C# non-nullable compiler warning when property was checked in a validation method

Question

I have a class that has a property declared as string? Email { get; set; } and a method that will return false if the email is empty or null for validation. There are other more complex checks in this method, so I am excluding them below for brevity. Although the Email was checked for null and will return False if it is null, the compiler still complains that it could be null.

public string? Email { get; set; }

private bool IsFormValid()
{
   if (string.IsNullOrEmpty(Email))
   {
      return false;
   }

   return true;
}

public void Submit()
{
    if (IsFormValid()) 
    {
        Submit(Email);     // Compiler warns that Email may be null here
    }
}

Does the compiler not recognize that Email would not be null in this scenario, or am I missing something? Is there a way to mark that Email is not null at this point other than rechecking Email is not null?

public void Submit()
{
    if (IsFormValid() && Email is not null)   // I don't want to do this since it was already checked
    {
        Submit(Email);     
    }
}

Answer 1

It does not work like that because it could be null. How can you be sure the Email is not changed to null since you called IsFormValid() by another thread? You either decide that you are sure to be in a single threaded context and overwrite the warning with !, or (preferred) you take a local reference to the Email, check it and then pass it to Submit.

public void Submit() {
    if (IsFormValid())
    {
        Submit(Email!);
    }
}

or

public void Submit() {
    string? email = Email;
    if (IsFormValid() && email is not null) {
        Submit(email);
    }
}

Answer 2

In this case it seems that you could use the MemberNotNullWhen attribute on your IsFormValid() method as such:

[MemberNotNullWhen(true, nameof(Email))]
private bool IsFormValid()
{
    if (string.IsNullOrEmpty(Email))
    {
        return false;
    }

    return true;
}

According to the documentation:

Specifies that the method or property will ensure that the listed field and property members have non-null values when returning with the specified return value condition.

This should get rid of the generated compiler warning.