How to get ValueType default for DBNull column using reflection?

  • Thread starter Thread starter Brian Brane
  • Start date Start date
B

Brian Brane

I have properties that wrap DataRow columns as in:
public int aNumber
{ get{ return m_DataRow["aColumnName"]; }
set{ m_DataRow["aColumnName"] = value; } }

If the column happens to contain DBNull, I get a cast
exception since DBNull cannot be converted to int. I
wrote the following method that looks up the column's
data type and if it is a ValueType, returns the default
value for the ValueType.

I want the method to use reflection to get the default
value, so I don't have to code for each data type. However,
GetConstructor always returns null. Is there some other
way to get the default value for a ValueType?

Thanks,

Brian Brane

Here's the method...

// Instance variables containing default values (work around)
bool m_Bool;
byte m_Byte;
DateTime m_DateTime;
Guid m_Guid;
int m_Int;

private object GetColumnValueOrDefault(DataRow dataRow, string columnName)
{
DataColumn column = dataRow.Table.Columns[columnName];
if (column == null)
throw new Exception("Invalid column name: " + columnName);

// If the column contains a value, then return the value
object value = dataRow[column];
if (value != System.DBNull.Value)
return value;

// If the column is not a ValueType, then return null
System.Type dataType = column.DataType;
if (!dataType.IsValueType)
return null;

// Use reflection to create a new ValueType
System.Reflection.ConstructorInfo constructor =
dataType.GetConstructor(System.Type.EmptyTypes);
if (constructor != null) //// IS ALWAYS NULL!
{
object defaultValue = constructor.Invoke(new object[0]);
return defaultValue;
}

// Return a ValueType that is initialized to its default value
if (dataType == typeof(System.Boolean))
return m_Bool;

if (dataType == typeof(System.Byte))
return m_Byte;

if (dataType == typeof(System.DateTime))
return m_DateTime;

if (dataType == typeof(System.Guid))
return m_Guid;

if (dataType == typeof(System.Int32))
return m_Int;

throw new Exception("Unable to determine default for " + columnName);
}
 
Hi Brian,

I think you are posting to the wrong newsgroup, I think you did want to be
2 rows higher (if you see the JavaScritp group above that is CSharp)

:-)

Cor
 
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