Convert.IsDBNull(Object) Convert.IsDBNull(Object) Convert.IsDBNull(Object) Convert.IsDBNull(Object) Method

Definizione

Indica se l'oggetto specificato è di tipo DBNull.Returns an indication whether the specified object is of type DBNull.

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

Parametri

value
Object Object Object Object

Oggetto.An object.

Restituisce

true se value è di tipo DBNull ; in caso contrario, false.true if value is of type DBNull; otherwise, false.

Esempi

L'esempio seguente usa un SqlDataReader oggetto per recuperare i dati dei sondaggi da un database.The following example uses a SqlDataReader object to retrieve survey data from a database. Assegna i valori di campo di ogni riga in una matrice e quindi passa ogni elemento di matrice al IsDBNull (metodo).It assigns each row's field values to an array, and then passes each array element to the IsDBNull method. Se il metodo restituisce true, nell'esempio viene assegnato la stringa "NA" per l'elemento della matrice.If the method returns true, the example assigns the string "NA" to the array element. La matrice viene quindi aggiunto al Rows raccolta di un System.Windows.Forms.DataGridView controllo.The array is then added to the Rows collection of a 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();
}
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

Commenti

Il IsDBNull metodo verifica se il value parametro è uguale a DBNull.Value.The IsDBNull method tests whether the value parameter is equal to DBNull.Value. È equivalente al codice seguente:It is equivalent to the following code:

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

Nota

DBNull.Value Consente di indicare un valore che non è presente.DBNull.Value is used to indicate a value that is missing. Non è equivalente a null o a String.Empty.It is not equivalent to null or to String.Empty. Pertanto, ad esempio di codice Convert.IsDBNull(null) in C# o Convert.IsDBNull(Nothing) in Visual Basic restituiscefalse.Therefore, code such as Convert.IsDBNull(null) in C# or Convert.IsDBNull(Nothing) in Visual Basic returnsfalse.

Si applica a

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