Compile all expressions at once when validating.

src/Test/TestCases.Workflows/ExpressionTests.cs

@@ -8,6 +8,7 @@
 using System.Activities.Validation;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace TestCases.Workflows;
@@ -47,7 +48,8 @@ static ExpressionTests()
         // There's no programmatic way (that I know of) to add assembly references when creating workflows like in these tests.
         // Adding the custom assembly directly to the expression validator to simulate XAML reference.
         // The null is for testing purposes.
-        VbExpressionValidator.Instance = new VbExpressionValidator(new() { typeof(ClassWithCollectionProperties).Assembly, null });
+        VbExpressionValidator.Instance.AddRequiredAssembly(typeof(ClassWithCollectionProperties).Assembly);
+        CSharpExpressionValidator.Instance.AddRequiredAssembly(typeof(ClassWithCollectionProperties).Assembly);
     }
 
     [Theory]
@@ -424,6 +426,67 @@ public void CSRoslynValidator_ValidatesMoreThan16Arguments()
         var result = ActivityValidationServices.Validate(sequence, _useValidator);
         result.Errors.ShouldBeEmpty();
     }
+    [Fact]
+    public void VB_Multithreaded_NoError()
+    {
+        var activities = new List<Activity>();
+        var tasks = new List<Task>();
+        var results = new List<ValidationResults>();
+        for (var i = 0; i < 20; i++)
+        {
+            var seq = new Sequence();
+            seq.Variables.Add(new Variable<int>("sum"));
+            for (var j = 0; j < 10000; j++)
+            {
+                seq.Activities.Add(new Assign
+                {
+                    To = new OutArgument<int>(new VisualBasicReference<int>("sum")),
+                    Value = new InArgument<int>(new VisualBasicValue<int>($"sum + {j}"))
+                });
+            }
+        }
+        foreach (var activity in activities)
+        {
+            tasks.Add(Task.Run(() =>
+            {
+                results.Add(ActivityValidationServices.Validate(activity, _useValidator));
+            }));
+        }
+        Task.WaitAll(tasks.ToArray());
+
+        results.All(r => !r.Errors.Any() && !r.Warnings.Any()).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void CS_Multithreaded_NoError()
+    {
+        var activities = new List<Activity>();
+        var tasks = new List<Task>();
+        var results = new List<ValidationResults>();
+        for (var i = 0; i < 20; i++)
+        {
+            var seq = new Sequence();
+            seq.Variables.Add(new Variable<int>("sum"));
+            for (var j = 0; j < 10000; j++)
+            {
+                seq.Activities.Add(new Assign
+                {
+                    To = new OutArgument<int>(new CSharpReference<int>("sum")),
+                    Value = new InArgument<int>(new CSharpValue<int>($"sum + {j}"))
+                });
+            }
+        }
+        foreach (var activity in activities)
+        {
+            tasks.Add(Task.Run(() =>
+            {
+                results.Add(ActivityValidationServices.Validate(activity, _useValidator));
+            }));
+        }
+        Task.WaitAll(tasks.ToArray());
+
+        results.All(r => !r.Errors.Any() && !r.Warnings.Any()).ShouldBeTrue();
+    }
 
     [Fact]
     public void VBReferenceTypeIsChecked()

src/UiPath.Workflow.Runtime/ActivityLocationReferenceEnvironment.cs

@@ -12,7 +12,10 @@ internal sealed class ActivityLocationReferenceEnvironment : LocationReferenceEn
     private Dictionary<string, LocationReference> _declarations;
     private List<LocationReference> _unnamedDeclarations;
 
-    public ActivityLocationReferenceEnvironment() { }
+    public ActivityLocationReferenceEnvironment()
+    {
+        Extensions = new();
+    }
 
     public ActivityLocationReferenceEnvironment(LocationReferenceEnvironment parent)
     {
@@ -22,6 +25,7 @@ public ActivityLocationReferenceEnvironment(LocationReferenceEnvironment parent)
             CompileExpressions = parent.CompileExpressions;
             IsValidating = parent.IsValidating;
             InternalRoot = parent.Root;
+            Extensions = parent.Extensions;
         }
     }
 

