将类型“ System.Int64”转换为类型“ System.Object”

Question

I'm trying to export a SQL query result to an CSV file, but i'm having some issues with the cast.

here's my code :

var query = string.Concat((from t in db.employees select t.ID + "," + t.LastName+"\n" ).Take(2000));

return File(new System.Text.UTF8Encoding().GetBytes(query), "text/csv", "Report.csv");

But i have the following error : Additional information: Unable to cast the type 'System.Int64' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types. Additional information: Unable to cast the type 'System.Int64' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

Can you help me on this ? Thanks

Answer 1

You may need to query the data and do the string projection in two steps:

var query = 
    (from t in db.employees 
     select new {t.ID , t.LastName})
    .Take(2000)
    .AsEnumerable();

var csv = string.Concat(query.Select(t => t.ID + "," + t.LastName + "\n"));

return File(new System.Text.UTF8Encoding().GetBytes(csv), "text/csv", "Report.csv");

Answer 2

It looks like EF is unable to translate your string concatenation call to SQL. You should be able to work around this issue by concatenating in memory:

var query = string.Concat(
    db.employees
        .Select(e => new {ID, LastName})
        .Take(2000)
        .AsEnumerable() // The rest of the query happens in memory
        .Select(p => p.ID + "," + p.LastName)
);

Answer 3

Just to post my theory:

var query = string.Concat(
    (from t in db.employees select t.ID + "," + t.LastName+"\n" )
    .Take(2000)
    .ToArray());

The difference is the final .ToArray() . string.Concat(IEnumerable<string>) was introduced in .NET 4.0 . Perhaps you are using an older version of .NET .