How to get ValueType default for DBNull column using reflection?

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  • Brian Brane
    #1

    How to get ValueType default for DBNull column using reflection?

    I have properties that wrap DataRow columns as in:
    public int aNumber
    { get{ return m_DataRow["aColumnNam e"]; }
    set{ m_DataRow["aColumnNam e"] = 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 GetColumnValueO rDefault(DataRo w dataRow, string columnName)
    {
    DataColumn column = dataRow.Table.C olumns[columnName];
    if (column == null)
    throw new Exception("Inva lid column name: " + columnName);

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

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

    // Use reflection to create a new ValueType
    System.Reflecti on.ConstructorI nfo constructor =
    dataType.GetCon structor(System .Type.EmptyType s);
    if (constructor != null) //// IS ALWAYS NULL!
    {
    object defaultValue = constructor.Inv oke(new object[0]);
    return defaultValue;
    }

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

    if (dataType == typeof(System.B yte))
    return m_Byte;

    if (dataType == typeof(System.D ateTime))
    return m_DateTime;

    if (dataType == typeof(System.G uid))
    return m_Guid;

    if (dataType == typeof(System.I nt32))
    return m_Int;

    throw new Exception("Unab le to determine default for " + columnName);
    }
    Tags: None
    • Cor
      #2
      Re: How to get ValueType default for DBNull column using reflection?

      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



      "Brian Brane" <brian_brane@ms n.com> schreef in bericht
      news:24e540ea.0 312030625.3d3c0 dbc@posting.goo gle.com...[color=blue]
      > I have properties that wrap DataRow columns as in:
      > public int aNumber
      > { get{ return m_DataRow["aColumnNam e"]; }
      > set{ m_DataRow["aColumnNam e"] = 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 GetColumnValueO rDefault(DataRo w dataRow, string columnName)
      > {
      > DataColumn column = dataRow.Table.C olumns[columnName];
      > if (column == null)
      > throw new Exception("Inva lid column name: " + columnName);
      >
      > // If the column contains a value, then return the value
      > object value = dataRow[column];
      > if (value != System.DBNull.V alue)
      > return value;
      >
      > // If the column is not a ValueType, then return null
      > System.Type dataType = column.DataType ;
      > if (!dataType.IsVa lueType)
      > return null;
      >
      > // Use reflection to create a new ValueType
      > System.Reflecti on.ConstructorI nfo constructor =
      > dataType.GetCon structor(System .Type.EmptyType s);
      > if (constructor != null) //// IS ALWAYS NULL!
      > {
      > object defaultValue = constructor.Inv oke(new object[0]);
      > return defaultValue;
      > }
      >
      > // Return a ValueType that is initialized to its default value
      > if (dataType == typeof(System.B oolean))
      > return m_Bool;
      >
      > if (dataType == typeof(System.B yte))
      > return m_Byte;
      >
      > if (dataType == typeof(System.D ateTime))
      > return m_DateTime;
      >
      > if (dataType == typeof(System.G uid))
      > return m_Guid;
      >
      > if (dataType == typeof(System.I nt32))
      > return m_Int;
      >
      > throw new Exception("Unab le to determine default for " + columnName);
      > }[/color]


        Comment

        • Brian Brane
          #3
          Re: How to get ValueType default for DBNull column using reflection?

          Will do. Thanks!

            Comment

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