src/UiPath.Workflow.Runtime/EnvironmentExtensions.cs

@@ -0,0 +1,74 @@
+// This file is part of Core WF which is licensed under the MIT license.
+// See LICENSE file in the project root for full license information.
+
+namespace System.Activities
+{
+    internal class EnvironmentExtensions
+    {
+        private readonly Dictionary<Type, object> _extensions = new();
+
+        /// <summary>
+        /// Gets the specified extension.
+        /// If the extension does not exist,
+        /// it will invoke the <paramref name="createExtensionFactory"/> parameter
+        /// </summary>
+        /// <typeparam name="T">The type of the extension</typeparam>
+        /// <param name="createExtensionFactory">The factory to create the extension</param>
+        /// <exception cref="ArgumentNullException"></exception>
+        public TInterface GetOrAdd<TInterface>(Func<TInterface> createExtensionFactory) 
+            where TInterface : class 
+        {
+            var type = typeof(TInterface);
+            if (_extensions.TryGetValue(type, out object extension))
+            {
+                return extension as TInterface;
+            }
+
+            return CreateAndAdd(createExtensionFactory, type);
+        }
+
+        /// <summary>
+        /// Retrieves the extension registered for the given type
+        /// or null otherwise
+        /// </summary>
+        /// <typeparam name="T">The type of the extension.</typeparam>
+        public T Get<T>() where T : class
+        {
+            if (_extensions.TryGetValue(typeof(T), out object extension))
+                return extension as T;
+            return null;
+        }
+
+        /// <summary>
+        /// Adds the specified extension to the list.
+        /// The extension is treated as a singleton, 
+        /// so if a second extension with the same type is added, it will
+        /// throw an <see cref="InvalidOperationException"/> 
+        /// </summary>
+        /// <typeparam name="TInterface">The type of the extension</typeparam>
+        /// <param name="extension">The extension</param>
+        /// <exception cref="InvalidOperationException"></exception>
+        public void Add<TInterface, TImplementation>(TImplementation extension) 
+            where TInterface : class
+            where TImplementation : class, TInterface
+        {
+            if (_extensions.ContainsKey(typeof(TInterface)))
+                throw new InvalidOperationException($"Service '{typeof(TInterface).FullName}' already exists");
+
+            _extensions[typeof(TInterface)] = extension;
+        }
+
+
+        #region private methods
+        private T CreateAndAdd<T>(Func<T> createExtensionFactory, Type type) where T : class
+        {
+            var extension = createExtensionFactory();
+            if (extension is null)
+                throw new ArgumentNullException(nameof(extension));
+
+            _extensions[type] = extension;
+            return extension;
+        }
+        #endregion
+    }
+}

src/UiPath.Workflow.Runtime/LocationReferenceEnvironment.cs

@@ -16,6 +16,8 @@ protected LocationReferenceEnvironment() { }
     /// </summary>
     internal bool IsValidating { get; set; }
 
+    internal EnvironmentExtensions Extensions { get; set; }
+
     public abstract Activity Root { get; }
 
     public LocationReferenceEnvironment Parent { get; protected set; }

src/UiPath.Workflow.Runtime/Validation/ActivityValidationServices.cs

@@ -10,8 +10,9 @@ namespace System.Activities.Validation;
 
 public static class ActivityValidationServices
 {
-    internal static readonly ReadOnlyCollection<Activity> EmptyChildren = new(Array.Empty<Activity>());
     private static readonly ValidationSettings defaultSettings = new();
+
+    internal static readonly ReadOnlyCollection<Activity> EmptyChildren = new(Array.Empty<Activity>());
     internal static ReadOnlyCollection<ValidationError> EmptyValidationErrors = new(new List<ValidationError>(0));
 
     public static ValidationResults Validate(Activity toValidate) => Validate(toValidate, defaultSettings);
@@ -245,7 +246,7 @@ private static string GenerateExceptionString(IList<ValidationError> validationE
         return exceptionString;
     }
 
-    static internal string GenerateValidationErrorPrefix(Activity toValidate, ActivityUtilities.ActivityCallStack parentChain, ProcessActivityTreeOptions options, out Activity source)
+    internal static string GenerateValidationErrorPrefix(Activity toValidate, ActivityUtilities.ActivityCallStack parentChain, ProcessActivityTreeOptions options, out Activity source)
     {
         bool parentVisible = true;
         string prefix = "";
@@ -428,6 +429,7 @@ internal InternalActivityValidationServices(ValidationSettings settings, Activit
         {
             _settings = settings;
             _rootToValidate = toValidate;
+            _errors = new List<ValidationError>();
             _environment = settings.Environment ?? new ActivityLocationReferenceEnvironment();
             _environment.IsValidating = !settings.ForceExpressionCache;
             if (settings.SkipExpressionCompilation)
@@ -452,8 +454,11 @@ internal ValidationResults InternalValidate()
                 // We want to add the CacheMetadata errors to our errors collection
                 ActivityUtilities.CacheRootMetadata(_rootToValidate, _environment, _options, new ActivityUtilities.ProcessActivityCallback(ValidateElement), ref _errors);
             }
-
-            return new ValidationResults(_errors);
+
+            return new ValidationResults(_errors.Concat(GetValidationExtensionResults(_environment)).ToList());
+
+            IEnumerable<ValidationError> GetValidationExtensionResults(LocationReferenceEnvironment environment) => 
+                environment.Extensions.Get<IValidationExtension>()?.Validate(_rootToValidate) ?? Array.Empty<ValidationError>();
         }
 
         private void ValidateElement(ActivityUtilities.ChildActivity childActivity, ActivityUtilities.ActivityCallStack parentChain)

src/UiPath.Workflow.Runtime/Validation/IValidationExtension.cs

@@ -0,0 +1,7 @@
+namespace System.Activities.Validation
+{
+    internal interface IValidationExtension
+    {
+        IEnumerable<ValidationError> Validate(Activity activity);
+    }
+}