Question

1. I created a Emp class

   class Emp
   {
       public Emp()
       {
       }
       public Emp(Int64 empId, string empName, double empSalary,int empDeptId)
       {
           this.EmpID = empId;
           this.EmpName = empName;
           this.EmpSalary = empSalary;
           this.EmpDeptId = empDeptId; 
       }
       public Int64 EmpID { get; set; }
       public string EmpName { get; set; }
       public double EmpSalary { get; set; }
       public Int32 EmpDeptId { get; set; }
   }
   

2. I just Apply Group by command into EmpList then at return statement i got runtime exception that

An unhandled exception of type ‘System.InvalidCastException’ occurred in ConsoleApplication5.exe

Additional information:

Unable to cast object of type ‘WhereSelectEnumerableIterator2[System.Linq.IGrouping2[System.Int32,ConsoleApplication5.Emp],<>f__AnonymousType03[System.Int32,System.Double,System.Double]]' to type 'System.Collections.Generic.IEnumerable1[ConsoleApplication5.EmpGroup]’.

I also share the code which i have written.

public IEnumerable<EmpGroup> GroupByDeptId()
        {

            var numberGroups =from result in empList
                              group result by result.EmpDeptId into groupingData
                              select new 
                              {
                                  EmpDeptId = groupingData.Key,
                                  SalarySum = groupingData.Sum(p => p.EmpSalary),
                                  AverageSalary = groupingData.Average(p => p.EmpSalary) 
                              };

               return (IEnumerable<EmpGroup>)numberGroups;

        }

`3. I also Created a class for EmpGroup to handle the grouping data.

class EmpGroup
    {
        public Int32 EmpDeptId { get; set; }

        public double SalarySum { get; set; }

        public double AverageSalary { get; set; }
    }

Answer 1

You cannot cast from anonymous to your type, even if they have the same fields. You should edit your LINQ select to create a new EmpGroup:

public IEnumerable<EmpGroup> GroupByDeptId()
        {

            var numberGroups =from result in empList
                              group result by result.EmpDeptId into groupingData
                              select new EmpGroup() // here
                              {
                                  EmpDeptId = groupingData.Key,
                                  SalarySum = groupingData.Sum(p => p.EmpSalary),
                                  AverageSalary = groupingData.Average(p => p.EmpSalary) 
                              };

               return (IEnumerable<EmpGroup>)numberGroups;

        }