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

Определение

Возвращает значение, указывающее, имеет ли заданный объект тип 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

Параметры

value
Object Object Object Object

Объект.An object.

Возвраты

true, если value имеет тип DBNull; в противном случае — false.true if value is of type DBNull; otherwise, false.

Примеры

В следующем примере используется SqlDataReader объект для извлечения данных опроса из базы данных.The following example uses a SqlDataReader object to retrieve survey data from a database. Он назначает значения полей для каждой строки в массив, а затем передает их массива элемент IsDBNull метод.It assigns each row's field values to an array, and then passes each array element to the IsDBNull method. Если метод возвращает true, этот пример назначает строку «Н/д» элемент массива.If the method returns true, the example assigns the string "NA" to the array element. Затем добавляется массив Rows коллекцию System.Windows.Forms.DataGridView элемента управления.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

Комментарии

IsDBNull Тесты метод ли value равен параметр DBNull.Value.The IsDBNull method tests whether the value parameter is equal to DBNull.Value. Это аналогично следующему коду:It is equivalent to the following code:

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

Примечание

DBNull.Value используется для указания значения, которое отсутствует.DBNull.Value is used to indicate a value that is missing. Это не эквивалентно null или String.Empty.It is not equivalent to null or to String.Empty. Таким образом, код например Convert.IsDBNull(null) в C# или Convert.IsDBNull(Nothing) в Visual Basic возвращаетfalse.Therefore, code such as Convert.IsDBNull(null) in C# or Convert.IsDBNull(Nothing) in Visual Basic returnsfalse.

Применяется к

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