Don't assume Params type

lib/PuppeteerSharp.Tests/ScreencastTests/PageScreencastTests.cs

@@ -0,0 +1,47 @@
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace PuppeteerSharp.Tests.ScreencastTests
+{
+    public class PageScreencastTests : PuppeteerBrowserContextBaseTest
+    {
+        [Test]
+        public async Task CanStartScreencast()
+        {
+            await using var page = await Context.NewPageAsync();
+            await page.SetViewportAsync(new ViewPortOptions
+            {
+                Width = 500,
+                Height = 500
+            });
+
+            var screencastFramesReceived = 0;
+            page.Client.MessageReceived += async (_, e) =>
+            {
+                if (e.MessageID == "Page.screencastFrame")
+                {
+                    Assert.That(e.MessageData.GetProperty("data").GetString(), Is.Not.Null.Or.Empty);
+                    screencastFramesReceived++;
+
+                    // acknowledge frame
+                    await page.Client.SendAsync("Page.screencastFrameAck", new
+                    {
+                        sessionId = e.MessageData.GetProperty("sessionId").GetInt32(),
+                    });
+                }
+            };
+
+            await page.Client.SendAsync("Page.startScreencast", new
+            {
+                format = "png",
+                everyNthFrame = 1
+            });
+
+            await page.GoToAsync(TestConstants.ServerUrl + "/grid.html");
+
+            await page.Client.SendAsync("Page.stopScreencast");
+
+            Assert.That(screencastFramesReceived, Is.GreaterThan(0));
+        }
+    }
+}

lib/PuppeteerSharp/Cdp/Connection.cs

@@ -303,10 +303,10 @@ private async Task ProcessMessage(MessageReceivedEventArgs e)
         private void ProcessIncomingMessage(ConnectionResponse obj)
         {
             var method = obj.Method;
-            var param = obj.Params?.ToObject<ConnectionResponseParams>();
 
             if (method == "Target.attachedToTarget")
             {
+                var param = obj.Params?.ToObject<ConnectionResponseParams>();
                 var sessionId = param.SessionId;
                 var session = new CdpCDPSession(this, param.TargetInfo.Type, sessionId, obj.SessionId);
                 _sessions.AddItem(sessionId, session);
@@ -320,6 +320,7 @@ private void ProcessIncomingMessage(ConnectionResponse obj)
             }
             else if (method == "Target.detachedFromTarget")
             {
+                var param = obj.Params?.ToObject<ConnectionResponseParams>();
                 var sessionId = param.SessionId;
                 if (_sessions.TryRemove(sessionId, out var session) && !session.IsClosed)
                 {