Use external expression in linq query - linq-to-sql

Question

I need to use an expression defined outside my query. This expression will be used in many other queries so I would like to reuse that.
This is a simple sample to illustrate.
repos.ItemDocumentoProprio.Select(s => new SaleDto {
Total = s.Total,
TotalSale = s.CalcTotalSale()
});
public class Sale
{
public int Id { get; set; }
public decimal Total { get; set; }
public decimal Discount { get; set; }
public Expression<Func<Sale, decimal>> CalcTotalSale()
{
return (s => s.Total - s.Discount);
}
}
This code doesn't compile because cannot implicity convert Expression> to decimal as I expected.

Answer 1

If you're looking to dynamically build your SELECT query in LINQ to Entities AND have reusable methods, that is a more advanced topic which I won't attempt to cover in this answer.
While it isn't clear what your "other queries" will look like, I'd like to point out an alternative approach that may help you think about Expressions differently. You could do the following:
repos.ItemDocumentoProprio.Select(Sale.GetSaleDto());
public class Sale
{
public int Id { get; set; }
public decimal Total { get; set; }
public decimal Discount { get; set; }
public static Expression<Func<Sale, SaleDto>> GetSaleDto()
{
// As mentioned in my response, if you need to build this expression dynamically at
// runtime for different data calls to the DB store, that is a more advanced task.
Expression<Func<Sale, SaleDto>> selectFunc = s => new SaleDto
{
Total = s.Total,
TotalSale = s.Total - s.Discount
};
return selectFunc;
}
}
Another alternative is making the TotalSale property in SaleDto a calculated property. If you want the TotalSale calculation (Total - Discount) to be available in other DTO classes, then make a new method elsewhere that accepts two arguments (Total, Discount) and returns a decimal and reuse that throughout. Better yet, move it into an interface/abstract base class that all of your Sale-like classes must implement.
public class SaleDto
{
public decimal Total { get; set; }
public decimal Discount { get; set; }
public decimal TotalSale
{
get { return Total - Discount; }
}
}
The only difference here is you are performing the TotalSale calculation in memory as opposed to in the RDBMS. Without doing any formal analysis, I'm guessing the performance difference is negligible and the data sent over the wire from the DB to app is going to be of similar size.