ASP.NET Core change model during ModelState check and return to View

Question

During ModelState.IsValid check, I need to change a property of the model and then return to View. Here is how my method looks like:

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> SaveProject(MainProjectViewModel model, string id = null)
        {
            if (string.IsNullOrEmpty(id))
            {
                if (!ModelState.IsValid)
                {    
                    bool validClientCheck = this.CheckIfClientNameAndIdAreValid(model.ProjectModel.ClientName, model.ProjectModel.ClientId);

                    if (!validClientCheck)
                    {
                        ModelState.AddModelError("ProjectModel.ClientId", $"Please select a valid client!");
                        model.ProjectModel.ClientId = null;
                    }

                    return View(model);
                }

                await this.projectService.SaveProject(model, "create");
            }
            return RedirectToAction("SaveProject", "Project", new { id = model.ProjectModel.Id });
        }

Even that I have model.ProjectModel.ClientId = null; in the view the hidden field for ClientId is still populated with the client id. How do I change the change the model before returning it to the view?

Answer 1

Model binding in View is not simple when you involve the validation with ModelState. For a specific field in post form, you could use a key to access it in the variable ModelState. If you want to modify the value of that field, you should modify the value in the variable ModelState instead of the page model.

In your case, you could add codes as below:

// Add custom error before the model validation...
ModelState.AddModelError("ProjectModel.ClientId", $"Please select a valid client!");
if (!ModelState.IsValid)
{
     // Modify the value to empty
     ModelState["ProjectModel.ClientId"].RawValue = "";  //add this line after set model error
}
return View(model);