ReadOnlyException DataTable DataRow "Column X is read only."

Question

I've got a short piece of code that originally created an SqlDataAdapter object over and over.

Trying to streamline my calls a little bit, I replaced the SqlDataAdapter with an SqlCommand and moved the SqlConnection outside of the loop.

Now, whenever I try to edit rows of data returned to my DataTable, I get a ReadOnlyException thrown that was not thrown before.

NOTE: I have a custom function that retrieves the employee's full name based on their ID. For simplicity here, I used "John Doe" in my example code below to demonstrate my point.

ExampleQueryOld works with the SqlDataAdapter; ExampleQueryNew fails with the ReadOnlyException whenever I try to write to an element of the DataRow:

• ExampleQueryOld

This works and has no issues:

public static DataTable ExampleQueryOld(string targetItem, string[] sqlQueryStrings) {
  DataTable bigTable = new DataTable();
  for (int i = 0; i < sqlQueryStrings.Length; i++) {
    string sqlText = sqlQueryStrings[i];
    DataTable data = new DataTable(targetItem);
    using (SqlDataAdapter da = new SqlDataAdapter(sqlText, Global.Data.Connection)) {
      try {
        da.Fill(data);
      } catch (Exception err) {
        Global.LogError(_CODEFILE, err);
      }
    }
    int rowCount = data.Rows.Count;
    if (0 < rowCount) {
      int index = data.Columns.IndexOf(GSTR.Employee);
      for (int j = 0; j < rowCount; j++) {
        DataRow row = data.Rows[j];
        row[index] = "John Doe"; // This Version Works
      }
      bigTable.Merge(data);
    }
  }
  return bigTable;
}

• ExampleQueryNew

This example throws the ReadOnlyException:

public static DataTable ExampleQueryNew(string targetItem, string[] sqlQueryStrings) {
  DataTable bigTable = new DataTable();
  using (SqlConnection conn = Global.Data.Connection) {
    for (int i = 0; i < sqlQueryStrings.Length; i++) {
      string sqlText = sqlQueryStrings[i];
      using (SqlCommand cmd = new SqlCommand(sqlText, conn)) {
        DataTable data = new DataTable(targetItem);
        try {
          if (cmd.Connection.State == ConnectionState.Closed) {
            cmd.Connection.Open();
          }
          using (SqlDataReader reader = cmd.ExecuteReader()) {
            data.Load(reader);
          }
        } catch (Exception err) {
          Global.LogError(_CODEFILE, err);
        } finally {
          if ((cmd.Connection.State & ConnectionState.Open) != 0) {
            cmd.Connection.Close();
          }
        }
        int rowCount = data.Rows.Count;
        if (0 < rowCount) {
          int index = data.Columns.IndexOf(GSTR.Employee);
          for (int j = 0; j < rowCount; j++) {
            DataRow row = data.Rows[j];
            try {
              // ReadOnlyException thrown below: "Column 'index'  is read only."
              row[index] = "John Doe";
            } catch (ReadOnlyException roErr) {
              Console.WriteLine(roErr.Message);
            }
          }
          bigTable.Merge(data);
        }
      }
    }
  }
  return bigTable;
}

Why can I write to the DataRow element in one case, but not in the other?

Is it because the SqlConnection is still open or is the SqlDataAdapter doing something behind the scene?

Answer 1

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; 

Answer 2

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";

Answer 3

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.