ReadOnlyException DataTable DataRow "Column X is read only."

In VB, don't pass a read-only DataRow Item by reference

The likelihood that you'll run into this is low, but I was working on some VB.NET code and got the ReadOnlyException.

I ran into this issue because the code was passing the DataRow Item to a Sub ByRef. Just the act of passing-byref triggers the exception.

Sub Main()

    Dim dt As New DataTable()
    dt.Columns.Add(New DataColumn With {
        .ReadOnly = True,
        .ColumnName = "Name",
        .DataType = GetType(Integer)
    })

    dt.Rows.Add(4)

    Try
        DoNothing(dt.Rows(0).Item("Name"))
        Console.WriteLine("All good")
    Catch ex As Exception
        Console.WriteLine(ex.Message)
    End Try 

End Sub

Sub DoNothing(ByRef item As Object) 
End Sub 

Output

Column 'Name' is read only

C-sharp

You can't even write code like this in C# . DoNothing(ref dt.Rows[0].Item["Name"]) gives you a compile time error.

I kept getting the same exception while trying different approaches. What finally worked for me was to set the column's ReadOnly property to false and change the value of the Expression column instead of row[index] = "new value";

using DataAdapter.Fill does not load the database schema, which includes whether a column is a primary key or not, and whether a column is read-only or not. To load the database schema, use DataAdapter.FillSchema, but then that's not your questions.

using DataReader to fill a table loads the schema. So, the index column is read-only (probably because it's the primary key) and that information is loaded into the DataTable. Thereby preventing you from modifying the data in the table.

I think @k3b got it right; by setting ReadOnly = false, you should be able to write to the data table.

foreach (System.Data.DataColumn col in tab.Columns) col.ReadOnly = false;