Custom Expression Handlers

Besides JavaScript and Liquid, you can implement your own workflow expression handler.

IExpressionHandler

To implement your own evaluator, implement IExpressionHandler:

public interface IExpressionHandler
{
    string Syntax { get; }
    Task<object?> EvaluateAsync(string expression, Type returnType, ActivityExecutionContext context, CancellationToken cancellationToken);
}

The Syntax property represents a moniker for the syntax you are implementing. For example, the JavaScriptExpressionHandler returns "JavaScript", while LiquidHandler returns "Liquid". When the user configures a workflow expression, they will select one of the available syntaxes.

Your handler should be able to evaluate the specified expression, and if specified, convert the result to the specified returnType.

Service Registration

To register your expression handler, use the TryAddProvider<T> extension method:

services.TryAddProvider<IExpressionHandler, MyCustomExpressionHandler>(ServiceLifetime.Scoped);

TryAddProvider<T> is like TryAddScoped<T>, except that it allows for multiple IExpressionHandlers registrations as long as the implementation type is different.