Convert.IsDBNull(Object) Método

Definición

Devuelve una indicación en la que se precisa si el objeto especificado es de tipo DBNull.

public:
 static bool IsDBNull(System::Object ^ value);
public static bool IsDBNull (object? value);
public static bool IsDBNull (object value);
static member IsDBNull : obj -> bool
Public Shared Function IsDBNull (value As Object) As Boolean

Parámetros

value
Object

Objeto.

Devoluciones

Boolean

true si value es de tipo DBNull; en caso contrario, false.

Ejemplos

En el ejemplo siguiente se usa un SqlDataReader objeto para recuperar datos de encuesta de una base de datos. Asigna los valores de campo de cada fila a una matriz y, a continuación, pasa cada elemento de matriz al IsDBNull método . Si el método devuelve true, el ejemplo asigna la cadena "NA" al elemento de matriz. A continuación, la matriz se agrega a la Rows colección de un System.Windows.Forms.DataGridView control .

private void Form1_Load(object sender, EventArgs e)
{
   // Define ADO.NET objects.
   SqlConnection conn = new SqlConnection(connectionString);
   SqlCommand cmd = new SqlCommand();
   SqlDataReader dr;

   // Open connection, and retrieve dataset.
   conn.Open();

   // Define Command object.
   cmd.CommandText = "Select * From Responses";
   cmd.CommandType = CommandType.Text;
   cmd.Connection = conn;

   // Retrieve data reader.
   dr = cmd.ExecuteReader();

   int fieldCount = dr.FieldCount;
   object[] fieldValues = new object[fieldCount];
   string[] headers = new string[fieldCount];

   // Get names of fields.
   for (int ctr = 0; ctr < fieldCount; ctr++)
      headers[ctr] = dr.GetName(ctr);

   // Set up data grid.
   this.grid.ColumnCount = fieldCount;

   this.grid.ColumnHeadersDefaultCellStyle.BackColor = Color.Navy;
   this.grid.ColumnHeadersDefaultCellStyle.ForeColor = Color.White;
   this.grid.ColumnHeadersDefaultCellStyle.Font = new Font(this.grid.Font, FontStyle.Bold);

   this.grid.AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders;
   this.grid.ColumnHeadersBorderStyle = DataGridViewHeaderBorderStyle.Single;
   this.grid.CellBorderStyle = DataGridViewCellBorderStyle.Single;
   this.grid.GridColor = Color.Black;
   this.grid.RowHeadersVisible = true;

   for (int columnNumber = 0; columnNumber < headers.Length;  columnNumber++)
      this.grid.Columns[columnNumber].Name = headers[columnNumber];

   // Get data, replace missing values with "NA", and display it.
   while (dr.Read())
   {
      dr.GetValues(fieldValues);

      for (int fieldCounter = 0; fieldCounter < fieldCount; fieldCounter++)
      {
         if (Convert.IsDBNull(fieldValues[fieldCounter]))
            fieldValues[fieldCounter] = "NA";
      }
      grid.Rows.Add(fieldValues);
   }
   dr.Close();
}
member this.Form1_Load(sender: obj, e: EventArgs) =
    // Define ADO.NET objects.
    use conn = new SqlConnection(connectionString)
    use cmd = new SqlCommand()

    // Open connection, and retrieve dataset.
    conn.Open()

    // Define Command object.
    cmd.CommandText <- "Select * From Responses"
    cmd.CommandType <- CommandType.Text
    cmd.Connection <- conn

    // Retrieve data reader.
    let dr = cmd.ExecuteReader()

    let fieldCount = dr.FieldCount
    let fieldValues = Array.zeroCreate<obj> fieldCount
    let headers = 
        // Get names of fields.
        Array.init fieldCount dr.GetName

    // Set up data grid.
    grid.ColumnCount <- fieldCount

    grid.ColumnHeadersDefaultCellStyle.BackColor <- Color.Navy
    grid.ColumnHeadersDefaultCellStyle.ForeColor <- Color.White
    grid.ColumnHeadersDefaultCellStyle.Font <- new Font(grid.Font, FontStyle.Bold)

    grid.AutoSizeRowsMode <- DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders
    grid.ColumnHeadersBorderStyle <- DataGridViewHeaderBorderStyle.Single
    grid.CellBorderStyle <- DataGridViewCellBorderStyle.Single
    grid.GridColor <- Color.Black
    grid.RowHeadersVisible <- true

    for columnNumber = 0 to headers.Length - 1 do
        grid.Columns[columnNumber].Name <- headers[columnNumber]

    // Get data, replace missing values with "NA", and display it.
    while dr.Read() do
        dr.GetValues fieldValues |> ignore
        for fieldCounter = 0 to fieldCount do
            if Convert.IsDBNull fieldValues[fieldCounter] then
                fieldValues[fieldCounter] <- "NA"
        grid.Rows.Add fieldValues |> ignore
    dr.Close()

override _.Dispose(disposing) =
    if disposing then
        grid.Dispose();
    base.Dispose disposing
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
   ' Define ADO.NET objects.
   Dim conn As New SqlConnection(connectionString)
   Dim cmd As New SqlCommand
   Dim dr As SqlDataReader

   ' Open connection, and retrieve dataset.
   conn.Open()

   ' Define Command object.
   cmd.CommandText = "Select * From Responses"
   cmd.CommandType = CommandType.Text
   cmd.Connection = conn

   ' Retrieve data reader.
   dr = cmd.ExecuteReader()

   Dim fieldCount As Integer = dr.FieldCount
   Dim fieldValues(fieldCount - 1) As Object
   Dim headers(fieldCount - 1) As String

   ' Get names of fields.
   For ctr As Integer = 0 To fieldCount - 1
      headers(ctr) = dr.GetName(ctr)
   Next

   ' Set up data grid.
   grid.ColumnCount = fieldCount

   With grid.ColumnHeadersDefaultCellStyle
      .BackColor = Color.Navy
      .ForeColor = Color.White
      .Font = New Font(grid.Font, FontStyle.Bold)
   End With

   With grid
      .AutoSizeRowsMode = _
          DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders
      .ColumnHeadersBorderStyle = DataGridViewHeaderBorderStyle.Single
      .CellBorderStyle = DataGridViewCellBorderStyle.Single
      .GridColor = Color.Black
      .RowHeadersVisible = True

      For columnNumber As Integer = 0 To headers.Length - 1
         .Columns(columnNumber).Name = headers(columnNumber)
      Next
   End With

   ' Get data, replace missing values with "NA", and display it.
   Do While dr.Read()
      dr.GetValues(fieldValues)

      For fieldCounter As Integer = 0 To fieldCount - 1
         If Convert.IsDBNull(fieldValues(fieldCounter)) Then
            fieldValues(fieldCounter) = "NA"
         End If
      Next
      grid.Rows.Add(fieldValues)
   Loop
   dr.Close()
End Sub

Comentarios

El IsDBNull método comprueba si el value parámetro es igual a DBNull.Value. Es equivalente al código siguiente:

return DBNull.Value.Equals(value);
DBNull.Value.Equals value
Return DBNull.Value.Equals(value)

Nota

DBNull.Value se usa para indicar un valor que falta. No es equivalente a null ni a String.Empty. Por lo tanto, el código como Convert.IsDBNull(null) en C# o Convert.IsDBNull(Nothing) en Visual Basic devuelvefalse.

Se aplica a

Consulte también