summary>
+    /// Tests for the CallMethodRequest class.
+    /// </summary>
+    [TestFixture, Category("CallMethodRequest")]
+    [SetCulture("en-us")]
+    [Parallelizable]
+    public class CallMethodRequestTests
+    {
+        /// <summary>
+        /// Verify that InputArguments is never null after construction.
+        /// </summary>
+        [Test]
+        public void InputArgumentsInitializedAfterConstruction()
+        {
+            var request = new CallMethodRequest();
+            Assert.IsNotNull(request.InputArguments);
+            Assert.AreEqual(0, request.InputArguments.Count);
+        }
+
+        /// <summary>
+        /// Verify that InputArguments is initialized to empty collection after decode
+        /// when the wire format indicates no arguments.
+        /// This tests the fix for calling methods with only output parameters (no input parameters).
+        /// </summary>
+        [Test]
+        public void InputArgumentsInitializedAfterDecode()
+        {
+            ITelemetryContext telemetry = NUnitTelemetryContext.Create();
+            var context = new ServiceMessageContext(telemetry);
+            var originalRequest = new CallMethodRequest
+            {
+                ObjectId = new NodeId(1000),
+                MethodId = new NodeId(2000),
+                InputArguments = new VariantCollection() // Empty collection
+            };
+
+            // Encode
+            using (var stream = new MemoryStream())
+            {
+                using (var encoder = new BinaryEncoder(stream, context, leaveOpen: true))
+                {
+                    originalRequest.Encode(encoder);
+                }
+
+                // Decode
+                stream.Position = 0;
+                using (var decoder = new BinaryDecoder(stream, context))
+                {
+                    var decodedRequest = new CallMethodRequest();
+                    decodedRequest.Decode(decoder);
+
+                    // InputArguments should not be null
+                    Assert.IsNotNull(decodedRequest.InputArguments);
+                    Assert.AreEqual(0, decodedRequest.InputArguments.Count);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Verify that InputArguments with values are correctly encoded and decoded.
+        /// </summary>
+        [Test]
+        public void InputArgumentsWithValuesEncodeDecode()
+        {
+            ITelemetryContext telemetry = NUnitTelemetryContext.Create();
+            var context = new ServiceMessageContext(telemetry);
+            var originalRequest = new CallMethodRequest
+            {
+                ObjectId = new NodeId(1000),
+                MethodId = new NodeId(2000),
+                InputArguments = new VariantCollection { new Variant(42), new Variant("test") }
+            };
+
+            // Encode
+            using (var stream = new MemoryStream())
+            {
+                using (var encoder = new BinaryEncoder(stream, context, leaveOpen: true))
+                {
+                    originalRequest.Encode(encoder);
+                }
+
+                // Decode
+                stream.Position = 0;
+                using (var decoder = new BinaryDecoder(stream, context))
+                {
+                    var decodedRequest = new CallMethodRequest();
+                    decodedRequest.Decode(decoder);
+
+                    Assert.IsNotNull(decodedRequest.InputArguments);
+                    Assert.AreEqual(2, decodedRequest.InputArguments.Count);
+                    Assert.AreEqual(42, decodedRequest.InputArguments[0].Value);
+                    Assert.AreEqual("test", decodedRequest.InputArguments[1].Value);
+                }
+            }
+        }
+    }
+}