Issue with expression evaluation

I’m new to Elsa and find the framework to be very robust. I was easily able to get the engine and workflow designer up and running. I was also able to develop a custom activity with an input parameter defined as follows:

public WorkflowExpression<string> FirstName { get => GetState(() => new WorkflowExpression<string>(LiteralEvaluator.SyntaxName, “”)); set => SetState(value); }

In this Activity i am setting an output variable as follows: Output.SetVariable(“ProviderOutput”, $“Person:{FirstName}”);

I’ve added this activity to the workflow and set the named instance to ProvOut. I’ve set the FirstName property with a literal value and it’s setting the value appropriately when i step through the custom activity.

I’ve then added the activity into the workflow again to test passing a value from one activity to another. The second activity is passing the FirstName property as follows (the expression type is Liquid): {{Activities.ProvOut.ProviderOutput}}

When i attempt to evaluate this i am getting an object reference error from the LiquidExpression evaluator. Here is a snippet of code from the OnExecuteAsync() method:

var firstName = await expressionEvaluator.EvaluateAsync( FirstName, workflowContext, cancellationToken );

My services are configured as follows:

services .AddElsa(elsa => elsa.AddEntityFrameworkStores<SqliteContext>(options => options.UseSqlite(Configuration.GetConnectionString(“Sqlite”)))) .AddHttpActivities(options => options.Bind(Configuration.GetSection(“Elsa:Http”))) .AddEmailActivities(options => options.Bind(Configuration.GetSection(“Elsa:Smtp”)))
.AddActivity<MapProvider>() .AddTimerActivities(options => options.Bind(Configuration.GetSection(“Elsa:Timers”))) .AddElsaDashboard();

This is the error i am receiving: “Error while evaluating Liquid expression "{{Activities.ProvOut.ProviderOutput}}". Message: Object reference not set to an instance of an object.”

And here is the stack trace " at Elsa.Expressions.WorkflowExpressionEvaluator.<EvaluateAsync>d__3.MoveNext()\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\n at System.Runtime.CompilerServices.TaskAwaiter1.GetResult()\n at Elsa.Extensions.WorkflowExpressionEvaluatorExtensions.<EvaluateAsync>d__01.MoveNext()\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\n at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\n at ElsaCustomActivities.MapProvider.<OnExecuteAsync>d__6.MoveNext() in /Users/larrymartin/Projects/ElsaTest/ElsaCustomActivities/Class1.cs:line 42"

[lmartinjr]

Can you check the F12 Developer Tools and see what the actual HTTP response is when requesting http://localhost:59003/_content/Elsa.Dashboard/assets/js/plugins/elsa-workflows/elsa-workflow-designer/elsa-workflow-designer.esm.js? Possibly you are getting a 404. In which case: make sure to follow these steps to build the client-side assets:

1. Run NPM install on src\dashboard\Elsa.Dashboard\Theme\argon-dashboard
2. Execute gulp build from the directory src\dashboard\Elsa.Dashboard\Theme\argon-dashboard

Let me know if this works or not.

Thanks! That’s exactly what i needed. I now have a dev environment set up and i’m able to step through the code.

[lmartinjr]

They way to do this is by creating a fork of this project and cloning that fork to your local machine (or update your local clone of this repo with the remote URL of the fork you will create), push your changes to your fork, and submit a PR.

This article explains the steps nicely.

Thanks Spike. I’ve now submitted a PR for review. I was able to confirm the code change worked as expected by creating an activity and assigning a name. I then checked the database (was using EF as persistence) and could confirm after making the change the Name column in the datastore was set. For testing i was using EF and SQLLite.