C# Linq中比较字符串使用 Equals 为什么报错

objGroList = objGroList.Where(s => s.BG_PriGroID.Equals(pId)).ToList();

        public override bool Equals(Object obj) {
            if (this == null)                        //this is necessary to guard against reverse-pinvokes and
                throw new NullReferenceException();  //other callers who do not use the callvirt instruction
 
            String str = obj as String;
            if (str == null)
                return false;
 
            if (Object.ReferenceEquals(this, obj))
                return true;
 
            if (this.Length != str.Length)
                return false;
 
            return EqualsHelper(this, str);
        }