Question

How can I calculate year in a nullable date?

partial void AgeAtDiagnosis_Compute(ref int result)
{
    // Set result to the desired field value
    result = DateofDiagnosis.Year - DateofBirth.Year;
    if (DateofBirth > DateofDiagnosis.AddYears(-result))
    {
      result--;
    }
}

The error is:

'System.Nullable<System.DateTime>' does not contain a definition for 'Year' and no 
 extension method 'Year' accepting a first argument of 
 type 'System.Nullable<System.DateTime>' could be found (are you missing a using 
 directive or an assembly reference?)   

Answer 1

Replace DateofDiagnosis.Year with DateofDiagnosis.Value.Year

And check the DateofDiagnosis.HasValue to assert that it is not null first.

I would write the code like this:

private bool TryCalculateAgeAtDiagnosis( DateTime? dateOfDiagnosis, 
                                         DateTime? dateOfBirth, 
                                         out int ageInYears)
{
    if (!dateOfDiagnosis.HasValue || !dateOfBirth.HasValue)
    {
        ageInYears = default;
        return false;
    }

    ageInYears = dateOfDiagnosis.Value.Year - dateOfBirth.Value.Year;

    if (dateOfBirth > dateOfDiagnosis.Value.AddYears(-ageInYears))
    {
        ageInYears--;
    }
    return true;
}