Having issues with Object comparison [modified]
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Hi, I'm tying to use code from the MS web site (Implement a DataSet GROUP BY Helper Class ): http://support.microsoft.com/kb/326145 The code works well with the DataTable sample they show, but when I use a DataTable populated from an Access database, the object comparison does not work. It only works if I convert to string... any idea why? ******************************************************************************** if (!ColumnEqual( LastSourceRow[Field.FieldName, SourceRow[Field.FieldName] )) {// yada } private bool ColumnEqual(object a, object b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a is DBNull) && (b is DBNull)) return true; //both are null if ((a is DBNull) || (b is DBNull)) return false; //only one is null return (a==b); //value type standard comparison } ********************************************************************************* I did this to make it work: if (!ColumnEqual( LastSourceRow[Field.FieldName].ToString(), SourceRow[Field.FieldName].ToString() )) {//yada } private bool ColumnEqual(string a, string b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a == "") && (b == "")) return true; //both are null if ((a == "") || (b == "")) return false; //only one is null return (a == b); //value type standard comparison } thanks, Ron -- modified at 3:07 Wednesday 21st March, 2007
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Hi, I'm tying to use code from the MS web site (Implement a DataSet GROUP BY Helper Class ): http://support.microsoft.com/kb/326145 The code works well with the DataTable sample they show, but when I use a DataTable populated from an Access database, the object comparison does not work. It only works if I convert to string... any idea why? ******************************************************************************** if (!ColumnEqual( LastSourceRow[Field.FieldName, SourceRow[Field.FieldName] )) {// yada } private bool ColumnEqual(object a, object b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a is DBNull) && (b is DBNull)) return true; //both are null if ((a is DBNull) || (b is DBNull)) return false; //only one is null return (a==b); //value type standard comparison } ********************************************************************************* I did this to make it work: if (!ColumnEqual( LastSourceRow[Field.FieldName].ToString(), SourceRow[Field.FieldName].ToString() )) {//yada } private bool ColumnEqual(string a, string b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a == "") && (b == "")) return true; //both are null if ((a == "") || (b == "")) return false; //only one is null return (a == b); //value type standard comparison } thanks, Ron -- modified at 3:07 Wednesday 21st March, 2007
it works here, with datatable and regular cells.
DataTable dt = new DataTable("temp"); dt.Columns.Add("col1"); dt.Rows.Add("row1"); dt.Rows.Add("row2"); dt.Rows.Add("row3"); Console.WriteLine(ColumnEqual(dt.Rows[1][0], dt.Rows[1][0])); Console.Read();--> This outputs True (can test it in an empty console app). You should check it with the debugger, using breakpoints, to see what values are actually compared.
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } } -
Hi, I'm tying to use code from the MS web site (Implement a DataSet GROUP BY Helper Class ): http://support.microsoft.com/kb/326145 The code works well with the DataTable sample they show, but when I use a DataTable populated from an Access database, the object comparison does not work. It only works if I convert to string... any idea why? ******************************************************************************** if (!ColumnEqual( LastSourceRow[Field.FieldName, SourceRow[Field.FieldName] )) {// yada } private bool ColumnEqual(object a, object b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a is DBNull) && (b is DBNull)) return true; //both are null if ((a is DBNull) || (b is DBNull)) return false; //only one is null return (a==b); //value type standard comparison } ********************************************************************************* I did this to make it work: if (!ColumnEqual( LastSourceRow[Field.FieldName].ToString(), SourceRow[Field.FieldName].ToString() )) {//yada } private bool ColumnEqual(string a, string b) { /* * Compares two values to see if they are equal. Also compares DBNULL.Value. * * Note: If your DataTable contains object fields, you must extend this * function to handle them in a meaningful way if you intend to group on them. */ if ((a == "") && (b == "")) return true; //both are null if ((a == "") || (b == "")) return false; //only one is null return (a == b); //value type standard comparison } thanks, Ron -- modified at 3:07 Wednesday 21st March, 2007
The comparison is not right. Having an empty string is not the same as having a null string. What you want to do is expand your checks for this:
private bool ColumnEqual(string a, string b) { if (string.IsNullOrEmpty(a) && string.IsNullOrEmpty(b)) return true; if (string.IsNullOrEmpty(a) || string.IsNullOrEmpty(b)) return false; return (a == b); }Deja View - the feeling that you've seen this post before.
