The workflow designer has a rich programming model that supports communication with the host. A rehosted workflow designer will automatically validate a workflow when it is changed; in order to programmatically call validation on demand on a rehosted designer, use the following method:

designer.Context.Services.GetService<ValidationService>().ValidateWorkflow();

To handle resulting validation errors, publish an implementation of IValidationErrorService, which will expose the set of errors through ValidationService.NavigateTo(). For information on how to do this, see the following topic:

How to: Display Validation Errors in a Rehosted Designer