Use Begin/End pattern to not break .Net 4 compatibility.

CefSharp.BrowserSubprocess/CefRenderProcess.cs

@@ -132,5 +132,37 @@ public Task<JavascriptResponse> EvaluateScriptAsync(int browserId, long frameId,
 
             return timeout.HasValue ? task.WithTimeout(timeout.Value) : task;
         }
+
+        public IAsyncResult BeginEvaluateScriptAsync(int browserId, long frameId, string script, TimeSpan? timeout, AsyncCallback callback, object state)
+        {
+            var tcs = new TaskCompletionSource<JavascriptResponse>(state);
+            var task = EvaluateScriptAsync(browserId, frameId, script, timeout);
+            task.ContinueWith(t =>
+            {
+                if (t.IsFaulted)
+                {
+                    tcs.TrySetException(t.Exception.InnerExceptions);
+                }
+                else if (t.IsCanceled)
+                {
+                    tcs.TrySetCanceled();
+                }
+                else
+                {
+                    tcs.TrySetResult(t.Result);
+                }
+
+                if (callback != null)
+                {
+                    callback(tcs.Task);
+                }
+            });
+            return tcs.Task;
+        }
+
+        public JavascriptResponse EndEvaluateScriptAsync(IAsyncResult result)
+        {
+            return ((Task<JavascriptResponse>)result).Result;
+        }
     }
 }

CefSharp/Internals/BrowserProcessServiceHost.cs

@@ -51,12 +51,11 @@ public void SetOperationContext(OperationContext operationContext)
         public Task<JavascriptResponse> EvaluateScriptAsync(int browserId, long frameId, string script, TimeSpan? timeout)
         {
             var operationContextTask = operationContextTaskCompletionSource.Task;
-
             return operationContextTask.ContinueWith(t =>
             {
                 var context = t.Result;
                 var renderProcess = context.GetCallbackChannel<IRenderProcess>();
-                return renderProcess.EvaluateScriptAsync(browserId, frameId, script, timeout);
+                return Task.Factory.FromAsync<JavascriptResponse>(renderProcess.BeginEvaluateScriptAsync(browserId, frameId, script, timeout, null, null), renderProcess.EndEvaluateScriptAsync);
             }).Unwrap();
         }
 

CefSharp/Internals/IRenderProcess.cs

@@ -11,7 +11,9 @@ namespace CefSharp.Internals
     [ServiceContract]
     public interface IRenderProcess 
     {
-        [OperationContract]
-        Task<JavascriptResponse> EvaluateScriptAsync(int browserId, long frameId, string script, TimeSpan? timeout);
+        [OperationContract(AsyncPattern=true)]
+        IAsyncResult BeginEvaluateScriptAsync(int browserId, long frameId, string script, TimeSpan? timeout, AsyncCallback callback, object state);
+
+        JavascriptResponse EndEvaluateScriptAsync(IAsyncResult result);
     }
 }