Convert.IsDBNull(Object) Metoda

Definicja

Zwraca wskazanie, czy określony obiekt jest typu 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

Parametry

value
Object

Obiekt.An object.

Zwraca

true, jeśli value jest typu DBNull; w przeciwnym razie false.true if value is of type DBNull; otherwise, false.

Przykłady

Poniższy przykład używa obiektu SqlDataReader, aby pobrać dane ankiety z bazy danych.The following example uses a SqlDataReader object to retrieve survey data from a database. Przypisuje każdemu wierszowi wartości pola do tablicy, a następnie przekazuje każdy element tablicy do metody IsDBNull.It assigns each row's field values to an array, and then passes each array element to the IsDBNull method. Jeśli metoda zwraca true, w przykładzie przypisze ciąg "NA" do elementu tablicy.If the method returns true, the example assigns the string "NA" to the array element. Tablica jest następnie dodawana do kolekcji Rows kontrolki System.Windows.Forms.DataGridView.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

Uwagi

Metoda IsDBNull sprawdza, czy parametr value jest równy DBNull.Value.The IsDBNull method tests whether the value parameter is equal to DBNull.Value. Odpowiada to następującemu kodowi:It is equivalent to the following code:

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

Uwaga

DBNull.Value służy do wskazywania braku wartości.DBNull.Value is used to indicate a value that is missing. Nie jest on równoznaczny z null lub String.Empty.It is not equivalent to null or to String.Empty. W związku z tym, kod, C# taki jak Convert.IsDBNull(null) w lub Convert.IsDBNull(Nothing) w Visual Basic zwracafalse.Therefore, code such as Convert.IsDBNull(null) in C# or Convert.IsDBNull(Nothing) in Visual Basic returnsfalse.

Dotyczy

Zobacz też