如何计算nullable日期中的年份？partialvoidAgeAtDiagnosis_Compute(refintresult){//Setresulttothedesiredfieldvalueresult=DateofDiagnosis.Year-DateofBirth.Year;if(DateofBirth>DateofDiagnosis.AddYears(-result)){result--;}}错误是:'System.Nullable'doesnotcontainadefinitionfor'Year'andnoextensionmethod'Year'accepting