Value Field
Collapse the table of content
Expand the table of content

DBNull.Value Field

 

Represents the sole instance of the DBNull class.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

public static readonly DBNull Value

Field Value

Type: System.DBNull

DBNull is a singleton class, which means only this instance of this class can exist.

If a database field has missing data, you can use the DBNull.Value property to explicitly assign a DBNull object value to the field. However, most data providers do this automatically.

To evaluate database fields to determine whether their values are DBNull, you can pass the field value to the DBNull.Value.Equals method. However, this method is rarely used because there are a number of other ways to evaluate a database field for missing data. These include the Visual Basic IsDBNull function, the Convert.IsDBNull method, the DataTableReader.IsDBNull method, the IDataRecord.IsDBNull method, and several other methods.

The following example calls the DBNull.Value.Equals method to determine whether a database field in a contacts database has a valid value. If it does, the field value is appended to the string output in a label.

private void OutputLabels(DataTable dt)
{
   string label; 

   // Iterate rows of table
   foreach (DataRow row in dt.Rows)
   {
      int labelLen;
      label = String.Empty;
      label += AddFieldValue(label, row, "Title");
      label += AddFieldValue(label, row, "FirstName");
      label += AddFieldValue(label, row, "MiddleInitial");
      label += AddFieldValue(label, row, "LastName");
      label += AddFieldValue(label, row, "Suffix");
      label += "\n";
      label += AddFieldValue(label, row, "Address1");
      label += AddFieldValue(label, row, "AptNo");
      label += "\n";
      labelLen = label.Length;
      label += AddFieldValue(label, row, "Address2");
      if (label.Length != labelLen)
         label += "\n";
      label += AddFieldValue(label, row, "City");
      label += AddFieldValue(label, row, "State");
      label += AddFieldValue(label, row, "Zip");
      Console.WriteLine(label);
      Console.WriteLine();
   }
}

private string AddFieldValue(string label, DataRow row, 
                             string fieldName) 
{                                
   if (! DBNull.Value.Equals(row[fieldName])) 
      return (string) row[fieldName] + " ";
   else
      return String.Empty;
}

Universal Windows Platform
Available since 10
.NET Framework
Available since 1.1
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Return to top
Show:
© 2016 Microsoft