linq query does not find the index of object from a particul

Question

I have a list of student type.I want to search intended student from the student list against studentID. My code snippet is given below

 

int index = StudentList.FindIndex(x => x.studentID == studId);

 

After execution of Linq Query ,I have 200 records in the student list when it applying the searching criteria on studentList it returns me unexpected results