Nullable<T>.HasValue 屬性

定義

取得值,指出目前的 Nullable<T> 物件是否具有其基礎類型的有效值。Gets a value indicating whether the current Nullable<T> object has a valid value of its underlying type.

public:
 property bool HasValue { bool get(); };
public bool HasValue { get; }
member this.HasValue : bool
Public ReadOnly Property HasValue As Boolean

屬性值

如果目前 Nullable<T> 物件有值,則為 true,如果目前 Nullable<T> 物件沒有值,則為 falsetrue if the current Nullable<T> object has a value; false if the current Nullable<T> object has no value.

範例

下列範例HasValue會使用Nullable<int>Nullable(Of Integer)在 Visual Basic)物件中的屬性,來判斷它是否應該顯示物件的Value屬性或其GetValueOrDefault屬性。The following example uses the HasValue property of a Nullable<int> (Nullable(Of Integer) in Visual Basic) object to determine whether it should display the object's Value property or its GetValueOrDefault property.

using System;

public class Example
{
   public static void Main()
   {
      Nullable<int> n1 = new Nullable<int>(10);
      Nullable<int> n2 = null;
      Nullable<int> n3 = new Nullable<int>(20);
      n3 = null;
      Nullable<int>[] items = { n1, n2, n3 }; 

      foreach (var item in items) {
         Console.WriteLine("Has a value: {0}", item.HasValue);
         if (item.HasValue) {
            Console.WriteLine("Type: {0}", item.GetType().Name);
            Console.WriteLine("Value: {0}", item.Value);
         }
         else {
            Console.WriteLine("Null: {0}", item == null);
            Console.WriteLine("Default Value: {0}", item.GetValueOrDefault());
         }
         Console.WriteLine();
      }                  
   }
}
// The example displays the following output:
//       Has a value: True
//       Type: Int32
//       Value: 10
//       
//       Has a value: False
//       Null: True
//       Default Value: 0
//       
//       Has a value: False
//       Null: True
//       Default Value: 0
Module Example
   Public Sub Main()
      Dim n1 As New Nullable(Of Integer)(10)
      Dim n2 As Nullable(Of Integer)
      Dim n3 As New Nullable(Of Integer)(20)
      n3 = Nothing
      Dim items() As Nullable(Of Integer) = { n1, n2, n3 }

      For Each item In items
         Console.WriteLine("Has a value: {0}", item.HasValue)
         If item.HasValue Then
            Console.WriteLine("Type: {0}", item.GetType().Name)
            Console.WriteLine("Value: {0}", item.Value)
         Else
            Console.WriteLine("Null: {0}", item Is Nothing)
            Console.WriteLine("Default Value: {0}", item.GetValueOrDefault())
         End If
         Console.WriteLine()
      Next                  
   End Sub
End Module
' The example displays the following output:
'       Has a value: True
'       Type: Int32
'       Value: 10
'       
'       Has a value: False
'       Null: True
'       Default Value: 0
'       
'       Has a value: False
'       Null: True
'       Default Value: 0

備註

如果屬性為true,則可以使用Value屬性來存取Nullable<T>目前物件的值。 HasValueIf the HasValue property is true, the value of the current Nullable<T> object can be accessed with the Value property. 否則,嘗試存取其值InvalidOperationException會擲回例外狀況。Otherwise, attempting to access its value throws an InvalidOperationException exception.

適用於