Unable to cast object of type 'System.Decimal' to type 'System.String'

Question

I wrote this code to find which one of dtExistCosts doesn,y exit in costsList.

var costMustbeDeleteDt = (
        from e in dtExistCosts.AsEnumerable()
        where !(from c in costsList 
                select Convert.ToString(c.CostRecordId))
                            .Contains(e.Field<string>("DETAIL_HAZ_PK"))
        select e).CopyToDataTable();

but when I run it I get the error:

Unable to cast object of type System.Decimal to type System.String

in

!(from c in costsList 
  select Convert.ToString(c.CostRecordId))
        .Contains(e.Field<string>("DETAIL_HAZ_PK"))

why?

Answer 1

.Contains(e.Field<string>("DETAIL_HAZ_PK"))

The exception should be at this line, you cannot do explicit conversion for a decimal to string as the exception suggested.

You may do:

.Contains(Convert.ToString(e.Field<decimal>("DETAIL_HAZ_PK")))

Of course, compare values directly by decimal should be the case but this answer your question on why.

Answer 2

It looks like you are trying to convert two numerical values (c.CostRecordId and the value of "DETAIL_HAZ_PK") to string before comparing them which is not only unnecessary but also leads to the error you described.

Try this instead:

var costMustbeDeleteDt = (from e in dtExistCosts.AsEnumerable()
  where !(from c in costsList select c.CostRecordId)
      .Contains(e.Field<decimal>("DETAIL_HAZ_PK"))
  select e).CopyToDataTable();

Additionally, I would suggest to improve the readability of your selection by introducing a variable for the costList IDs and converting your LINQ queries to the method chain syntax:

var costsListIds = costsList.Select (c => c.CostRecordId);

var costMustbeDeleteDt = dtExistCosts
    .Where (existCost => !costsListIds.Contains (existCost.Field<decimal>("DETAIL_HAZ_PK"))
    .CopyToDataTable();