为什么结构在vb和c＃中表现不同？ [英] Why structure behaves differently in vb and c#?

问题描述

结构是一个值类型，不能有空值，那么为什么它在vb和c＃中表现不同？

structure is a value type and can't have null values then why it behaves different in vb and c#?

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim c As System.Drawing.Color
        If c <> Nothing Then
            MessageBox.Show(c.Name)
        Else
            MessageBox.Show("null")
        End If
    End Sub
    Public Enum Align
        Left
        Center
        Right
    End Enum
    Public Alignment As Align
    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        If Alignment <> Nothing Then
            MessageBox.Show("true")
        Else
            MessageBox.Show("false")
        End If
    End Sub

private void button1_Click(object sender, EventArgs e)
        {
            System.Drawing.Color c = default(System.Drawing.Color);
            if (c != null)
            {
                MessageBox.Show(c.Name);
            }
            else
            {
                MessageBox.Show("null");
            }
        }
        public enum Align
        {
            Left,
            Center,
            Right
        }

        public Align Alignment;

        private void button2_Click(object sender, EventArgs e)
        {
            if (Alignment != null)
            {
                MessageBox.Show("true");
            }
            else
            {
                MessageBox.Show("false");
            }
        }

推荐答案

为什么结构在vb和c＃中表现不同？



它没有。您在代码中看到的是C＃中 null 与VB.NET中 Nothing 之间的区别。 />


null 是非参考，仅限于此。结构和枚举是值类型，而不是引用类型，因此您不能为它们指定null。



Nothing 是您分配给它的任何内容的默认值。对于引用类型，它与 null 相同。对于值类型，它是它们的默认值，因此通常为0。结构和枚举具有默认值，因此您可以为它们分配 Nothing 。



所以， VB.NET中没有基本上是 null 和默认（T） C＃中的一个。

Why structure behaves differently in vb and c#?

It doesn't. What you're seeing in your code is the difference between null in C# and Nothing in VB.NET.

null is a non-reference and only that. Structures and Enums are value types, not reference types, so you can't assign null to them.

Nothing is the default value of whatever you assign it to. For reference types, it's the same as null. For value types, it's their default value, so 0 in general. Structures and Enums have a default value, so you can assign Nothing to them.

So, Nothing in VB.NET is basically the null and default(T) of C# in one.

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