Convert.IsDBNull(Object) Methode

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

Parameters

Retouren

true als value van het type DBNullis; anders, false.

Voorbeelden

In het volgende voorbeeld wordt een SqlDataReader object gebruikt om enquêtegegevens op te halen uit een database. Hiermee worden de veldwaarden van elke rij toegewezen aan een matrix en wordt elk matrixelement vervolgens doorgegeven aan de IsDBNull methode. Als de methode retourneert true, wijst het voorbeeld de tekenreeks 'NA' toe aan het matrixelement. De matrix wordt vervolgens toegevoegd aan de verzameling Rows van een besturingselement System.Windows.Forms.DataGridView.

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

Opmerkingen

De IsDBNull methode test of de value parameter gelijk is aan DBNull.Value. Deze is gelijk aan de volgende code:

return DBNull.Value.Equals(value);

DBNull.Value.Equals value

Return DBNull.Value.Equals(value)

Zie ook

• DBNull