How can I solve an Asp.Net Custom Identity Error?

I used CustomIdentityUser library in ASP.NET Core project, I wanted to create a user update page. But even though there is no code error, when the user wants to change account data, the system gives error like "Incorrect password".

But I used the same password as when registering the user. What can be the cause of this error? Can you give me an idea? Thank you in advance.

My controller is

public ActionResult UpdateUser(ChangeUserViewModel changeUserViewModel)
{
     if (ModelState.IsValid)
     {
         
            CustomIdentityUser member = new CustomIdentityUser
            {
                UserName = changeUserViewModel.UserName,
                Email = changeUserViewModel.Email
            };

             var checkPassword = _userManager.CheckPasswordAsync(member, changeUserViewModel.NewPassword).Result;
             IdentityResult changePasswordResult = _userManager.ChangePasswordAsync(member, changeUserViewModel.CurrentPassword, changeUserViewModel.NewPassword).Result;
             if (changePasswordResult.Succeeded)
             {
                _signInManager.RefreshSignInAsync(member);
                return RedirectToAction("Login", "Account");
            }
            else
            {
                foreach (var error in changePasswordResult.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.Description);
                } 
            }
            
        }
        return View(changeUserViewModel);
 }

> But I used the same password as when registering the user. What can be the cause of this error?

In this scenario, you should better use CheckPasswordAsync before calling ChangePasswordAsync method bcause then you would understand if the password is correct.

>
> I wanted to create a user update page. But even though there is no
> code error, when the user wants to change account data, the system
> gives error like "Incorrect password".

Actually, based of your shared code snippet and descriptions at the birds eye view few inconsistencies appeared which might causing the error.

The first and most importantly, if you would like to update Identity user information then why you are creating new object of CustomIdentityUser rather you should update the information by finding existing user.

Thus, instead of creating new user instance you should find the existing user by your user information you would like to update. So, you can use your current context to pull that user information. You can do:
<!-- language: c# -->
var user = await _userManager.GetUserAsync(User);

            if (user == null)
            {
                return NotFound($"Unable to load user with ID '{_userManager.GetUserId(User)}'.");
            }

            var changePasswordResult = await _userManager.ChangePasswordAsync(user, Input.OldPassword, Input.NewPassword);
            if (!changePasswordResult.Succeeded)
            {
                foreach (var error in changePasswordResult.Errors)
                {
                    ModelState.AddModelError(string.Empty, error.Description);
                }
                return Page();
            }

            await _signInManager.RefreshSignInAsync(user);

Note: It requires users from user claim principal. You can get more details here in official sample and document