Add error message when invoking async methods synchronously in JSInterop

src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetDispatcher.cs

@@ -7,6 +7,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Reflection.Metadata;
+using System.Runtime.CompilerServices;
 using System.Runtime.ExceptionServices;
 using System.Text;
 using System.Text.Json;
@@ -55,7 +56,7 @@ public static class DotNetDispatcher
             targetInstance = jsRuntime.GetObjectReference(invocationInfo.DotNetObjectId);
         }
 
-        var syncResult = InvokeSynchronously(jsRuntime, invocationInfo, targetInstance, argsJson);
+        var syncResult = InvokeSynchronously(jsRuntime, invocationInfo, targetInstance, argsJson, isAsyncContext: false);
         if (syncResult == null)
         {
             return null;
@@ -94,7 +95,7 @@ public static void BeginInvokeDotNet(JSRuntime jsRuntime, DotNetInvocationInfo i
                 targetInstance = jsRuntime.GetObjectReference(invocationInfo.DotNetObjectId);
             }
 
-            syncResult = InvokeSynchronously(jsRuntime, invocationInfo, targetInstance, argsJson);
+            syncResult = InvokeSynchronously(jsRuntime, invocationInfo, targetInstance, argsJson, isAsyncContext: true);
         }
         catch (Exception ex)
         {
@@ -153,7 +154,7 @@ private static void EndInvokeDotNetAfterTask(Task task, JSRuntime jsRuntime, in
         jsRuntime.EndInvokeDotNet(invocationInfo, new DotNetInvocationResult(resultJson));
     }
 
-    private static object? InvokeSynchronously(JSRuntime jsRuntime, in DotNetInvocationInfo callInfo, IDotNetObjectReference? objectReference, string argsJson)
+    private static object? InvokeSynchronously(JSRuntime jsRuntime, in DotNetInvocationInfo callInfo, IDotNetObjectReference? objectReference, string argsJson, bool isAsyncContext)
     {
         var assemblyName = callInfo.AssemblyName;
         var methodIdentifier = callInfo.MethodIdentifier;
@@ -183,6 +184,13 @@ private static void EndInvokeDotNetAfterTask(Task task, JSRuntime jsRuntime, in
             (methodInfo, parameterTypes) = GetCachedMethodInfo(objectReference, methodIdentifier);
         }
 
+        // If the method is async but is not called asynchronously, throw to indicate the misuse
+        // We need to check the asyncContext flag since this method is used for both sync and async calls
+        if (!isAsyncContext && IsAsyncMethod(methodInfo))
+        {
+            throw new InvalidOperationException($"The method '{methodIdentifier}' cannot be invoked synchronously because it is asynchronous. Use '{nameof(BeginInvokeDotNet)}' instead.");
+        }
+
         var suppliedArgs = ParseArguments(jsRuntime, methodIdentifier, argsJson, parameterTypes);
 
         try
@@ -211,6 +219,8 @@ private static void EndInvokeDotNetAfterTask(Task task, JSRuntime jsRuntime, in
         }
     }
 
+    private static bool IsAsyncMethod(MethodInfo methodInfo) => methodInfo.GetCustomAttribute<AsyncStateMachineAttribute>() != null;
+
     [UnconditionalSuppressMessage("Trimming", "IL2026", Justification = "We expect application code is configured to ensure return types of JSInvokable methods are retained.")]
     internal static object?[] ParseArguments(JSRuntime jsRuntime, string methodIdentifier, string arguments, Type[] parameterTypes)
     {

src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetDispatcherTest.cs

@@ -886,6 +886,34 @@ public void ReceiveByteArray_Works()
         Assert.Equal(byteArray, jsRuntime.ByteArraysToBeRevived.Buffer[0]);
     }
 
+    [Fact]
+    public void CannotInvokeAsyncMethodSynchronously()
+    {
+        // Arrange: Track some instance plus another object we'll pass as a param
+        var jsRuntime = new TestJSRuntime();
+        var targetInstance = new SomePublicType();
+        var arg2 = new TestDTO { IntVal = 1234, StringVal = "My string" };
+        var arg1Ref = DotNetObjectReference.Create(targetInstance);
+        var arg2Ref = DotNetObjectReference.Create(arg2);
+        jsRuntime.Invoke<object>("unimportant", arg1Ref, arg2Ref);
+
+        // Arrange: all args
+        var argsJson = JsonSerializer.Serialize(new object[]
+        {
+                new TestDTO { IntVal = 1000, StringVal = "String via JSON" },
+                arg2Ref,
+        }, jsRuntime.JsonSerializerOptions);
+
+        var callId = "123";
+
+        // Act/Assert
+        var ex = Assert.Throws<InvalidOperationException>(() =>
+        {
+            DotNetDispatcher.Invoke(jsRuntime, new DotNetInvocationInfo(null, "InvokableAsyncMethod", 1, callId), argsJson);
+        });
+
+        Assert.Equal($"The method 'InvokableAsyncMethod' cannot be invoked synchronously because it is asynchronous. Use '{nameof(DotNetDispatcher.BeginInvokeDotNet)}' instead.", ex.Message);
+    }
     internal class SomeInteralType
     {
         [JSInvokable("MethodOnInternalType")] public void MyMethod() { }