Exception: Specified cast is not valid.

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Hello.

I have DataTable object with columns
bool, Bitmap, string, string, string, string.

I want to use Linq so that I could make some kind of filtering on the first column (bool).

Here is what I use:

C#

var qqq = row in DataTable.AsEnumerable()
          where row.Field<bool>(ColumnNames[1]).ToString() == false.ToString()
          select row;

foreach (DataRow dataRow in qqq.ToList())
{
    dataRow.Delete();
}

Then I get the exception that I can't cast the field, why is that?

Posted 11-Nov-14 3:48am

Zhivko Kabaivanov

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ZurdoDev 11-Nov-14 9:53am

You said you wanted first column but then referenced ColumnNames[1] which would really be the second column. Perhaps all you need is to change to ColumnNames[0].

Zhivko Kabaivanov 11-Nov-14 9:59am

No, ColumnNames is array that have the names of the columns in the DataTable.
The zero index ColumnName[0] is not initialized and the others goes like this ColumnName[1] = boolean,
ColumnName[2] = Read (Bitmap).

ZurdoDev 11-Nov-14 10:02am

I'd debug it and make sure ColumnNames[1] is what you actually think it is.

2 solutions

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Thomas Daniels · Answer 1 · 2014-11-11T03:53:00

Solution 1

C# arrays are zero-based, and you tell that the first column is a bool, so you should use ColumnNames[0] instead of ColumnNames[1]:

C#

where row.Field<bool>(ColumnNames[0]).ToString() == false.ToString()

Posted 11-Nov-14 3:53am

Thomas Daniels

Richard Deeming · Answer 2 · 2014-11-11T04:05:00

Solution 2

It sounds like your bool column is nullable, and contains a null value. Try using .Field<bool?>(...) instead.

Also, there's no need to convert a bool or bool? to a string to compare it with another value of the same type.

C#

var qqq = row in DataTable.AsEnumerable()
          where row.Field<bool?>(ColumnNames[1]) == false
          select row;

Nullable Types (C# Programming Guide)[^]

Posted 11-Nov-14 4:05am

Richard Deeming

Comments

Zhivko Kabaivanov 11-Nov-14 11:50am

Nope, actually the solution was that when I init the data table: DataTable.Columns.Add(ColumnNames[1]);
there is no type specified and maybe the default type is String.
So when I compare the values in the Linq I get the expception.