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The comparison is not right. Having an empty string is not the same as having a null string. What you want to do is expand your checks for this:
private bool ColumnEqual(string a, string b) { if (string.IsNullOrEmpty(a) && string.IsNullOrEmpty(b)) return true; if (string.IsNullOrEmpty(a) || string.IsNullOrEmpty(b)) return false; return (a == b); }Deja View - the feeling that you've seen this post before.
Thanks Pete, Yes, I was also worried about null strings too, but I did a test to see what output I would get. When you convert a DBNull.Value to string, the result is String.Empty. I'll use IsNullOrEmpty, just to be safe though. Ron if (DBNull.Value.ToString() == null) { MessageBox.Show("null"); } else if (DBNull.Value.ToString() == "") { MessageBox.Show("empty string"); } else { MessageBox.Show("not null or empty"); }
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it works here, with datatable and regular cells.
DataTable dt = new DataTable("temp"); dt.Columns.Add("col1"); dt.Rows.Add("row1"); dt.Rows.Add("row2"); dt.Rows.Add("row3"); Console.WriteLine(ColumnEqual(dt.Rows[1][0], dt.Rows[1][0])); Console.Read();--> This outputs True (can test it in an empty console app). You should check it with the debugger, using breakpoints, to see what values are actually compared.
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } }Hi joo, I used MessageBox to show me the comparison, row by row, and they do have the same string in the object (object.ToString()). They don't compair to true if left in the object (This only happends if the DataTable is populated by ADO). If building the DataTable by code (as in your example), this works fine. Ron
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Hi joo, I used MessageBox to show me the comparison, row by row, and they do have the same string in the object (object.ToString()). They don't compair to true if left in the object (This only happends if the DataTable is populated by ADO). If building the DataTable by code (as in your example), this works fine. Ron
It still works when I use ADO... Your problem appears to be very strange.
SqlConnection conn = new SqlConnection("connstring"); SqlDataAdapter da = new SqlDataAdapter("select * from [table]",conn); DataSet ds = new DataSet(); da.Fill(ds); dataGridView1.DataSource = ds; Console.WriteLine(ds.Tables[0].Rows[1][1] == ds.Tables[0].Rows[1][2]); //returns false Console.WriteLine(ds.Tables[0].Rows[1][1] == ds.Tables[0].Rows[1][1]); //returns true
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } } -
It still works when I use ADO... Your problem appears to be very strange.
SqlConnection conn = new SqlConnection("connstring"); SqlDataAdapter da = new SqlDataAdapter("select * from [table]",conn); DataSet ds = new DataSet(); da.Fill(ds); dataGridView1.DataSource = ds; Console.WriteLine(ds.Tables[0].Rows[1][1] == ds.Tables[0].Rows[1][2]); //returns false Console.WriteLine(ds.Tables[0].Rows[1][1] == ds.Tables[0].Rows[1][1]); //returns true
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } }Yes, I spent hours trying to figure out why it was not grouping. Finally, I tried changing the object to a string right at the compairison (a.ToString() == b.ToString()) and magically the grouping worked. Damn, I guess I have to go to work (day job). Ron
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Yes, I spent hours trying to figure out why it was not grouping. Finally, I tried changing the object to a string right at the compairison (a.ToString() == b.ToString()) and magically the grouping worked. Damn, I guess I have to go to work (day job). Ron
Yes, it is probably a good solution. But I still think it's strange that it didn't work before, because here in my sandboxapp it does. Weird. Have fun working ;)
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } } -
Yes, it is probably a good solution. But I still think it's strange that it didn't work before, because here in my sandboxapp it does. Weird. Have fun working ;)
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } }Hi again joon, I've been playing around with the stock code again... I've noticed that if you try and Group By the other columns (OrderID",Int32) [I did change a few rows so some are the same value], it will not group. The stock code only seems to group on a string column? Could you check it our with your sandboxapp? thanks, Ron
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Yes, it is probably a good solution. But I still think it's strange that it didn't work before, because here in my sandboxapp it does. Weird. Have fun working ;)
Visual Studio can't evaluate this, can you?
public object moo { __get { return moo; } __set { moo = value; } }OK, I see now... I'm a fairly new programmer (with no formal training), so I'm not familiar with object comparison. The other methods (min, max) use IComparable, but they don't bother using it for the ColumnEqual method. I've changed: return (a==b); //value type standard comparison to: if (((IComparable)a).CompareTo(b) == 0) return true; else return false; works great now, on the different data types and my OleDbCommand string results. Ron