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    <p>I ended up using the following activity. It can't access the workflow's variables, instead it accepts a single argument 'Value' that can be used by the same name inside the dynamic expression. Other than that it works pretty well.</p> <pre><code>public class Evaluate&lt;TIn, TOut&gt; : NativeActivity&lt;TOut&gt; { [RequiredArgument] public InArgument&lt;string&gt; ExpressionText { get; set; } [RequiredArgument] public InArgument&lt;TIn&gt; Value { get; set; } protected override void Execute(NativeActivityContext context) { var result = new ExpressionEvaluator&lt;TIn, TOut&gt;(ExpressionText.Get(context)).EvalWith(Value.Get(context)); Result.Set(context, result); } } public class ExpressionEvaluator&lt;TIn, TOut&gt; : Activity&lt;TOut&gt; { [RequiredArgument] public InArgument&lt;TIn&gt; Value { get; set; } public ExpressionEvaluator(string predicate) { VisualBasic.SetSettingsForImplementation(this, VbSettings); Implementation = () =&gt; new Assign&lt;TOut&gt; { Value = new InArgument&lt;TOut&gt;(new VisualBasicValue&lt;TOut&gt;(predicate)), To = new ArgumentReference&lt;TOut&gt;("Result") }; } public TOut EvalWith(TIn value) { return WorkflowInvoker.Invoke(this, new Dictionary&lt;string, object&gt; { { "Value", value } }); } private static readonly VisualBasicSettings VbSettings; static ExpressionEvaluator() { VbSettings = new VisualBasicSettings(); AddImports(typeof(TIn), VbSettings.ImportReferences); AddImports(typeof(TOut), VbSettings.ImportReferences); } private static void AddImports(Type type, ISet&lt;VisualBasicImportReference&gt; imports) { if (type.IsPrimitive || type == typeof(void) || type.Namespace == "System") return; var wasAdded = imports.Add(new VisualBasicImportReference { Assembly = type.Assembly.GetName().Name, Import = type.Namespace }); if (!wasAdded) return; if (type.BaseType != null) AddImports(type.BaseType, imports); foreach (var interfaceType in type.GetInterfaces()) AddImports(interfaceType, imports); foreach (var property in type.GetProperties()) AddImports(property.PropertyType, imports); foreach (var method in type.GetMethods()) { AddImports(method.ReturnType, imports); foreach (var parameter in method.GetParameters()) AddImports(parameter.ParameterType, imports); if (method.IsGenericMethod) { foreach (var genericArgument in method.GetGenericArguments()) AddImports(genericArgument, imports); } } if (type.IsGenericType) { foreach (var genericArgument in type.GetGenericArguments()) AddImports(genericArgument, imports); } } } </code></pre> <p><strong>EDIT</strong>: Updated the class to include complete assembly and namespace imports, lest you get the dreaded (and unhelpful) error message:</p> <blockquote> <p>'Value' is not declared. It may be inaccessible due to its protection level.</p> </blockquote> <p>Also, moved the ExpressionEvaluator class outside and made it public, so you can used it outside of WF, like so:</p> <pre><code>new ExpressionEvaluator&lt;int, double&gt;("Value * Math.PI").EvalWith(2); </code></pre> <p>Which will return:</p> <blockquote> <p>6.28318530717959</p> </blockquote>
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