I need to send a lamda in function parameter
but when i do that i’ve error say

Expression of type ‘System.Func2[KokazGoodsTransfer.Models.Receipt,System.Decimal]' cannot be used for parameter of type 'System.Linq.Expressions.Expression1[System.Func2[KokazGoodsTransfer.Models.Receipt,System.Decimal]]' of method 'System.Decimal Sum[Receipt](System.Linq.IQueryable1[KokazGoodsTransfer.Models.Receipt], System.Linq.Expressions.Expression1[System.Func2[KokazGoodsTransfer.Models.Receipt,System.Decimal]])’ (Parameter ‘arg1′)’

example of code

var sum = await context.Receipts.GroupBy(c => c.ClientId).Select(c => new { c.Key, Sum = c.Sum(c=>c.Amount) }).ToListAsync();

it’s work fine

but when i try this i see the error

Func<Receipt, Decimal> func = c => c.Amount;
var sum = await context.Receipts.GroupBy(c => c.ClientId).Select(c => new { c.Key, Sum = c.Sum(func) }).ToListAsync();

thank you

>Solution :

EF usually requires an expression tree to be able to translate the code into actual SQL query.

You can try something like this (though not tested, but in some cases such tricks worked as far as I remember):

Expression<Func<Receipt, Decimal>> func = c => c.Amount;
var sum = await context.Receipts
    .GroupBy(c => c.ClientId)
    .Select(c => new { c.Key, Sum = c.AsQueryable().Sum(func) })
    .ToListAsync();

Otherwise you maybe will need either to build select statement expression manually (which is not that easy) or look into 3rd party library like LINQKit which allows to use Func‘s with some magic. Something along this lines:

Expression<Func<Receipt, Decimal>> func = c => c.Amount;
var sum = await context.Receipts
    .AsExpandable() // or WithExpressionExpanding on the context DI set up
    .GroupBy(c => c.ClientId)
    .Select(c => new { c.Key, Sum = c.Sum(func.Compile()) })
    .ToListAsync();